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Integrated use of terrestrial laser scanning and advanced numerical methods for a total slope analysis.. 2006

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INTEGRATED USE OF TERRESTRIAL LASER SCANNING AND ADVANCED NUMERICAL METHODS FOR A TOTAL SLOPE ANALYSIS OF AFTERNOON CREEK, WASHINGTON by A L E X A N D E R B R I A N STROUTH B.A.Sc , Colorado School of Mines, 2004 A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF M A S T E R OF APPLIED SCIENCE in THE F A C U L T Y OF G R A D U A T E STUDIES (Geological Engineering) THE UNIVERSITY OF BRITISH C O L U M B I A August 2006 © Alexander Brian Strouth, 2006 A B S T R A C T On November 9, 2003 a rock slide, involving approximately 750,000 m 3 of jointed, orthogneiss occurred at the Afternoon Creek ridge adjacent to the North Cascades Highway in northwest Washington. Most of the failed blocks slid into the shallowly-sloping Afternoon Creek, and did not reach the highway. However, a small fraction of the failed volume traveled down the west side of the ridge, and entered the steep, bedrock-exposed Falls Creek. This debris fell more than 600 meters and impacted the highway, destroying a section of the roadway. There is potential for future rock slope failures to occur at this site, thereby presenting a hazard to the highway below. The purpose of this research was to investigate the November, 2003 rock slope failure and post-failure motion, and use the results of the investigation to assess the location, volume, and effects of future slope failures at Afternoon Creek. A Total Slope Analysis methodology was followed, which linked the failure initiation analysis with runout analysis. The structural and topographic controls to failure were analyzed with limit equilibrium, and the 2-D and 3-D distinct element codes, UDEC and 3DEC. Runout was analyzed with the 3- D dynamic analysis code, DAN3D. A data collection methodology designed specifically for numerical modeling was implemented. A laser scanner and field survey were used to collect discontinuity characterization data at the inaccessible, hazardous site. The analysis suggested that the past event involved a single-stage, extremely-rapid translational failure on a highly-persistent, moderately-dipping joint set. Material that entered Falls Creek was most likely caused by secondary rockfall immediately following the slide. A future rock slope failure up to 300,000 m 3 emanating from the existing failure scarp is possible; however runout debris from an event of this volume is not expected to reach the highway at Afternoon Creek. In contrast, retrogression of the failure scarp crest by sliding or toppling of individual columns may cause rockfall to enter the steeper Falls Creek travel path where it will most likely impact the highway embayment. ii T A B L E O F C O N T E N T S ABSTRACT .' i i T A B L E OF CONTENTS. i i i LIST OF TABLES vii LIST OF FIGURES ix A C K N O W L E D G E M E N T S xv 1 INTRODUCTION 1 1.1 Problem Statement 1 1.2 Research Objectives 2 1.3 Organization of the Paper 2 1.4 Literature Review 3 1.4.1 Slope Failure Initiation 3 1.4.2 Rock Mass Characterization 8 1.4.3 Runout Prediction of Failed Slopes 10 2 PROJECT SETTING: The Afternoon Creek Rockslide 13 2.1 Introduction 13 2.2 Geological and Geomorphological Setting 15 2.3 History of Activity 17 2.4 Chronology of Recent Mass Wasting Events 18 2.5 Continued Monitoring, Hazard Mitigation, and Future Work 20 2.6 Conclusions 22 3 M E T H O D O L O G Y 23 3.1 Total Slope Analysis Procedure 23 3.2 Traditional Data Collection Techniques 24 3.2.1 Desk Study 25 3.2.2 Field Mapping 27 3.2.3 Discontinuity Mapping 28 3.3 Terrestrial Laser Scanning (LiDAR) 30 3.3.1 L i D A R Data Acquisition 30 3.3.2 L i D A R Data Processing 34 3.3.3 L i D A R Data Analysis 36 3.4 Computer Analysis Techniques 41 i n 3.4.1 Geographical Information System (GIS) 41 3.4.2 Spheristat 2.2 41 3.4.3 RocLab 42 3.4.4 Limit Equilibrium Methods (RocPlane, Swedge) 42 3.4.5 Universal Distinct Element Code (UDEC) 42 3.4.6 Three Dimensional Distinct Element Code (3DEC) 42 3.4.7 Surfer 43 3.4.8 Three-Dimensional Dynamic Analysis (DAN3D) 43 3.4.9 Three-Dimensional Rockfall Simulation.(PIR3D) 43 3.5 Numerical Modeling Procedure 43 4 PROCESSING OF FIELD D A T A FOR N U M E R I C A L M O D E L I N G 45 4.1 Physiographic & Geologic Setting 45 4.1.1 Regional Setting 45 4.1.2 Local Geology 46 4.2 Geometric Parameters 53 4.2.1 Topography 53 4.2.2 Description of Joint Sets 54 4.2.2.1 Orientation 54 4.2.2.2 Spacing 58 4.2.2.3 Persistence ••• 58 4.3 Mechanical Properties 59 4.3.1 Rock Mass Properties 59 4.3.2 Discontinuity Properties 61 5 B A C K A N A L Y S I S OF THE 2003 AFTERNOON C R E E K SLOPE F A I L U R E 63 5.1 Failure Initiation in Afternoon Creek 63 5.1.1 Kinematic Analysis 64 5.1.2 Limit Equilibrium Analysis 67 5.1.3 Two-Dimensional Numerical Modeling 71 5.1.3.1 2-D Representation 71 5.1.3.2 Modeling Methodology 74 5.1.3.3 Baseline Model Results 78 5.1.3.4 Trial Model Results 85 5.1.4 Three-Dimensional Numerical Modeling 95 5.1.4.1 Modeling Methodology 95 5.1.4.2 3-D Model Results 98 5.2 Runout Analysis along the Afternoon Creek Travel Path 105 5.2.1 Runout Analysis Methodology 105 5.2.2 Runout Analysis Results 107 5.3 Falls Creek Travel Path 112 5.3.1 Mechanism 1: Translational Failure 113 5.3.2 Mechanism 2: Toppling Failure 117 5.3.3 Mechanism 3: Spreading of a Flowing Mass 121 iv 5.3.4 Mechanism 4: Rockfall caused by dilation of the failed mass 124 5.4 Total Slope Back Analysis Summary 127 5.4.1 Afternoon Creek Travel Path 127 5.4.2 Falls Creek Travel Path 130 6 ANALYSIS OF FUTURE EVENTS AT AFTERNOON C R E E K 133 6.1 Failure Volume Assessment 133 6.1.1 Methodology 134 6.1.2 Locations of Slope Instabilities 136 6.1.3 Summary of Potential Rock Slope Hazard Sources 145 6.2 Analytical Runout Assessment 146 6.2.1 Source Rank 1: Toppling and Rockfall at Slope Crest 147 6.2.2 Source Rank 2: Mid-Slope Failure Scarp 150 6.2.3 Source Rank 3: Translational Failure at Slope Crest 154 6.4 Hazard Assessment Summary 157 7 DISCUSSION OF TOOLS A N D METHODS 158 7.1 Terrestrial Laser Scanning (LiDAR) 158 7.1.1 L i D A R Benefits 158 7.1.2 L i D A R Limitations 159 7.1.3 L i D A R Recommendations 160 7.2 Numerical Modeling Tools 162 7.2.1 UDEC 162 7.2.2 3DEC 163 7.2.3 DAN3D 164 7.3 Total Slope Analysis Method 165 8 CONCLUSIONS A N D RECOMMENDATIONS 167 8.1 Conclusions 167 8.2 Recommendations for Further Work 169 REFERENCE LIST 172 APPENDIX A: SCANLINE S U R V E Y D A T A 179 APPENDIX B: L i D A R POINT CLOUDS A N D PHOTOGRAPHS 181 APPENDIX C: VERIFICATION OF METHODS 183 C . l Discontinuity Orientation 183 C.2 Discontinuity Set Spacing 186 v APPENDIX D: SPACING A N D PERSISTENCE C A L C U L A T I O N S 188 D . l Average Joint Set Spacing 188 D. 2 Average Joint Set Persistence 188 APPENDIX E: N U M E R I C A L M O D E L I N G INPUT FILES 193 E. l UDEC Baseline Model A - A ' 193 E.2 3DEC Model 195 vi L I S T O F T A B L E S Table 1.1 Comparison of classification schemes 6 Table 3.1 Data requirements for numerical modeling 25 Table 3.2 Terrestrial laser scans attempted 32 Table 4.1 Summary of parameters that describe discontinuities in the Afternoon Creek rock slope 54 Table 4.2 Estimates of persistence and exposed persistence in structural Zone 3 59 Table 4.3 Estimated range of rock mass mechanical properties in each structural zone 60 Table 4.4 Estimated range of discontinuity strength and stiffness parameters for numerical modeling 62 Table 5.1 Geometric and mechanical parameters for baseline U D E C models 77 Table 5.2 Mechanical parameters of Zone B modeled as an equivalent continuum 85 Table 5.3 Range of reasonable mechanical parameters for zone 87 Table 5.4 Range of geometric parameters used in the trial models 89 Table 5.5 Geometric and mechanical parameters for the 3DEC model : 98 Table 5.6 Mechanisms that allow debris to enter Falls Creek - Ranked in order of importance 132 Table 6.1 Geometric configurations of the forward analysis U D E C models, Afternoon Creek 135 Table 6.2 Summary of the three hazard sources at Afternoon Creek 145 Table 6.3 Summary of runout models of the source volume at the middle of the failure scarp. 151 Table 6.4 Summary of runout models of the failure scarp crest source volume 154 Table C . l The 10 meter portion of scanline survey 1 considered in this example 183 Table D.2 Discontinuity persistence estimated from photograph trace length measurements. 190 Table D.3 Discontinuity exposed persistence estimated by direct measurement on the 3-D point cloud 191 vii Table D.4 Discontinuity exposed persistence estimated by relating the exposed persistence to patch area in the 3-D point cloud 192 v i i i L I S T O F F I G U R E S Figure 2.1 Afternoon Creek rockslide above SR 20 near Newhalem, WA. Photograph provided by John Scurlock, Concrete, W A 14 Figure 2.2 Location of Afternoon Creek rockslide. Three kilometers east of Newhalem, Washington, USA. Failure area outlined in yellow 15 Figure 2.3 Sub-vertical, open fractures parallel to Afternoon Creek. Photograph provided by URS Corporation (photograph date, March 1, 2004) 17 Figure 2.4 Historical photographs of debris flows that reached the state route 20 highway. (a) Afternoon Creek debris flow material (March 23, 1949). (b) Falls Creek Chute debris flow covered with fresh snow (1990). (c) Afternoon Creek (1999 or 2000). Photographs provided by WSDOT 18 Figure 2.5 Afternoon Creek rockslide slope before the November 9, 2003 event. Photograph provided by WSDOT (date unknown) 18 Figure 2.6 Afternoon Creek failure scarp from Afternoon Creek, (photograph date April, 2005) 19 Figure 2.7 Digital elevation model of the Afternoon Creek rockslide. Failed mass is shown in gray. Arrows indicate direction of movement. Dotted blue line indicates the extent of path 20 Figure 3.1. Flow chart of the repeated Total Slope Analysis. Back analysis and characterization of current hazard 24 Figure 3.2 URS structural mapping location compared to failure zone 26 Figure 3.3 Schematic cross-sections perpendicular and oblique to Afternoon Creek 28 Figure 3.4. Oblique photo of the Afternoon Creek scarp, showing location of scanline survey 1 (75 meters length) and scanline survey 2 29 Figure 3.5 Terrestrial laser scanning stations 31 Figure 3.6 Histograms of dip direction vs. frequency for four scans of structural Zone 3 showing a bias in the orientation of automatically generated patches. Joint planes that strike perpendicular to the scanner position tend to reflect more laser strikes. Line-of-sight of the scanner is superimposed over the histogram 33 Figure 3.7 Point cloud processing procedure 35 I X Figure 3.8 Comparison of automatically generated patches in point clouds 2/1, 6/1, 7/1, 7/2. Minimum patch size =10; minimum neighbor angle = 4 degrees (top); minimum neighbor angle =10 degrees (bottom). Pole size is scaled to patch area 37 Figure 3.9 Schematic, annotated patch 40 Figure 4.1 Geologic map of Afternoon Creek 47 Figure 4.2 Structural domain Zone 1 49 Figure 4.3 Structural domain Zone 2 50 Figure 4.4 Structural domain Zone B 51 Figure 4.5 Structural domain Zone 3 viewed from three directions 52 Figure 4.6 Contour map and topographic profiles derived from the L i D A R D E M after the November, 2003 event. Arrows indicate the path of steepest descent 53 Figure 4.7 Joint sets A and B in structural domain Zone 3 55 Figure 4.8 Structural Zone 3. Contoured stereographic projection of automatically generated patches that represent discontinuity surfaces measured in scanline survey JS2 and in point clouds 2/1, 6/1, 7/1, 7/2. minimum neighbor angle = 6° and minimum patch size=20. Vectors indicate the L i D A R scan line-of-sight 56 Figure 4.9 Structural Zone 2. Stereographic projection of discontinuities exposed at the base of the failure scarp. Measured during scan line survey JS1 and point cloud analysis of scan 3/4 57 Figure 4.10 Stereographic projection of automatically generated patches that represent discontinuity surfaces in point clouds 2/1, 6/1, 7/1, 7/2. Pole size is scaled to patch area. Joint set orientation is the average orientation of poles included in the set 57 Figure 5.1 Topographic map of Afternoon Creek after the November, 2003 failure, showing the average and local slope orientation used in the kinematic analysis 65 Figure 5.2 Stereographic projection of the important discontinuity sets compared with the average orientation of the Afternoon Creek slope 66 Figure 5.3 Stereographic projection of the important discontinuity sets compared with the local orientation of the Afternoon Creek slope at the interface of Zone B and Zone 3 67 Figure 5.4. Illustration of the simplified limit equilibrium analysis models. The Rocscience program Swedge was used for wedge stability analysis; Rocplane was used for planar stability analysis 68 x Figure 5.5 Comparison of the joint strength properties necessary to achieve stability for the three failure modes 69 Figure 5.6 Summary of the parametric study - Percent Change versus Factor of Safety 70 Figure 5.7 Two-stage failure hypothesis showing failure masses and movement directions. 72 Figure 5.8 Map of Afternoon Creek (after the November, 2003 failure) showing location of cross-sections and source volume (contour units = meters) 72 Figure 5.9 Cross-section A - A ' before and after the November, 2003 failure. The post- glacial topographic profile is inferred 73 Figure 5.10 Cross-section B - B ' before and after the November, 2003 failure. The post- glacial topographic profile is inferred 73 Figure 5.11 Flow chart of modeling procedure 74 Figure 5.12 Block outline for cross sections A - A ' and B - B ' 75 Figure 5.13 Horizontal (Sxx) and vertical.(Syy) stress contours for baseline models A and B. .' 76 Figure 5.14 Baseline model A - A ' results, damage states 2 and 4 relative to November, 2003 slide surface; horizontal displacement contours (top); total displacement vectors and joint shear indicators (red) (bottom) • 81 Figure 5.15 Baseline model B - B ' results, damage states 3 and 4 relative to November, 2003 slide surface; horizontal displacement contours (top); total displacement vectors and joint shear indicators (red) (bottom) 82 Figure 5.16 Baseline model A - A ' results, damage state 2 and 4 relative to the November, 2003 slide surface; plasticity indicators 83 Figure 5.17 Baseline model B - B ' results, damage state 2 and 4 relative to the November, 2003 slide surface; plasticity indicators 84 Figure 5.18 Zone B modeled as an equivalent continuum, damage state 4 showing horizontal displacement contours and plasticity indicators; cross-section A - A ' (top); cross- section B - B ' (bottom) 86 Figure 5.19 Comparison of tensile yielding for 3 different rock mass tensile strength values. Damage state 4, cross-section B - B ' models 88 Figure 5.20 Trial models of reduced joint set spacing, damage state 4, showing horizontal displacement 90 Figure 5.21 Trial models with variable joint set B persistence and orientation, damage state 4 showing horizontal displacement 92 x i Figure 5.22 Trial models with joint set B dip = 70°, joint set A dip = 40°, damage state 4 showing horizontal displacement. Joint friction angle = 28° 93 Figure 5.23 Trial models with one joint set, dip = 50°, damage state 4, showing horizontal displacement and plasticity indicators 94 Figure 5.24 The 3-D block model of Afternoon Creek, colored by region 96 Figure 5.25 3-D model results, (a) 3-D view of the failed mass from 'inside' the slope 99 Figure 5.25 continued 3-D model results, (b) Cross-section of failed volume showing filled contours of total displacement. Primary direction of sliding is out of the page, (c) Indicators of plastic yielding of the rock mass, showing the approximate location of the November, 2003 sliding surface 100 Figure 5.26 3-D model plot colored by plastic yielding state of surface zones 101 Figure 5.27 Plastic yielding indicators and total displacement vectors compared with the actual sliding surface. Equivalent location as cross-section A - A ' (top) and B - B ' (bottom) 102 Figure 5.28 Cross-section showing plastic yielding in Zone B 103 Figure 5.29 Difference map overlying the before D E M . Increased mass shown in blue, decreased mass shown in red. Contour interval = 20 m 107 Figure 5.30 Photographs of the Afternoon Creek debris. Note person for scale 108 Figure 5.31 Calibrated DAN3D analysis results. Single-stage analysis results 109 Figure 5.32 Calibrated DAN3D analysis results. Two-stage analysis I l l Figure 5.33 Illustration of the translational failure mechanism in Falls Creek 113 Figure 5.34 Photograph of the crest of the Afternoon Creek/ Falls Creek ridge from the Falls Creek side 114 Figure 5.35 Map of pre-failure topography showing location of cross-section C - C . Filled red contours indicate location and thickness of the failed zone 115 Figure 5.36 U D E C models of the translational failure mechanism before and after removal of Afternoon Creek material. (A) and (B) show Factor of Safety and horizontal velocity contours at failure. (C) and (D) show contours of the major principal stress 116 Figure 5.37 Column toppling towards Afternoon Creek. Photograph date December, 2004; provided by URS Corporation 118 xn Figure 5.38 UDEC model with the measured joint orientations (run 1) showing potential for toppling towards Falls Creek.(A) and (B) show total displacement vectors and plasticity indicators. (C) and (D) show contours of x-displacement 119 Figure 5.39 U D E C model with cross-joints orthogonal to set B (run 2) showing potential for toppling towards Falls Creek.(A) and (B) show total displacement vectors and plasticity indicators. (C) and (D) show contours of x-displacement 120 Figure 5.40 DAN3D simulations during runout. Red contours indicate source zone. Filled blue contours indicate deposit thickness. Dotted black line indicates the trimline mapped on air photos 123 Figure 5.41 The crest of the Aftenoon Creek-Falls Creek ridge showing scarring caused by rockfall 124 Figure 6.1 Post-failure block model, cross-section A - A ' and B - B ' 134 Figure 6.2 Cross-section A - A ' failure volume assessment for future events; horizontal displacement contours. Geometric configurations 1, 2, and 3 137 Figure 6.3 Cross-section B - B ' failure volume assessment for future events; horizontal displacement contours. Geometric configurations 1, 2, and 3 139 Figure 6.4 Two potentially unstable areas of the Afternoon Creek slope. Photograph by Erik Eberhardt (August 2005) 142 Figure 6.5 Map and schematic cross-section D-D' showing estimated location, thickness, and volume of the slope hazard sources 144 Figure 6.6 Example of an unstable column that has potential to cause rockfall in Falls Creek. August ,2005 145 Figure 6.7 Completed embayment on Washington State Route 20 at Falls Creek in February, 2006. Photograph provided by WSDOT 148 Figure 6.8 Rockfall simulation with PIR3D, showing plan view and 3-D view 149 Figure 6.9 Map showing the Falls Creek rockfall source zone boundary line 149 Figure 6.10 DAN3D runout assessment of an Afternoon Creek rock avalanche originating from middle of the failure scarp, showing source volume depth (meters), deposit depth (meters), and maximum velocity. Benchmark model results (top); "worst- case" trial model 3 results (bottom) 153 Figure 6.11 DAN3D runout assessment of an Afternoon Creek rock avalanche originating from the failure scarp crest, showing source volume depth (meters), deposit depth (meters), and maximum velocity. Baseline model results (top); 'worst case scenario' trial model 3 results (bottom) 156 xi i i Figure A l . Photograph of scanline 1 from position 0 to 2000 cm 180 Figure B l . Digital image and 3-D point cloud of scan 2/1. Scan area is outlined in red on the digital photo 181 Figure B2. Digital image and 3-D point cloud of scan 6/1. Scan area is outlined in red on the digital photo 181 Figure B3. Digital image and 3-D point cloud of scan 7/1. Scan area is outlined in red on the digital photo 182 Figure B4. Digital image and 3-D point cloud of scan 7/2. Scan area is outlined in red on the digital photo 182 Figure CI . A9 point cloud showing inserted patches 184 Figure C2. Stereonet showing joint surface orientation estimated from the two methods - point cloud patch method (blue triangles); and hand measurements (black circles) from table B l 185 Figure C3. Comparison of point cloud 3/4 patches (blue triangles) with entire scanline survey in Zone 2 185 Figure C4. 3-D point cloud with virtual scan line 187 Figure C5. Scanline survey JS1 187 x i v A C K N O W L E D G E M E N T S Firstly I would like to thank my wife and best-friend, Amanda, for her encouragement, support, and love during the course of this research. Without her I would be lost. This work would not have been possible without the guidance and funding of my supervisors Dr. Erik Eberhardt and Dr. Oldrich Hungr. Dr. Eberhardt suggested the topic, organized the project, and directed my research. Both provided countless hours of individual and classroom instruction, covering landslides, rock mechanics, engineering, computer modeling, thesis writing and more. Thank you most of all for keeping an open door and allowing so much time for discussion and instruction. Thank you also for funding and organizing research related trips to the U.S., Nepal, and Chile. These are experiences that I will never forget. I would like to thank my committee members Dr. R. Beckie, and Dr. D. Stead, and external examiner Tom Badger for providing their time, guidance, instruction, and discussion throughout the course of this research. The Washington State Department of Transportation funded the site visits and provided open access to information regarding Afternoon Creek. URS Corporation and Wyllie & Norrish Rock Engineers generously provided open access to all information regarding the Afternoon Creek site. Special thanks to Dr. R.L. Burk from URS Corp. for his guidance and support. Rosie Cobbett, Ming Yang, Suzanne Chalindar, and my supervisors assisted during site visits and laser scanning. Amandine Brosse assisted with the 3-D rockfall computer simulation. Finally, I would like to thank my office mates Andrea Kay and Scott McDougall for their friendship and help. Andrea provided answers, software support, and entertainment. Scott was my mentor and general role model. xv 1 I N T R O D U C T I O N 1.1 Problem Statement On November 9, 2003 a rock slope failure occurred at Afternoon Creek in northwest Washington State that involved approximately 750,000 m 3 of jointed, orthogneiss rock. The source for this event was located at the crest of a ridge. Most of the debris fell to the east of the ridge into the shallow sloping Afternoon Creek. This material moved as a rock avalanche for approximately 500 meters before coming to rest. The rock avalanche did not intersect the highway. A small percentage of the debris fell to the west of the ridge down the steep Falls Creek and Falls Creek Chute. Much of this material traveled all the way to Washington State Route 20 (SR 20), destroying portions of the roadway and guardrail, and depositing up to 4- meter diameter boulders on the road. The Afternoon Creek rock slope continues to threaten SR 20 at two locations due to the topography of the slope. If another slope failure does occur, the debris could potentially impact the highway as a rock avalanche from Afternoon Creek or as rockfall from Falls Creek. Due to the imminent threat posed by the slope, research was undertaken through this thesis study to investigate the November, 2003 Afternoon Creek rock slope failure and post- failure motion, and to characterize the hazard that currently exists at Afternoon Creek. The investigation focused on the structural and topographic controls of failure and runout. An analysis methodology was used that integrated several numerical methods linking failure initiation to runout, termed "Total Slope Analysis" (Stead and Coggan 2005; Stead et al. 2006). The analysis was preceded by a data collection program designed specifically for the numerical modeling application. A secondary purpose of the research was to evaluate integrated use of the state-of-the-art data collection and numerical modeling tools utilized during the investigation, including: terrestrial laser scanning (LiDAR) for collecting rock mass characterization data of an inaccessible slope; two-dimensional (2-D) and three- dimensional (3D) discontinuum numerical modeling of the failure initiation processes; and three-dimensional dynamic analysis of the post-failure motion. 1 1.2 Research Objectives The objectives of this research were as follows: 1. Investigate the structural and topographic controls to the operative deformation and failure mechanisms of the November 9, 2003 rock slope failure at Afternoon Creek. 2. Characterize the post-failure motion of the debris. 3. Estimate the location and volume of future slope failures at Afternoon Creek, and determine the effects of the future failures on the highway (SR 20). 4. Evaluate the integrated use of state-of-the-art techniques in rock slope characterization and stability analysis including: a. Light Detection And Ranging (LiDAR) scanning for collecting rock mass characterization data, and b. Two-dimensional and three-dimensional discontinuum numerical modeling. 1.3 Organization of the Paper Chapter 1 presents the research objectives, and Chapter 2 then presents a description of the Afternoon Creek rockslide case history. The case history has been published as the following peer-reviewed journal article: Strouth, A. , Burk, R.L., Eberhardt, E. 2006. The Afternoon Creek rockslide near Newhalem, Washington. Landslides, 3 (2): 175 -179. Chapter 3 is a discussion of the methods used to complete this research. It includes a description of the Total Slope Analysis procedure, numerical modeling procedure, and the data collection methods. The computer programs used during the research are briefly described. Chapter 4 describes the results of the data collection program. The data collection program was designed specifically to acquire information required to build the distinct element numerical models. Chapter 5 presents the back analysis performed for the November 9, 2003 slope failure at Afternoon Creek. The Total Slope Analysis of the Afternoon Creek and Falls Creek travel paths are described including the 2-D and 3-D distinct element modeling, coupled with the rock avalanche runout analysis. Mechanisms that cause rocks to enter Falls Creek are investigated and the results of an earlier rockfall analysis are summarized. Chapter 6 is an analysis of future events at Afternoon Creek, using 2 the Total Slope Analysis as a forward analysis to estimate the location, size, and effects of future slope failures at Afternoon Creek. Chapter 7 is a discussion of the state-of-the-art tools used in these analyses, specifically terrestrial laser scanning, 2-D and 3-D distinct element modeling, and 3-D dynamic analysis. Benefits, limitations, and recommendations concerning each tool are discussed. Chapter 8 summarizes the most important conclusions of this research and provides recommendations for future work that relate to the Afternoon Creek case study and the tools used. 1.4 Literature Review 1.4.1 S lope Failure Initiation Mechanisms of Failure In 'Mechanism of Landslides' (1950), Terzaghi describes the processes contributing to landslides, and discusses their dynamics. He divides the causes of landslides into internal and external ones. The external causes are those that produce an increase in the shearing stresses, and internal causes are those that decrease the shear strength (resistance) of the slope material. Undercutting the foot of a slope, loading the top of a slope, or adding earthquake loading are examples of external causes. Common internal causes are increased pore-water pressures and decreased slope material strength. Terzaghi observed that most slope failures take place during periods of exceptionally heavy rainfall, and asks us to remember that "exposure to rain or melting snow belongs to the normal existence of a slope. Hence, i f a slope is old, heavy rainstorms or rapidly melting snow can hardly be the sole cause of a slope failure". Most landslides are a result of the combination of several causes. It is common for the shear strength of an old slope to decrease through time by weathering and fracture growth, and failure to be triggered by exceptionally large pore water pressures. In a subsequent paper focused on the stability of rock slopes (1962), Terzaghi discusses the significance of joints in hard unweathered rock, noting that the slope stability is determined by the shear strength of joints and faults, and not by the strength of the rock itself. The cohesion of a joint is partly a result of intact rock-bridges between discontinuous sections of the joint. Pore water pressure is also an important component to the shear strength of rock joints, however the water table is often poorly defined in a rock slope 3 because the secondary permeability of jointed rock commonly varies erratically across a slope.. , Numerous authors have researched the shear strength of joints and stress-strain relationships of laboratory rock samples in further detail. Some attempts are made to compare these with larger scale rock mass failures in tunnels (Hajiabdolmajid and Kaiser 2002; Hajiabdolmajid et al. 2003; Lockner 1995). Lockner discusses brittle failure mechanisms in rock in the context of the Coulomb failure criterion. The conceptual model of brittle fracture is described for samples that are loaded in tension, and in compression. Hajiabdolmajid and others focus on the center portion of the stress-strain curve between the onset of microcracking and movement along a shear plane. The relationship between loss of cohesional strength and mobilization of frictional strength is discussed for rock samples and around tunnel openings. Other authors consider stress-strain relationships, and progressive brittle failure in natural rock slopes with discontinuous joints (Cording et al. 2002; Eberhardt et al. 2004; Hajiabdolmajid and Kaiser 2002; Sjoberg 1999). Each of these authors distinguish failures on continuous joints from those that require shear surface development through intact (although weakened/deformed) rock. Cording et al. (2002) investigate the relationship between the shape of the failure plane and the dip of discontinuous joints. The other authors investigate the degradation of rock slope stability with time as the rock slope deforms and fractures propagate. Varga (2003) divides the pre-collapse process of a slope into 3 different stages: (1) primary elasto-plastic stage, (2) secondary plastic deformation stage, (3) Tertiary viscoelastic movement. Walder and Hallet (1985) present a model of fracture propagation in rock during freezing. Calculations and empirical data indicate that sustained freezing is most effective in producing crack growth when ample water is available, and temperatures range from -4° to - 15° C. Failure Classifications Several classifications of failure mechanisms for large rock slopes exist. As stated by (Martin and Kaiser 1984) and echoed by several others, " A proper understanding of the mechanisms taking place during the failure process... is a fundamental necessity for the 4 selection of the appropriate method of analysis." For each classification unstable rock masses are classified according to different parameters. Lansheng & Zhouyjan (1984), Lansheng et al. (1992), Poisel & Preh (2004), and Hungr & Evans (2004) consider the type of deformation and failure mechanism common to different structural settings. Table 1.1 is a comparison of these classification schemes. Lansheng and others classify slope failure and deformation by the slope structure and five 'geomechanical deformation models'. The geomechanical deformation models are: (1) sliding (or creep sliding) - cracking (SC or CSC); (2) sliding and compressed cracking (SCC); (3) sliding and bending (SB); (4) bending and cracking (or toppling) (BC); and (5) plastic-flowing and cracking (PFC) (Lansheng and Zhuoyuan 1984). Hungr and Evans describe the typical failure behavior (i.e., slow, rapid, or catastrophic) of each failure mode in strong and weak rock. For example, strong, brittle rock is more likely to fail catastrophically, and weak, ductile rock commonly fails slowly. Translational slides usually are extremely rapid in strong and weak rock (Hungr and Evans 2004). Poisel and Preh (2004) have created a catalogue of rock slope failure mechanisms and point out that an ideal model would simulate both the initiation of failure and the run out. The catalogue considers the geological setting, geometry of the slope, joint structure, configuration of the rock blocks, and mechanical behavior of the rock mass. Varga & Gorbushina (1988) point out that the type of structure is important to the failure mechanism as well as the orientation. Five main types are identified. The controlling structure may be any one of the following or a combination of several: bedding, faults, joints, schistosity, injective structure (magmatic intrusion contact). Martin & Kaiser (1984) and Eberhardt et. al. (2004) classify slope failures by the amount of deformation in the moving rock mass. Martin and Kaiser separate failures into 3 classes: (Class I) rigid body motion; (Class II) local yielding; (Class III) yielding of the entire rock mass. Eberhardt and others note that the analysis method depends largely on the extent of internal deformation. Examples of failure mechanism are given for minimal and extensive internal rock mass deformation. 5 Table 1.1 Comparison of classification schemes Joint orientation/ orientation of the failure surface (Lansheng et al. 1992) (Hungr& Evans 2004) (Poisel & Preh 2004) Type of Slope Structure Model of deformation Failure mode Failure mode Behavior Martin and Kaiser Failure Class Failure mode Martin and Kaiser Failure Class Homogeneous Soil, weak rock mass CSC Curved rotary slide Rock slump Slow - rapid, self stabilizing I (III ?) Slope Creep III Horizontal - sub horizontal sec Horizontally pushing slide; rotary slide Compound slide Slow, self stabilizing Ill Gentle dip out of SC Block slide Translational rock block Extremely rapid, potentially large 1 Sliding of a rock block on a single discontinuity; I slope Compound slide Slow, self stabilizing III Sliding of several rock blocks on a polygonal sliding plane III Consequent Kink band slumping III Moderate dip out of slope SB slide; rotary slide; rockfall Translarional rock block Extremely rapid, large 1 Sliding of a rock block on a single discontinuity I Steep dip out of B C Topples and rockfalls; rotary slide Block topple • - Gradual piecemeal disintegration I I I Buckling of column or slab- shaped rock blocks; Kink III One controlling discontinuity or Joint Set slope - large catastrophic rock avalanche i , n band slumping; Rock Slumping Topples and rockfalls, rotary slide in depth Flexural toppling Self stabilizing; ductile behavior in Flexural Toppling III Near vertical dip into slope B C or CSC Block topple - Gradual piecemeal disintegration i • i i toppling of column or slab- i - I I - large catastrophic rock avalanche i , i i shaped block i , t i Moderate dip into slope CSC or B C Surface slide, rotary slide Flexural toppling Self stabilizing; ductile behavior III Toppling of column or slab- shaped block I; I I Varying dip angle towards slope SB Consequent and rotary Structurally defined compound slide Extremely rapid III slide Block slide with toe breakout Depends on rock strength II; III Blocky mass 2 or more controlling Joint sets SC Wedge slide or flowslide Translational rock block or wedge slide Extremely rapid, potentially large 1 Falling rock blocks; sliding of blocks on two discontinuities; rotation of a single rock block 1 Broken Mass (Many controlling joint sets) Numerous joint sets, no prominent weak plane SC or CSC Wedge slide or flowslide Rock Collapse Extremely rapid, generally small volume III (I?) Sliding of a fractional body on a shelly, newly formed sliding surface III (I?) Slope with soft foundation PFC Block slide; rock fall; rotary slide Block Topple Gradual piecemeal disintegration I; II Translational or rotational descent of tower blocks of competent rock upon an incompetent base i; I I CSC or SC: (creep) sliding - cracking; SCC: Sliding - compression cracking; SB: Sliding - bending; BC: bending - cracking; PFC: plastic flowing - cracking (Langsheng et al. 1992). (Class I): Rigid body motion (translation or rotation) along a planar or circular failure surface. No internal yielding of the rock mass required. (Class II): Local yielding inside the rock mass required allowing mass movement along the irregular basal slip surface. (Class III): Yielding of the rock mass along pervasive critically oriented internal shear surfaces required to allow motion along the basal slip surface (Martin & Kaiser 1984). 6 Numerical Modeling Stability analyses of rock slopes are routinely performed with numerical techniques. Stead et. al. (2001) describe which numerical techniques are best suited for different classes of failure and/or deformation. Common simple techniques, such as limit equilibrium analysis and empirical methods, have relevant applications and are appropriate for situations where a 2-D rigid block assumption is valid. However, these techniques have many important limitations - for example, they are not able to analyze creep, progressive deformation, and extensive internal disruption that precedes or follows a sliding failure (Stead et al. 2001). Where it is necessary to include the stress state within the rock mass and the influence of complex deformation and brittle fracture, numerical modeling techniques should be used. Three categories of numerical methods exist: continuum, discontinuum, and hybrid models. Continuum models are best suited for analysis of slopes comprised of massive, intact rocks, weak rocks or soil like rock masses. Discontinuum models are best suited for blocky mediums and are the most commonly applied numerical approach to rock slope analysis. The rock mass is treated as an assemblage of rigid or deformable blocks. Hybrid techniques are being adopted in rock slope analysis. These techniques are a combination of continuum and discontinuum codes. The propagation of cracks through the finite element mesh can be simulated (Stead et al. 2001). Many authors (Benko and Stead 1998; Bovis and Stewart 1998; Eberhardt et al. 2004; Esaki et al. 1999; Nichol et al. 2002; Stead and Eberhardt 1997) have studied individual cases of rock slope failure with the continuum code F L A C (Itasca 2002) and the discontiuum code UDEC (Itasca 2000). The numerical modeling procedure for various scenarios is recommended by several other authors (Hart 1993; Itasca 2000; Itasca 2002; Ranjith and Saravanan 2002). Failure Prediction Terzaghi (1950) claims that the only slides that occur without warning are caused by earthquakes and spontaneous liquefaction, "all others are preceded by.. .progressive deformation...(And) i f a landslide comes as a surprise to the eyewitness, it would be more accurate to say that the observer failed to detect the phenomena that preceded the slide." Surface movements before a slide include downhill creep and movement along joints/faults, 7 both of which may contribute to the formation of tension cracks along the upper boundary of the slide area. The rate of displacement accelerates from a nearly constant creep to a more rapid sliding. The rate of acceleration from creep to slide motion depends on the thickness of the sliding surface/shear zone. Slides with thin shear zones accelerate very quickly (i.e., brittle), while those with thicker shear zones tend to accelerate more slowly (i.e., ductile). Glawe & Lotter (1996) describe methods for time prediction of rock slope failures based on monitoring results, including geotechnical investigations, displacement monitoring, limit equilibrium analysis, and seismic monitoring. They also observe that a period of steady state movement of the slide mass is followed by an accelerated phase of movement before failure. They note that sensitivity to external influences (such as water supply) increases before failure, and caution that significantly decreasing rates of displacement do not necessarily indicate stability. One must make a comparison of short- medium-, and long- term velocities. Zvelebil (1984) gives an example of a toppling failure that was successfully predicted based on crack extensometer data. The critical displacement to cause toppling was calculated, and the displacement versus time curve was extrapolated to predict a time of failure where the curves intersect. The actual failure occurred only seven days after predicted. Fukuzono (1985) proposed a simple method of predicting the time of failure using the reciprocal of mean velocity. Rose and Hungr (2006) successfully applied the Fukuzono method to the prediction of three large slope failures in open pit mines. The predictions were forecasted two weeks to three months prior to failure. 1.4.2 Rock Mass Characterization Reliable estimates of rock mass strength and deformation characteristic are necessary for nearly all stability analysis procedures. Marinos et al. (2005) describe the applications and limitations of the Geological Strength Index (GSI). The GSI is a method for obtaining estimates of the strength of jointed rock masses, based upon an assessment of the interlocking of rock blocks and the condition of the surfaces between these blocks. It is used to reduce the Hoek-Brown Failure Criterion (Hoek et al. 2002) material constants from 8 intact, laboratory values to appropriate in situ values (Marinos and Hoek 200,0). Rocscience has created software called RocLab for determining rock mass strength parameters, based on the latest version of the generalized Hoek-Brown failure criterion (Rocscience 2005). Barton (1976) describes the shear strength of rock joints based on empirical data. A list of basic friction angles is compiled for various rock types. The actual friction angle for a joint depends on the rock type and the degree of roughness. Kulhawy (1975) presents the results of an extensive literature survey on the stress deformation properties of rock materials and rock discontinuities. Typical values of density, porosity, cohesion, friction angle, and more are tabulated for different rock types. The results are for samples tested under uniaxial and triaxial conditions. Romana Ruiz (2002), and Hoek and Diederichs (2006) compare the many methods for determining the deformation modulus of rock masses. The current practice is to estimate the deformation modulus according to one of several empirical formulations. Hoek and Diedrichs propose a new relationship based on a large number of in situ measurements from China and Taiwan. The (US) National Research Council committee on fracture characterization and fluid flow (NRC 1996) describe investigation methods (geologic and geophysical) for characterizing rock fracture patterns and properties at the surface and at depth. Current understanding of how fluids travel through the fracture network and how these fluids affect the stress in the rock mass is also described. Berkowitz (2002) presents a short review of characterizing flow in fractured rock. Roberts and Poropat (2000) give an example of using 3-D spatial data, digital images, and software to create detailed structural maps of a rock slope. These tools significantly reduce the cost and risk associated with completing a traditional joint survey. Several authors, including (Alba et al. 2005; Pringle et al. 2004; Rosser et al. 2005; Rowlands et al. 2003) show that Light Detection and Ranging (LiDAR) is a key technological development that improves our capacity to collect reliable rock mass and landslide data. Several researchers, including (Kemeny et al. 2006; Kemeny et al. 2004; Monte 2004; Slob et al. 2005), have been exploring and developing 3-D laser mapping methodologies specifically for rock mass characterization and discontinuity analysis. Kemeny et al. (2006) tested the accuracy of joint orientation measurements made digitally from LiDAR-derived point clouds. They showed that errors are associated with 1) the instrument, 2) the procedures for scanning 9 in the field, and 3) processing the resulting point clouds. They claimed that instrument errors are typically less than 1.5 cm, and that processing errors for discontinuity dip and dip direction are less than 0.5 degrees. In a simple case study that compared LiDAR-derived measurements with hand measurements using a Brunton compass, the L i D A R results were within 2-4 degrees of the hand measurement results. 1.4.3 Runout Predict ion of Failed S lopes Empirical Methods The 'angle of reach' (fahrboschung) concept for a rock avalanche was first described by Heim (1932). The 'angle of reach' is a line that connects the top edge of the source area to the distal edge of the deposit. Heim observed that the angle of reach decreases with increased volume of the falling mass. Several other authors further investigate this relationship (Corominas 1996; Evans and Hungr 1993; Hsu 1975; Hungr 1990; Legros 2002; Li 1983; Scheidegger 1973; Voight et al. 1983). They suggest causes for the correlation and causes of scattering in the relationship. Scheidegger (1973) relates the volume of landslides to the coefficient of friction. This correlation can be used to calculate the expected reach and velocity of an imminent slide. L i (1983) presents the statistical analysis of 76 major rockfalls in the Alps. He shows that a correlation exists between the rockfall volume and area covered by the slide, and between the volume and fahrboschung angle. Hungr (1990) also shows that the area of a landslide deposit correlates with the volume: Area = Volume (- 2 / 3\ Hungr also provides a summary of the mechanism of movement proposed by previous authors. Corominas (1996) summarizes the volume threshold observed in landslide mobility by several other authors. He points out that since there is a lack of agreement among researchers, a direct inference from the plots of volume versus fahrboschung can not be made. He claims that the scattering in the relationship is mostly due to mechanisms of motion and to obstacles and topographic constraints on the path. Only slides that have similar composition and follow a similar flow path should be compared. The decrease in the reach angle with volume suggests that scale effects should be taken into account. He also shows that the height of fall has no control over the angle "of reach and notes that some 10 landslides did not occur as a single event, but as several events over a longer time frame. Therefore the total volume can not be considered equivalent to a single event because the location of the source zone and fahrboschung angle changes. Legros (2002) summarizes the mechanisms that have been proposed by other authors to explain the long runout of landslides. The central idea developed is that the apparent coefficient of friction (tangent of the fahrboschung angle) is physically meaningless. It is proposed that the runout distance depends primarily on the volume and not on the fall height, which just adds scatter to the correlation. He points out that there is considerable error in the estimation of thickness (and therefore volume) of a landslide. Data from 203 subaerial, submarine, Martian slides, and debris flows are used in the analysis. Following the example of Lied (1977), Evans and Hungr (1993) propose the use of the 'shadow angle', defined as the angle from the top of talus cone to the distal limit of rockfall, for rockfall investigations. They claim that the shadow angle is preferable to the use of the fahrboschung because it does not necessitate the start and end point of each rockfall to be located. Analytical Methods Numerical models for the dynamic analysis of rapid landslides, D A N (Hungr 1995) and DAN3D (McDougall and Hungr 2004), have been developed. These programs are used for risk assessment and design of remedial measures against rapid landslides such as debris flows and rock avalanches. Each model is based on a Lagrangian solution of the equations of motion and allows for a variety of material rheologies including plastic, frictional, viscous, Bingham and Voellmy that can vary along the slide path or within the moving mass. The approach is semi-empirical in that the complex, heterogeneous fluid is replaced by an equivalent, homogeneous fluid whose bulk properties approximate the behavior of the moving mass. A typical procedure is to calibrate the model by back analysis of known cases and to predict the behavior of new events as required (Hungr 1995). D A N is pseudo-3D; the surface width along the slide path must be assumed beforehand as an input function (Hungr 1995). DAN3D has the ability to simulate rapid landslide motion across complex 3-D terrain in which the material spreads, contracts, changes direction, splits or joins in response to local topography. The model accepts spatial 11 input in the form of user-created grid files specifying bed elevation, source landslide depth, erosion depth, and basal rheology. It has the ability to account for complex anisotropic internal stress states, material entrainment, and rheology variations within the slide mass and along the slide path (McDougall and Hungr 2004). 12 2 P R O J E C T S E T T I N G : T h e A f t e r n o o n C r e e k R o c k s l i d e 1 Abstract A series of mass wasting events occurred above a Washington, U S A highway in the Cascade Mountains in November and December, 2003. The largest event was a rockslide involving approximately 750,000 m 3 that occurred on November 9, 2003. The source zone for this event was located at the crest of a ridge. Most of the debris fell to the east of the sharp ridge and was deposited in the relatively shallow sloping Afternoon Creek without causing damage to the highway. Lesser amounts of debris fell to the west of the ridge, falling 600 meters down the steeper Falls Creek and impacting the road. There is evidence of one or more historical rock avalanches at this location. Displacement of reference points, ground vibration, crack extension, and tilting are being monitored due to concerns that future slope failures or remobilization of debris might again damage or block the highway. 2.1 Introduction On November 9, 2003, a rockslide occurred above Washington State Route 20 (SR 20) near Newhalem, Washington, USA. Rock avalanche debris fell more than 600 meters in elevation down Falls Creek, Falls Creek Chute and Afternoon Creek (Figure 2.1). Most of the debris (approximately 750,000 m3) fell to the east of a sharp ridge and was deposited in the relatively shallow sloping Afternoon Creek without causing damage to the highway. Lesser amounts of debris fell to the west of the ridge down the steeper Falls Creek and Falls Creek Chute. Much of this material traveled all the way to SR 20, destroying portions of the roadway and guardrail, and depositing up to 4-meter diameter boulders on the road. This rockslide was followed by a series of smaller mass wasting events in November and December, 2003. 1. This chapter has been published in a modified form as the peer-reviewed paper: Strouth, A. , Burk, R.L. , Eberhardt, E. 2006. The Afternoon Creek rockslide near Newhalem, Washington. Landslides, 3(2): 175-179. 13 Figure 2.1 Afternoon Creek rockslide above SR. 20 near Newhalem, W A . Photograph provided by John Scurlock, Concrete, W A . The slope is composed of orthogneiss of the Skagit Gneiss Complex, and it appears that the instigating factors underlying the rock slope hazard are glacial over-steepening of the slope, multiple crosscutting faults and fractures, and decreased rock mass strength due to weathering. The November 9th event was triggered by elevated groundwater conditions created by rainfall events in October, 2003 (URS and Wyllie & Norrish Rock Engineers 2004). This chapter presents the results of a field investigation conducted immediately following the Afternoon Creek rockslide, including a description of the geological setting, history of landslide activity in the area, and its implications with respect to future rockslide hazards that threaten the highway. Details of the ongoing monitoring and hazard mitigation work are also discussed. 14 2.2 Geological and Geomorphological Setting The source zone for the Afternoon Creek rockslide is located 600 meters above SR 20, the northernmost route through the Cascade mountain range in the United States. Afternoon Creek is located in the heart of these exceptionally steep and rugged mountains (Figure 2.2); the peaks near the Afternoon Creek rockslide have nearly 1600 meters of vertical relief. Figure 2.2 Location of Afternoon Creek rockslide. Three kilometers east of Newhalem, Washington, USA. Failure area outlined in yellow. The North Cascades is a complex region of accreted Mesozoic and Paleozoic terrains that were assembled during the early to middle Cretaceous. The geology of the region is further complicated by late Cretaceous through Eocene thrusting, plutonism, regional metamorphism, strike-slip faulting, extensional faulting and basin development. Quaternary glaciation created the chiseled peaks and open parabolic-shaped valleys that exist today. The most recent Cordilleran glaciation covered the area 15,000 years ago with a continuous ice 15 cap (Tabor et al. 2003). The Afternoon Creek slope is over-steepened as a result of this most recent glaciation. A l l of the rocks involved in the rockslide are hornblende-biotite tonalite orthogneiss of the Skagit Gneiss Complex (Tabor et al. 2003 map). The tonalite is most likely intrusive igneous material with original igneous crystallization from the late Cretaceous to early Paleocene. The cause of metamorphism is still under debate, but it is agreed that the mechanism is related to some sort of crustal thickening and that ductile deformation ceased by the early Oligocene (Tabor et al. 2003). The average uniaxial compressive strength (UCS) of the intact orthogneiss is approximately 90 MPa according to UCS tests performed by GeoTest Unlimited, and from point load correlation and field observations (URS and Wyllie & Norrish Rock Engineers 2004). This corresponds to 'R4-strong rock' according to Brown (1981). Multiple cross cutting fractures and faults divide the slope into several structural zones. The rock mass in each zone ranges from 'disintegrated' with 'fair' surface conditions to 'very blocky' with 'good' surface conditions corresponding to a Geological Strength Index ranging from 30 to 60 (Marinos and Hoek 2000). Dozens of northeast-southwest trending fracture lineaments that are visible on a regional scale cut across the Afternoon Creek rock slope; additionally, locally persistent fractures trend northwest-southeast, parallel to Afternoon Creek. Near the failed slope, many of these discontinuities are filled with soil and rock rubble debris. Two important joint sets were apparent in the preliminary investigation. The most common set (plane A) dips parallel to the larger Skagit valley (i.e., strikes perpendicular to the Afternoon Creek slope). The second set is sub-vertical and parallel to Afternoon Creek (plane B). Plane B joints are widely spaced and highly persistent. Some are open and filled with soil and rock rubble (Figure 2.3). 16 Figure 2.3 Sub-vertical, open fractures parallel to Afternoon Creek. Photograph provided by URS Corporation (photograph date, March 1, 2004). 2.3 History of Activity Debris flows and snow avalanches are common to both Afternoon Creek and Falls Creek and have forced road closure on several occasions in the past (e.g. Figure 2.4). Although the November 9, 2003 event was the first large rock avalanche on record, undated photographs and aerial photos from 1998 indicate ongoing rockfall activity several years before the event. Significant rockfall scars near the top of the slope and fresh boulder-sized debris in Afternoon Creek can be seen in undated oblique photographs (Figure 2.5). Rockfall scars of the same magnitude can be seen in aerial photos taken in 1998. Some of the rock rubble that is now covered by vegetation (Figure 2.5) may be debris from a single rock avalanche, or a series of historical rock avalanches that have occurred since deglaciation (late Pleistocene). This evidence indicates that slope instabilities in the form of isolated rockfall and larger rock avalanches are common in Afternoon Creek since deglaciation. 17 Figure 2.4 Historical photographs of debris flows that reached the state route 20 highway, (a) Afternoon Creek debris flow material (March 23, 1949). (b) Falls Creek Chute debris flow covered with fresh snow (1990). (c) Afternoon Creek (1999 or 2000). Photographs provided by WSDOT. Figure 2.5 Afternoon Creek rockslide slope before the November 9, 2003 event. Photograph provided by WSDOT (date unknown). 2.4 Chronology of Recent Mass Wasting Events The November 9th Afternoon Creek rockslide, and subsequent smaller mass wasting events that occurred through November and December, 2003, were preceded by record rainfall in October. The week of October 16 through 21 was one of the wettest weeks in western Washington history; a rain gauge just 6.5 km from the landslide measured more than 18 400 mm of rain in these six days. The soil and rock rubble filled fractures that cut across the Afternoon Creek slope readily allow surface runoff to enter the fracture system. This water backed up against hydraulic barriers such as clay-rich shear planes, soil and possibly ice, creating high pore-water pressures. It is likely that the increased pore water pressures in the slope due to the influx of water in mid-October triggered the initial collapse of the slope. The failure mechanism of the November 9th rockslide is complex. A preliminary interpretation is that the initial collapse occurred in the southern half of the failure zone where the rock mass is most highly fractured and dilated. This material collapsed toward Afternoon Creek. The event unloaded the toe of larger, more competent blocks that slid on plane A, initially in a direction parallel/oblique to Afternoon Creek (Figure 2.6). Rockfall and/or toppling in the upper portion of the failure zone followed after the loss of lateral support provided by the large blocks. Plane B (parallel to Afternoon Creek) allowed toppling and provided the lateral release necessary for planar sliding. Given the geometry of the ridge, a small percentage of the total rockslide material (<10%) traveled down the west side of the ridge along Falls Creek and Falls Creek Chute impacting the highway below (Figure 2.7). Figure 2.6 Afternoon Creek failure scarp from Afternoon Creek, (photograph date April , 2005). 19 Figure 2.7 Digital elevation model of the Afternoon Creek rockslide. Failed mass is shown in gray. Arrows indicate direction of movement. Dotted blue line indicates the extent of path. This initial slope failure was followed by several smaller events during the following weeks. Heavy rain a week later washed smaller rock and debris out of Falls Creek, and on November 19, 2003, a debris flow (approximately 35,000 - 60,000 m 3 in size) came down Falls Creek Chute and closed the road. Increased rockfall activity in Afternoon Creek was observed beginning in early December and leading up to December 19th. On December 19, 2003, a rockfall of approximately 35,000 m 3 in size occurred. This event deposited boulders up to 15-meter in diameter in Afternoon Creek. A series of smaller rockfall events continued during the following week. Large scale fractures were recognized in the intact bedrock near the failed face following these events. On several occasions workers heard loud explosion-like booms and felt the ground vibrate beneath their feet. Although the sounds emanated from the slide area, they were not followed by rolling rocks or dust clouds. These events may have been the result of intact brittle rock fracturing in the slope. 2.5 Continued Monitoring, Hazard Mitigation, and Future Work After the November 9th collapse, there were concerns that additional slope failures and remobilization of the rock avalanche debris might further damage or block the highway (SR 20). The Washington State Department of Transportation (WSDOT) contracted with 20 URS Corporation and Wyllie & Norrish Rock Engineers to begin a monitoring and investigation program. As part of this investigation, four monitoring techniques were employed to locate areas of ongoing slope movement, measure movement vectors and enable prediction of future slope failures: 1. Displacement monitoring consisted of regularly surveying the location of reference points to track cumulative movement of those points. Fifteen geodetic prisms were placed near or within the area of slope failure as reference points. 2. Vibration monitoring was used to correlate ground vibrations with rockfall activity triggered by precipitation events. Geophones were buried adjacent to Afternoon Creek, in Falls Creek Chute and along the upper head scarp of the failure plane. 3. Displacement monitoring was used to monitor crack dilation to assist in predicting potential rock failure. Two extensometers were installed across cracks within intact bedrock just above the upper head scarp of the failure. 4. Rock tilt monitoring was used to assist in interpreting i f further head scarp development was occurring. Two tiltmeters were installed on intact bedrock faces near the upper head scarp of the failure. The tiltmeters were used to monitor rotation of the bedrock adjacent to the area of rock failure. Three of the geodetic prisms in the upper slope were destroyed by rockfall and have not been replaced. One of these was lost in the rockfall event of December 19, 2003. Based on the survey data, this prism accelerated to failure, traveling more than 2.5 m prior to failure in a period of 27 days. The remaining prisms in the upper and lower slope show no measurable movements within the error limits of the survey method (URS and Wyllie & Norrish Rock Engineers 2004b). Geophone background values typically increase slightly during precipitation events. The majority of peak vibrations are two to four times larger than background values. The rockfall event of December 19, 2003 had a peak vibration 35 times larger than typical background values. There is a correlation between rainfall and increased event activity. The 21 extensometer and tiltmeter data show diurnal changes, however, no long-term trends related to movement of the rock has been observed (URS and Wyllie & Norrish Rock Engineers 2004b). 2.6 Conclusions A series of mass wasting events occurred at Afternoon Creek in November and December, 2003. The largest event was a 750,000 m 3 rockslide on November 9th, originating near the top of a sharp ridge. Most of this volume of large boulder debris landed in the Afternoon Creek without causing damage to the highway; however a very small portion of the material traveled down the backside of the ridge and impacted the Washington SR 20 roadway. Glacial over-steepening of the slope, multiple cross-cutting fractures and shear zones, and decreased rock mass strength due to weathering were key factors in conditioning the slope for failure. Heavy precipitation leading to high joint water conditions triggered the rockslide. The slope is currently being monitored with a regular survey of reference points, geophones, crack extensometers, and tiltmeters. 22 3 M E T H O D O L O G Y 3.1 Total Slope Analysis Procedure A procedure that combines several numerical techniques, linking failure initiation processes to runout, termed "Total Slope Failure Analysis" by Stead et al. (2006), was followed to characterize the past slope failure, and hazard that currently exists at Afternoon Creek. Stead et al. point out that the traditional engineering approach is to analyze either the failure initiation mechanism or the transport/deposition stage. They suggest, however, that if true risk is to be ascertained then the deformation characteristics prior to failure and the post- failure movement must be linked. In this thesis, traditional methods, including kinematic analysis and limit equilibrium analysis, were combined with advanced numerical methods, including the discontinuum numerical codes, UDEC (Itasca 2000) and 3DEC (Itasca 2003), to assess the failure initiation process. The results of the assessment provided understanding of the slope deformation and failure mechanisms, the failure volume, and helped to reduce uncertainty regarding the physical properties of an unstable rock mass. These guided the post-failure motion analysis. Post-failure motion, including the runout path, runout distance, and velocity, were analyzed with the dynamic/rheological flow code DAN3D (McDougall and Hungr 2004). The Total Slope Analysis procedure was followed to back analyze the November, 2003 event, and then repeated as a forward analysis of the current slope configuration. The Total Slope Analysis procedure is schematically illustrated in Figure 3.1. 23 fir (A C si O U CO Past s lope fai lure Mechanism Discontinuity parameters Rock mass parameters Runout Rheology Base friction angle Internal friction angle '55 Q . ( o O c </) < O Potential s lope ; fai lure Failure Mechanism Volume of failed mass R u n o u t P r e d i c t i o n Figure 3.1. hazard. Flow chart of the repeated Total Slope Analysis. Back analysis and characterization of current 3.2 Traditional Data Collection Techniques A key objective of the data collection program was to obtain input parameters required for numerical modeling. The data required for a proper numerical analysis can be divided into two categories: (1) contextual information, and (2) input parameters. The contextual information includes the physiographic and geological setting; it helps the modeler choose the appropriate boundary conditions, model size, and physical properties. Most of this information is not explicitly included in the model, but provides a connection between the model and reality, and aids in the interpretation of the model results. The input parameters are essentially numbers, which are used to describe the rock mass system. There are a minimum number of parameters required by the modeling software for a solution to be derived. A vast amount of additional data can be added to the model to more closely recreate the complexity of the exact circumstances. The basic parameters required for a distinct element numerical model were assembled and presented in the deliverable format described in Table 3.1: 24 Table 3.1 Data requirements for numerical modeling. Data Requirements for Numerical Modeling Deliverable Format Physiographic & geologic setting Regional/ local geology map Geometry Topographic profile Digital elevation model Discontinuity characterization - Orientation, spacing, persistence LiDAR analysis Mechanical Properties Rock mass - Density, bulk and shear modulus - Cohesion, friction angle, tensile strength Table of reasonable values Discontinuity - Joint normal and shear stiffness, joint cohesion, joint friction angle Table of reasonable values Data was collected in four basic phases: desk study, field mapping, scan-line joint mapping, and terrestrial laser scanning. The data was then assembled and converted into the required deliverable format. 3.2.1 Desk Study The initial investigation began with a desk study of available information concerning the Afternoon Creek rockslide. Most of this information was generously made available by the Washington State Department of Transportation (WSDOT), URS Corporation, and Wyllie & Norrish Rock Engineers, Inc. The analysis would not have been feasible without the support of these organizations. Immediately following the initial rockslide in November 2003, WSDOT contracted with URS Corporation and Wyllie & Norrish Rock Engineers to begin a monitoring and investigation program. As part of this program, these companies produced a geotechnical report that includes a lengthy description of their site characterization results (URS and Wyllie & Norrish Rock Engineers 2004). These results provided a starting point for further data collection. The site characterization results include the following: overview of the general geology; geologic map of the site; overview of geologic hazards; description of subsurface borings drilled adjacent to the failed zone; structural mapping results adjacent to the failed zone; and rock strength testing. The structural mapping and boreholes were completed near the intersection of Falls Creek and State Route 20 in order to design a rockfall embayment to reduce the rockfall hazard to motorists on SR 20. However, the discontinuity pattern here is different from the 25 discontinuity pattern in the upper reaches of the Afternoon Creek slope where failure occurred (Figure 3.2). Equal Ar«a N ^ Figure 3.2 URS structural mapping location compared to failure zone. URS Corporation also made available digital data that was assembled as part of their investigation. This data includes ortho-rectified aerial photographs taken approximately one month after the slide, and Digital Elevation Models (DEMs) from before and after the November, 2003 event. The "before" D E M was produced by the U.S. Geological Survey, presumably from aerial photographs. The "after" D E M was created from aerial L i D A R flown one month after the event by Terra Remote Sensing Inc. The ortho-rectified photos and digital elevation models provided a base for all of the subsequently created maps. 26 Topographic profiles necessary for numerical modeling cross-sections were created from the digital elevation models. The desk study also included a review of the regional geology and geologic history, primarily from aerial photograph interpretation, and the U.S. Geological Survey map and pamphlet of the Mount Baker 30- by 60- minute quadrangle (Tabor et al. 2003). Aerial photographs taken in 1998 were acquired from the U.S. National Parks Service. The scale of the photos is approximately 1:10,000. The photos are evidence of the pre-failure morphology of the slope. They were used to make a preliminary geological map of the site. Material properties of the Orthogneiss rock mass and discontinuities were estimated from tabulated values of intact rock properties (Kulhawy 1975) and point load and Unconfmed Compressive Strength (UCS) test performed during the WSDOT investigation (URS and Wyllie & Norrish Rock Engineers 2004). A total of 41 point load tests and five UCS tests were performed on core samples from boreholes in Falls Creek, near SR 20. The intact properties were corrected for the influence of discontinuities using the Geological Strength Index (GSI; (Marinos and Hoek 2000). 3.2.2 Field Mapping A field mapping campaign was carried out over the Afternoon Creek fan, Afternoon Creek rock avalanche deposit, and parts of Falls Creek Chute that were accessible by foot. The failure scarp was photographed and described from as many angles as possible. It was not possible to traverse most sections of the failure scarp because it is too steep for walking and there is considerable rockfall hazard. Only portions of the failure scarp that can be reached while standing on the rock avalanche deposit were accessed. Joint sets visible in the failure scarp were qualitatively described from vantage points on top of the Afternoon Creek deposit. The orientation of the sets was estimated by measuring accessible joint planes in the very lowest portion of the failure scarp. The failure scarp was subjectively divided into four structural domains. A structural domain is an area with similar rock mass quality (measured by the GSI), structure, and strength characteristics. The GSI in each domain was assigned based on the lithology, structure, and surface conditions of the discontinuities. The intact rock strength was verified by means of rock hammer blows - a technique proposed by Marinos and Hoek (2000) to estimate rock strength in the field. 27 A geological map that was created during air photo interpretation was field checked. The surficial materials were described and photographed. Cross sections parallel to the direction of mass movement were sketched in the field, to be used as a guide when creating cross sections for numerical modeling. These cross sections showed the primary joint sets, and orientation/ location of the weaker, highly-fractured rock mass zone (Figure 3.3). Perpendicular to Afternoon Creek Oblique to Afternoon Creek Figure 3.3 Schematic cross-sections perpendicular and oblique to Afternoon Creek. 3.2.3 Discontinuity Mapping Two scan line surveys were completed in Afternoon Creek at the base of the failure zone: Scanline survey 1 (JS1) sampled 42 discontinuities over 75 meters of structural Zone 2; Scanline survey 2 (JS2) sampled 6 discontinuities over 10 meters of structural Zone 3 (see Figure 3.4 for scan line locations; see Appendix A: Scanline Survey Data). Scan lines were measured by stretching a tape along the failure zone wall, parallel to the Afternoon Creek 28 debris where it could be reached without the aide of climbing ropes. Discontinuity orientations were measured with a Brunton compass with its declination set at 19 degrees east. Figure 3.4. Oblique photo of the Afternoon Creek scarp, showing location of scanline survey 1 (75 meters length) and scanline survey 2. A scan line survey is biased towards features that are perpendicular to the scan line. A survey should be made with at least two perpendicular scan lines to remove this bias (Hudson and Priest 1979). However in this case, perpendicular scan lines were not possible as the required traverse up the near-vertical failure scarp was inaccessible and hazardous. It is recognized that this may have caused a bias in the collected data. The mapped discontinuities were plotted on a stereonet; joint sets were defined by discontinuity orientation. The average spacing (S) of each joint set, was calculated by Equation 3.1, for each scan line: [3.1] S = — N-\ Where N = corrected number of discontinuities within the set; L = scan line distance between the first and last discontinuity within the set; 29 The Terzaghi correction was applied to the number of discontinuities counted as follows: [3.2] Napp sinf? N = Where Napp = apparent N ; number of discontinuities counted; 6 = Angle between the rock face and strike of the joint set The average persistence of each joint set is assumed to be the mean of the persistence of each joint estimated in the field. It was recognized that more rigorous methods for estimating persistence do exist (Mauldon 1998; Zhang and Einstein 1998), however these methods were not attempted. 3.3 Terrestrial Laser Scanning (LiDAR) 3.3.1 L iDAR Data Acquis i t ion A total of 21 terrestrial L i D A R scans of the Afternoon Creek and Falls Creek Chute slopes were attempted. These scans were completed from five different stations in the Afternoon Creek deposit zone, and from two stations on the slope south of the Skagit River (Figure 3.5; Table 3.2). Several of these scans involved repeat surveys of the same area but at different distances and resolutions in order to test the limits of the instrument and compare the quality of the resulting three-dimensional (3-D) point cloud. For each scan, the L i D A R scanning unit was positioned on a tripod, leveled (perpendicular to the line of sight), and aimed at the desired location on the slope. The plunge and trend of the instrument's line-of-site was then measured. This information was used in the data analysis phase to orient the point cloud with respect to true North. The scanner was connected to a laptop via a wireless connection. Using the controller software, the digital camera settings of the instrument were adjusted for the present lighting conditions, and the region to be scanned was selected. The point cloud resolution was set by adjusting the 'spot-spacing', which is a parameter that defines the scan density. A small spot-spacing provides high resolution, while a large spot-spacing provides low resolution. Point cloud 30 data is collected by the instrument at a rate of 2,000 points per second (Optech 2002), allowing a typical scene of a few million data points to be captured in 10 - 20 minutes. Figure 3.5 Terrestrial laser scanning stations 31 Table 3.2 Terrestrial laser scans attempted survey station/ scan target line-of- sight (azimuth) tilt (degrees)** spot spacing approx. distance* (m) approx. resolution (cm) relative quality 1/1 fai lure scarp 304 25 7 600 11 moderate 1/2 lower slope 268 15 20 300 16 good 2/1 fai lure scarp 301 29 8 500 11 good 2/2 fa i lure scarp 301 29 15 500 20 moderate 2/3 fa i lure scarp 301 29 30 500 40 moderate 3/1 scanl ine 1 165 -5 60 60 10 moderate 3/2 scanl ine 1 165 -5 20 60 3 good 3/3 scanl ine 1 193 8 20 50 3 good 3/4 scanl ine 1 230 23 20 50 3 good 4/1 fai lure scarp 333 7 10 1000 27 very poor 4/2 Fa l l s C reek runout path 333 7 10 300 - 1000 8 - 2 7 poor 5/1 fai lure scarp 334 6 10 1000 27 very poor 5/2 lower slope 334 6 10 500 13 good 5/3 Fa l l s C reek runout path 334 6 10 750 20 moderate 5/4 range test 334 6 10 > 1200 32 very poor 5/5 range test 326 14 10 > 1500 40 no data 6/1 fai lure scarp - zone 3 203 20 16 250 11 good 7/1 fa i lure scarp - zone B, zone 3 214 11 20 150 8 good 7/2 fa i lure scarp - zone 3 256 27 20 100 5 good *This is the estimated line-of-sight distance from the scanner station to the center of the target ** (-) tilt indicates that the line-of-sight is below horizontal For useful, good quality point clouds, the strike-line of the target area should be approximately perpendicular to the line-of-sight of the scanner, and the target should be within the maximum operating range of the instrument which is a function of the atmospheric visibility and target reflectivity (typically 800 meters for a target with 20% reflectivity using the Optech ILRIS3D laser). Several of the scans completed for the Afternoon Creek survey resulted in 'very poor' quality point cloud data, specifically those used to test the limitations of the instrument (Table 3.2). A poor quality point cloud means that few spot reflections were received by the instrument; therefore the resolution of the resulting point cloud was so coarse that no useful joint orientation data can be extracted. 32 The laser scanner is a line-of-sight instrument, meaning that the position of the first object in the light's path is recorded. Therefore small (and large) variations in topography, and vegetation create shadows - areas of the topography where no data is collected - in the resulting point cloud. Additionally, surfaces that strike sub-parallel to the line-of-sight of the scanner tend to reflect few laser strikes; therefore joint sets that strike parallel are poorly sampled, while sets that strike perpendicular are well sampled, introducing a bias to the final data. This point is illustrated by Figure 3.6, showing rose diagrams of automatically- generated patches found in four different point clouds of the same target (structural Zone 3). Southeast dipping surfaces were preferentially recognized in the scan from survey station 2 because this joint set was orthogonal to the scanner line-of-sight. Northeast dipping surfaces were preferentially recognized in the scans from survey stations 6 and 7. This bias can be removed by scanning the target from all possible angles and analyzing all of the scans (either individually or by aligning the scans into a single point cloud). 0 0 0 0 Figure 3.6 Histograms of dip direction vs. frequency for four scans of structural Zone 3 showing a bias in the orientation of automatically generated patches. Joint planes that strike perpendicular to the scanner position tend to reflect more laser strikes. Line-of-sight of the scanner is superimposed over the histogram. Ideally, the entire failure scarp and other areas of interest would have been scanned at a similar resolution from several angles with considerable overlap; this would have enabled the different point clouds to be joined, creating a single 3-D model of the entire slope. A single, continuous point cloud would have made the data analysis simpler, and more objective. However, in this case, overlapping data from all portions of the failure scarp was not achieved due to access limitations in the narrow Afternoon Creek. 33 Good quality point clouds for the failure scarp were collected from Stations 2, 6, and 7; however Stations 6 and 7 were too close to the failure scarp to allow for the entire zone to be captured, and therefore the resulting point clouds do not overlap. The scans from Stations 6 and 7 do however provide data for shadowed portions of the point cloud from the Station 2 scan. Another limitation imposed by the narrow confines of the Afternoon Creek channel was that the scanner was steeply inclined when directed towards the base of the failure scarp; 'false' summits in the failure scarp created a complete shadow of the upper portions of the failure zone. This shadow could have been removed by performing a scan midway up the valley wall opposite the failure scarp; however no safe access path or setup point could be found. 3.3.2 L iDAR Data P rocess ing Split-FX™ software, developed by Split Engineering L L C , was used to visualize the 3-D point clouds, and extract the orientation, spacing, and persistence of dominant discontinuity sets. Because it was not possible to create a single, comprehensive point cloud of the entire slope, each scan's point cloud was analyzed separately. Nine scans of various portions of the failure scarp were completed; however only four scans were considered to be of sufficient quality to be incorporated into the analysis: 2/1, 6/1, 7/1, 7/2 (see Table 3.2; Appendix B). The remaining five scans were not used because they were either 'very poor' quality or coarser resolution copies of one of the included scans. Each of the four point clouds was processed and analyzed in the same way using the following procedure (Figure 3.7): 1. Orient the point cloud. Input the true bearing of the scanner's line of sight and components of tilt (as measured in the field at the time of scanning). 2. Edit the point cloud. Remove points that are not related to joint orientation (e.g. reflections from vegetation, talus, soil-cover, etc.). 3. Create a mesh. The Split-FX™ software drapes a polygonal surface mesh over the point cloud. The analyst decides the mesh grid size, which controls the size of the cells, the number of points per cell, and the precision of the polygonal surface model (Split_Engineering 2005). 34 4. Automatic patch generation. Patches are planes fit to the discontinuity surfaces present in the point cloud. Patches are found first by grouping neighboring mesh triangles together based on the similarity of their vector normals, and then by using least squares to fit a plane through the points bounded by the grouped triangles (SplitEngineering 2005). User controls include the minimum patch size and minimum neighbor angle, which are tolerance parameters used to group neighboring mesh triangles. 5. Edit patches. The analyst visually inspects the patches and deletes erroneous patches and adds missing patches, i f necessary. 6. Stereonet Analysis. The patch orientation, size, and roughness are recorded by the software. These can be exported to any stereonet analysis package or analyzed with the Split-FX™ stereonet software. mesh mesh with patches Figure 3.7 Point cloud processing procedure A bias is introduced to the final data set through discontinuity surfaces that are small relative to the point cloud resolution and mesh size; these are not sampled by the method described above. This biasing is unavoidable, although the influence of the small, unsampled discontinuities can still be accounted for through the designation of rock mass quality. 35 3.3.3 L iDAR Data Ana lys i s To determine the influence of the mesh density and patch control parameters on the processing accuracy, with respect to the true joint surfaces measured, a qualitative parametric study was conducted. The mesh density recommended by the software manual yields approximately 30 points per mesh grid cell. The results of the parametric study showed that this was an appropriate point density for the Afternoon Creek slope. A coarser mesh density did not capture important (small) features of the rock slope, whereas a finer mesh did not add any features of significance to the data set. Also the finer mesh resulted in more patches that were smaller in size, that in turn were more difficult to visually inspect. The minimum patch size parameter is used to filter out small patches that are difficult to visually inspect and which add significant noise to the stereonets. This parameter was typically set to a value between 10 and 40 grid cells per patch. The minimum neighbor angle determines which grid cells are included in the patch. A relatively large value allows adjacent grid cells of an undulating, rough discontinuity to be grouped into a single patch at the average grid cell orientation. This parameter was typically set to a value between 4° and 8°. A value less than 4° typically yielded very few patches, and excluded surfaces in the point cloud that were obvious joint surfaces. A value greater that 8° typically yielded numerous erroneous patches that were removed during visual inspection (Figure 3.8). When a relatively fine mesh density was used the minimum patch size and minimum neighbor angle were typically set to the upper end of the described ranges. 36 2/1 Area !<n l̂ GO 180.0 • 6/1 Area [rn^l • 0.0 * 180.0 • 7/1 Area lm-'] 1.1.0 1 7/2 Area [nv*| 0.0 :• 180.0 + Plane A shallow * PbntAsutp o Plane H 4 5 6 0 Joint sat Dip Dip direction Plane A shal low 42 127 Plane A steep S9 107 Plane B 59 58 0 8 1 Equal area projection -- Lower hemisphere Figure 3.8 Comparison of automatically generated patches in point clouds 2/1, 6/1, 7/1, 7/2. Minimum patch size = 10; minimum neighbor angle = 4 degrees (top); minimum neighbor angle = 10 degrees (bottom). Pole size is scaled to patch area. 37 Orientation Each joint set was defined by its orientation. The. pole to each patch was plotted on the Split-FX™ stereonet. Joint sets were visually identified and then defined on the stereonet. The visual identification of the sets was verified by contouring the poles, and by highlighting all of the patches within a set and then visually inspecting the patches on the point cloud image. The selection of joint sets was repeated and compared for a range of minimum patch sizes and minimum neighbor angles. Spacing The average spacing of joints within a set was estimated by inserting a "virtual scan line" of known length into the point cloud approximately normal to the joint set, and counting the number of joints belonging to the set that crossed the line. The procedure below was followed, under the assumption that all discontinuities within each set formed an exposed surface: 1. Isolate patches belonging to the same joint set. This involves an initial grouping based on the stereonet analysis and pole plot contouring, and then visually inspecting and deleting all patches that do not belong to the joint set of interest. 2. Record the average orientation of the joint set in terms of its unit normal vector, n. 3. Insert the scan line. The scan line should be contained within parts of the cloud that are highly populated with points. Select a point on the slope at the beginning and end of the scan line; these two points define the line. Orient the scan line so that it is approximately perpendicular to the joint set. 4. Record the unit vector in the direction of the scanline, /; and the length of the scanline, L. 5. Orient the point cloud so that the entire scan line is visible. Select all patches (i.e., joint planes) that intersect the scan line. 6. Count the number of patches (N) that intersect the scan line. 7. The average true spacing (s) can be approximated with Equation 3.3: 38 L * cos 0 [3.3] v • $ — — ; Where 6 is the angle between the scanline unit vector, 1, and the joint set unit normal vector, n. The value of cos(0) can be found with the dot product of 1 and n: [3.4] / • « = |/||«|*cosf9 Persistence No feature is currently present within Split-FX™ to automatically calculate persistence. As such, the following three methods were considered for estimating the average persistence of joints within each set: (1) measure trace length on photographs; (2) measure the maximum dimension of exposed planes in the 3-D point cloud; (3) relate the persistence to area of patches in the 3-D point cloud. Each method involved measuring all of the visible discontinuities in the point cloud that meet some size and orientation criteria, and then assuming that the average persistence is equal to the mean of the measurements. The first method was to identify representative discontinuities for the different joint sets in digital photographs taken of the slope, and then calculate their trace lengths. The scale of the rock slope in the photograph is a function of the distance between the rock slope and the camera lense. However, it was difficult to keep track of these distances/scales. Photographs of the Afternoon Creek slope have a widely varying scale because the slope is composed of numerous benches and near-vertical faces and because many of the photographs were taken at an oblique angle to the slope. Where possible, the scale was estimated by the height of trees. However trees do not exist within the main failure scarp. Split-FX™ has a function that estimates trace length based on a user defined scale. The program assumes that the photographed rock face is a single continuous slope. Photographs of the Afternoon Creek slope generally do not meet this assumption. In some cases, photographs of smaller portions of the slope do meet this assumption; however the smaller frame of reference means that many of the discontinuity traces are truncated. Overall, this method was able to provide an estimate of discontinuity trace length, but the accuracy of those estimates is suspect. 39 The second and third methods applied were based on the dimensions of exposed planes in the 3-D point clouds. The term "exposed persistence" was adopted in recognition that the joint may be more persistent than that suggested by the plane exposed/visible on the slope face In this sense, the exposed persistence can be treated as an estimate of the minimum persistence, or lower bound of the average persistence for each particular joint set. 'Method 2' involved the direct measurement of the exposed persistence. The average exposed persistence was assumed to be the mean of the measurements. Accurate measurement of the joint dimensions is made possible by selecting points on the surface of the discontinuity and then calculating the distance between them. This method is similar to 'Method 1' in that it requires good geological engineering judgment when selecting, and measuring relevant discontinuity surfaces; however distance measurements on the point cloud are far more accurate than persistence measurements made on the photographs. 'Method 3' was an attempt to partially automate the procedure for estimating exposed persistence. The implied assumption is that there is a relationship between the area of the exposed discontinuity plane and the exposed persistence of the discontinuity. In the Afternoon Creek point clouds, the patches (i.e., the discontinuity planes) appear to be rectangular in shape; therefore the relationship between area and exposed persistence ('ep') is based on a subjective estimate of the aspect ratio (A) of most of the patches. The area of the patch is equal to the aspect ratio times the square of the minimum dimension (x) (Figure 3.9). Given the aspect ratio and area of the patch, the minimum dimension (x) is found with Equation 3.5. An estimate of the exposed persistence is easily calculated using Equation 3.6. [3.5] [3.6] Ax Figure 3.9 Schematic, annotated patch. 40 Problems with 'Method 3' are that it requires the patches to be rectangular in shape, and the aspect ratio to be accurately estimated for all of the patches. Additionally, the patches must completely cover the exposed joint surfaces in the point cloud, a condition that is not often met by the automatic patch generator. This necessitates that the patches be manually adjusted, a procedure that requires considerable time and effort. In summary, based on these experiences, it was found that it was easier to directly measure the exposed persistence using 'Method 2'. 'Method 1' was ineffective in measuring the persistence of joint sets striking sub-parallel with the rock face as these joints do not form prominent traces in the photographs. 3.4 Computer Analysis Techniques 3.4.1 Geographica l Information Sys tem (GIS) ArcGIS 9.0, from ESRI Inc., was used to manage and analyze maps, ortho-rectified aerial photographs, and digital elevation models (that were generously provided by URS Corporation for the analysis). Contour maps at many different scales were made from the DEMs. These served as base maps for field mapping and all subsequent analysis. Additionally slope maps were made that were used to determine accessibility for field mapping and L i D A R scanning. The ortho-rectified aerial photographs were draped over the digital elevation models and viewed in three-dimensions. This provided important insight to the failure volume and geomorphology of the area. It also helped to determine accessibility and feasible L i D A R setup locations. 3.4.2 Spheristat 2.2 Spheristat 2.2, created by Pangaea Scientific, is a windows-based analytical tool for creating stereonets and rose diagrams. The orientation of discontinuities was plotted on the spherical projection and contoured by the program. Based on the contouring, discontinuities were grouped into sets. The program calculated the average orientation and spacing of the set. Kinematic analysis for planar, wedge, or toppling failure compared the discontinuity sets with the slope orientation. 41 3.4.3 RocLab RocLab, created by Rocscience Inc., is a free software program used to determine rock mass strength parameters based on the Hoek-Brown rock mass classification parameters (Rocscience 2002). The required classification parameters were the unconfmed compressive strength of the intact rock, the geological strength index (GSI), and the intact rock parameter, mj. RocLab calculated the Hoek-Brown failure criterion parameters and the equivalent Mohr-Coulomb strength parameters (cohesion and friction angle) for a given stress range. The Mohr-Coulomb strength parameters for the rock mass were required input to the limit equilibrium and numerical modeling analyses. 3.4.4 Limit Equi l ibr ium Methods (RocPlane, Swedge) RocPlane (Rocscience 2001) and Swedge (Rocscience 2002), both by Rocscience Inc., were used to perform 2-D limit equilibrium analyses, based on the assumption of a rigid rock block system. RocPlane was used to analyze the planar failure mechanism. Swedge was used to analyze the wedge failure mechanism. These preliminary analyses guided subsequent development of the more complicated numerical models. 3.4.5 Universal Distinct Element C o d e (UDEC) The Universal Distinct Element Code (UDEC) is a two-dimensional numerical program based on the distinct element method for discontinuum modeling. UDEC simulates the response of jointed rock masses to loading. The rock mass is divided into an assemblage of discrete blocks that are able to move independently of each other. The blocks can be made deformable by dividing them into a mesh of finite difference elements (Itasca 2000). UDEC was the primary tool for numerical modeling of the Afternoon Creek slope. 3.4.6 Three Dimens iona l Distinct Element C o d e (3DEC) 3DEC is the three1dimensional extension of the 2-D distinct-element program, UDEC. As in UDEC, the rock mass is divided into discrete blocks that can be made deformable. 3DEC simulates the response of the jointed rock masses to loading (Itasca 2003). 3DEC models of Afternoon Creek were compared to the 2-D U D E C results to determine the influence and effects of the third-dimension in stability analysis. 42 3.4.7 Surfer Surfer 8.0, created by Golden Software, Inc., is a contouring and 3-D surface mapping program (Golden_Software 2002). It was used to visualize and extract information from the digital elevation models (DEM). Due to its ease of use, Surfer was used to create cross-sections that were used in numerical modeling and the DEMs required for the DAN3D analysis. 3.4.8 Three-Dimensional Dynamic Ana lys is (DAN3D) DAN3D, developed by Scott McDougall and Oldrich Hungr at the University of British Columbia, is a numerical model for the dynamic analysis of rapid flow sides, debris flows, and avalanches (McDougall and Hungr 2004). It has the ability to model rapid landslide motion across complex three-dimensional terrain. It is based on the Lagrangian solution of the equations of motion and depth-averaged hydrodynamic theory. The model has been used to simulate several landslide case histories (McDougall and Hungr 2004; McDougall and Hungr 2005). A DAN3D model was calibrated by the analysis of the November, 2003 rock avalanche. The calibrated model was used to predict the runout path, distance, and velocity of future events at the site. 3.4.9 Three-Dimensional Rockfal l S imulation (PIR3D) PIR3D is a lumped-mass, 3-D rockfall simulation program. It was developed by Mag-Informatique, France. Input required by PIR3D includes a digital elevation model, restitution coefficients, bounce variation angle, and initial fall height. In this study, the PIR3D analysis was completed by Amandine Brosse from Ecole Nationale des Travaux Publiques de l'Etat. 3.5 Numerical Modeling Procedure The procedure followed for numerical modeling analysis was based on the recommendations of Starfield and Cundall (1988), and the U D E C user's manual (Itasca 2000). First it was recognized that the Afternoon Creek rock slope analysis was a data- 43 limited problem due to practical constraints. Thus, rather than using the analysis methods and numerical models to develop an exact prediction, they were used as tools for performing parametric studies to better understand the physical properties of the slope, and the failure mechanism. To gain this understanding, the following procedure was followed to perform the numerical experiments (modified from (Itasca 2000)): 1. Define the objectives of the analysis: The objectives dictated the model design. Complicating features in the natural slope, such as the precise topography, groundwater, and individual discontinuities that were irrelevant to the objectives were omitted, especially in early stages of modeling. The influence of complexities was investigated during the parametric studies. 2. Create a conceptual picture of the slope: The conceptual model of the slope was based on the initial reconnaissance phase of data collection, including information gained from the desk study and initial site visit. 3. Simple, idealized slope stability analysis: The experience gained from simple, idealized model runs guided the design of the detailed data collection program and the design of detailed numerical models. Parametric studies of the simple models showed the parameters to which the slope failure was most sensitive. These parameters became the focus of the data collection program. 4. Detailed data collection: A reasonable range of values for each input parameter was selected based on field mapping and published examples; Large uncertainties were associated with many of the parameters. 5. Detailed slope stability analysis: Kinematic analysis, limit equilibrium analysis, and detailed numerical models incorporating the parameter values ascertained during the detailed data collection. Numerical experiments designed to meet the project objectives were run. Parametric studies were completed to verify the model results. 44 4 P R O C E S S I N G O F F I E L D D A T A F O R N U M E R I C A L M O D E L I N G The data collection program described in chapter 3 provided initial estimates of input parameters required by the numerical models and other computer analysis techniques used in the study. Chapter 4 describes the results of the data collection phase of the study, including a description of the regional and local geology, a description of the local topography and joint sets, and a description of the mechanical properties that describe the response of the rock mass and discontinuities to loading. 4.1 Physiographic & Geologic Setting The physiographic and geologic setting is important because it describes the context of the numerical models. It was the basis for the selection of appropriate physical properties, boundary conditions, and model size. 4.1.1 Regional Setting The Afternoon Creek rock slope is located in the Northwest quadrant of Washington State in the exceptionally steep and rugged North Cascade mountains. The rockslide failure zone is located more than 600 meters above Washington State Route 20 (SR 20) near Newhalem, Washington; the peaks near Newhalem have nearly 1.5 km of vertical relief. Afternoon Creek is a small perennial stream that disappears beneath bouldery debris below the failure scarp, then reappears briefly before passing beneath SR 20 and flowing into the Skagit River (Figure 2.2). The North Cascades is a complex region of accreted Mesozoic and Paleozoic terrains that were assembled during the early to middle Cretaceous. The geology of the region is further complicated by late Cretaceous through Eocene thrusting, plutonism, regional metamorphism, strike-slip faulting, extensional faulting and basin development. Quaternary glaciation created the chiseled peaks and open parabolic-shaped valleys that exist today. The most recent Cordilleran glaciation covered the area 15,000 years ago with a continuous ice 45 cap (Tabor et al. 2003). The Afternoon Creek slope is over-steepened as a result of this most recent glaciation. A l l of the rocks involved in the rockslide are hornblende-biotite tonalite orthogneiss of the Skagit Gneiss Complex (Tabor et al. 2003 map). The tonalite is most likely intrusive igneous material with original igneous crystallization from the late Cretaceous to early Paleocene. The cause of metamorphism is still under debate, but it is agreed that the mechanism is related to some sort of crustal thickening and that ductile deformation ceased by the early Oligocene (Tabor et al. 2003). 4.1.2 Loca l Geo logy The geology of the Afternoon Creek site is dominated by the exposed strong tonalite orthogneiss of the Skagit Gneiss Complex (SGN) with a variety of surficial deposits including alluvium, colluvium, and rock avalanche deposits. The local geology is best described with a map (Figure 4.1). The following descriptions refer to units and features of the geologic map. 46 47 Artificial Fill (Qaf) - Road base and embankment materials for State Route 20. Recent Rock Avalanche Deposits (Qrra) - Rock avalanche debris from the November and December, 2003 rock avalanches. The deposit is very coarse with grain sizes ranging from sand to 25-meter diameter boulders. The debris is composed of orthogneiss from the SGN; it originated from the zone labeled "Failure Area." Pre-historic Rock Avalanche Deposits (Qpra) - Older rock avalanche debris deposited since the most recent deglaciation (15 ka). The deposit is very coarse and clast supported with grain sizes ranging from sand to angular 4-meter diameter boulders. One to two-meter diameter boulders are common. The debris is uniformly composed of orthogneiss from the SGN; most of the debris probably originated from the west side of Afternoon Creek, near the zone labeled "Failure Area." Quaternary Fan Deposits (Qf) - Undifferentiated deposits at the mouth of the Afternoon Creek drainage channel that were transported by fluvial, or mass wasting processes, such as debris flows. The deposit is generally unsorted and matrix supported with grain sizes ranging from silty-sand to boulders. Quaternary Alluvium (Qal) - Fluvial deposits in the Skagit River channel. Also contains large boulders that have been transported to the river channel by snow avalanches, debris flows, and rockfall. Cretaceous/ Tertiary Skagit Gneiss Complex (SGN) - Strong hornblende-biotite tonalite orthogneiss. Dozens of northeast-southwest trending fracture lineaments that are visible on a regional scale cut the SGN at the Afternoon Creek slope. Many of these lineaments are high-angle shear zones. A second set of locally persistent fractures trend northwest-southeast, parallel to Afternoon Creek. Many of these discontinuities are filled with soil and rock rubble debris at the surface. The November and December, 2003 failure scarp, labeled "Failure Area" was divided into four structural domains - zones 1, 2, B, 3 - based on rock mass quality and structural pattern. Zones 2, B, and 3 are separated by shear zones. Zone B 48 a n d Z o n e 3 w e r e d i r e c t l y i n v o l v e d i n the N o v e m b e r a n d D e c e m b e r , 2 0 0 3 f a i l u r e s . D e t a i l e d d e s c r i p t i o n o f these z o n e are as f o l l o w s : Zone 1 i s c o m p o s e d o f v e r y - b l o c k y , w e a t h e r e d - g r e y g n e i s s . W e d g e - s h a p e d b l o c k s are c o m m o n i n Z o n e 1. M a n y f r ac tu res a re d i l a t e d . F r a c t u r e s u r f a c e s a re s m o o t h a n d m o d e r a t e l y w e a t h e r e d ; t he r e fo r e a G S I o f 4 5 - 5 0 w a s a s s i g n e d . F r e s h su r f a ce s do e x i s t i n th i s z o n e , i n d i c a t i n g that r o c k s h a v e r e c e n t l y f a l l e n f r o m th i s z o n e o r that the a rea w a s d a m a g e d b y the i m p a c t o f l a rge b l o c k s d u r i n g the N o v e m b e r , 2 0 0 3 f a i l u r e . Z o n e 1 is s m a l l a n d p r o b a b l y o f l i t t l e i m p o r t a n c e t o the o v e r a l l s t a b i l i t y o f the s l o p e . It w a s n o t d i r e c t l y i n v o l v e d i n the N o v e m b e r / D e c e m b e r , 2 0 0 3 f a i l u r e s e q u e n c e ( F i g u r e 4 .2 ) . Figure 4.2 Structural domain Zone 1. Zone 2 i s c o m p o s e d o f b l o c k y , w e a t h e r e d g r e y g n e i s s . T h e r o c k m a s s i n Z o n e 2 i s m a s s i v e a n d c l i f f - f o r m i n g . T h e s u r f a c e i s m o s s - c o v e r e d i n d i c a t i n g that i t w a s no t i n v o l v e d i n the N o v e m b e r / D e c e m b e r , 2 0 0 3 f a i l u r e s e q u e n c e . F r a c t u r e su r f a ce s are r o u g h , a n d s l i g h t l y w e a t h e r e d , t he r e fo r e a G S I o f 6 0 - 6 5 w a s a s s i g n e d to Z o n e 2. D i s c o n t i n u i t i e s i n t h i s z o n e w e r e m a p p e d w i t h a 7 5 - m e t e r l o n g s c a n l i n e - J o i n t s u r v e y 1 ( F i g u r e 4 .3 ) . 4 9 Figure 4.3 Structural domain Zone 2. Zone B is separated from Zone 2 by the Base shear zone and separated from Zone 3 by the Tower shear zone. Zone B is composed of dark-grey to black, mica-rich, high- schistosity gneiss. The zone is highly fractured. Fracture intensity increases with proximity to the Tower and Base shear zones. Zone B has a parabolic-shaped weathering profile indicating its relatively low resistance to erosion. The surface of the shallow slope formed by Zone B is covered by a veneer of coarse, unconsolidated, matrix-supported debris. The matrix is coarse sand. The cobble to boulder-sized clasts are angular. The debris and exposed rock is typically stained an orange, iron- oxide color. A GSI of 30-35 was assigned to Zone B (Figure 4.4). 50 Figure 4.4 Structural domain Zone B. Zone 3 is composed of massive, blocky fresh gneiss. This zone was exposed by the November, 2003 failure. The joints in this zone are widely spaced (on the order of 7 to 15 meters) and highly persistent (up to 100 meters) - the November, 2003 failure released blocks up to 8000 m 3 . The lower portions of this zone, at the level of Afternoon Creek are weathered and moss covered - this portion of Zone 3 is below the failure scarp. Portions of Zone 3 involved in the November, 2003 failure are inaccessible, therefore observations of this zone were made from a distance. A GSI of 60-65 was assigned to Zone 3. Much of Zone 3, especially the upper portions, was inaccessible and hidden from view; therefore further refinement was not possible. One distinctive section of Zone 3 should however be noted - the upper portion of Zone 3 appears to be very blocky with dilated fractures, which may be an indication of extensive internal deformation. Some of the material in this portion of Zone 3 is 51 unconsolidated debris similar to what exists in Zone B. The GSI of the upper portion of Zone 3 is probably in the range of 45-55 (Figure 4.5). Figure 4.5 Structural domain Zone 3 viewed from three directions. Base Shear Zone - The lower boundary of the Zone B wedge. It forms a discrete, undulating plane that averages 15 cm in thickness and dips between 10° and 30° to the West. The zone is orange in color and soft enough to be easily broken with a hammer corresponding to Rl-very weak strength classification. Quartz infilling exists in some places along with sharp rock fragments. At one location, groundwater seeps from this zone at an approximate rate of a few liters per minute (estimated on a dry day in April , 2005). Tower Shear Zone - The northern boundary of the Zone B wedge. It is composed of several sub-parallel shear zones that dip steeply (75° - 90°) to the southeast . Each zone is approximately 3 cm to 30 cm wide, infilled with clayey fault gouge, planar, smooth, and occasionally slickensided. 52 North Shear Zone - Near vertical zone that forms the northern boundary of the failure zone, visible on both sides of Afternoon Creek. 4.2 Geometric Parameters 4.2.1 Topography Digital elevation models (DEM) from before and after November, 2003 were generously made available for this project by URS Corporation. The D E M from before the rockslope failure were created by the U.S. Geological Survey. The size of the raster cells is 10 meters by 10 meters. The D E M from after the rockslide was created from aerial L i D A R flown one month after the event. The spacing of the L i D A R points is approximately 1 meter (Figure 4.6). 700 600 500 400-f 300 200 100 s Ii \ V N \ \ V \ \ \ meters Figure 4.6 Contour map and topographic profiles derived from the L i D A R D E M after the November, 2003 event. Arrows indicate the path of steepest descent. 53 4.2.2 Descr ipt ion of Joint Sets Structural mapping was done in structural domains Zone 2 and 3, where the rock mass is blocky to very blocky. Mapping in Zone 2 was done by means of a 75-meter scan line survey. A 15-meter wide section of Zone 2 was also scanned with the terrestrial laser scanner, enabling a comparison of the two mapping methods. Several terrestrial laser scans were made of structural Zone 3; parameters that describe the pattern of discontinuities, including joint set orientation, spacing, and persistence, were extracted from the resulting 3- D point clouds. The results of the structural mapping in zones 2 and 3 are summarized in Table 4.1. These results were used in subsequent numerical analyses. Each parameter is discussed in further detail below. Table 4.1 Summary of parameters that describe discontinuities in the Afternoon Creek rock slope. Joint Set A (Zone 3) Joint Set B (Zone 3) Joint Set C (Zone 2) Relative confidence** mean standard deviation mean standard deviation mean standard deviation Orientation (Dip direction/ dip) (degrees) 116/51 010/9 053/62 009/8 291/66 005/9 High Spacing (meters) 4 - 7 - 12-15 - 8 - Moderate Persistence (meters) 42 28 84 20 34 21 Low ** 'Relative confidence' is subjective. It refers to confidence in the accuracy of the estimates. 4.2.2.1 Orientation Joint sets were defined by orientation as derived from analysis of 3-D point cloud models of structural Zone 3, and analysis of traditional scan line discontinuity data collected at the base of the failure scarp in Zone 2 (location shown in Figure 3.4). A comparison of joint orientation estimates measured by hand with a Brunton compass and derived from the 3-D point clouds can be found in Appendix C: Verification of Methods. Discontinuities were divided into three joint sets (A, B, C). Joint set A was found in Zone 2 and Zone 3 by the point cloud analysis and the scan line survey. Joint set B was found only in Zone 3 by the point cloud analysis (figure 4.7). Joint set C was found only in Zone 2 by the scan line survey. 54 Figure 4.7 Joint sets A and B in structural domain Zone 3. The orientation of joint set A in Zone 3, estimated primarily from the point clouds, is (dip direction/dip) 116751° (Figure 4.8); while the orientation in structural Zone 2, estimated by the scan line survey, is 144760° (Figure 4.9) based on recorded joints. This difference is partially due to an actual change in orientation across the slope; however it is possible that a small systematic error caused by inaccurate georeferencing of the point cloud exists. The scan line survey at Afternoon Creek was conducted in a portion of the slope that was not directly involved in the failure, while the point cloud analysis considered the actual failure scarp; therefore the orientation estimated from the point cloud analysis was used during subsequent slope stability analysis. The automatically generated patches of joint set A typically have a large surface area (Figure 4.10), indicating that these joints are highly persistent and relatively smooth. Planar sliding is a kinematically feasible failure mechanism for this joint set. The orientation of joint set B estimated from the point clouds is 053762° (Figure 4.8). These discontinuity surfaces provided the lateral release necessary for planar sliding on joint set A. The automatically generated patches were typically small. Many of these discontinuity surfaces are highly persistent. When the minimum neighbor angle was increased to about 10° during automatic patch generation, the joint set B patches were typically as large as the joint set A patches (Figure 3.8b) indicating that the small patch size 55 is probably due to the rough or undulating texture of these joint surfaces. The strike of joint set B is approximately coincident with the trend of the scan line; therefore it did not cross the scan line. The orientation of joint set C estimated from six discontinuities sampled by the traditional scanline survey (Figure 4.9) is 291766°. Joint set C was not visible in Zone 3, nor was it found in the point clouds of the failure scarp. This set does not form exposed discontinuity surfaces in Zone 2. Additionally, the dip direction of joint set C is in nearly the same direction as the line of sight of scan 2/1; therefore the footwall of this discontinuity will invariably be shadowed, and overhanging faces are uncommon in this slope. Joint set C was probably not a controlling discontinuity set. Where this joint set does exist, it provides lateral and rear release for planar sliding on joint set A. Figure 4.8 Structural Zone 3. Contoured stereographic projection of automatically generated patches that represent discontinuity surfaces measured in scanline survey JS2 and in point clouds 2/1, 6/1, 7/1, 7/2. minimum neighbor angle = 6° and minimum patch size=20. Vectors indicate the L i D A R scan line-of-sight. Axial _ „ . . . „ . . N = 47 Zone 2 Discontinuity Orientation Figure 4.9 Structural Zone 2. Stereographic projection of discontinuities exposed at the base of the failure scarp. Measured during scan line survey JS1 and point cloud analysis of scan 3/4 . Figure 4.10 Stereographic projection of automatically generated patches that represent discontinuity surfaces in point clouds 2/1, 6/1, 7/1, 7/2. Pole size is scaled to patch area. Joint set orientation is the average orientation of poles included in the set. 5 7 4.2.2.2 Spacing Spacing of the joints within each set was estimated using two different techniques: (1) the virtual scanline technique using a 3-D point cloud; and (2) the standard scanline technique with the Terzaghi correction. The two techniques are compared in Appendix C, for a portion of structural Zone 2 that was sampled by the traditional scanline method, and with the terrestrial laser scanner. This comparison shows a good correlation of the results. Further comparisons at a more accessible site should probably be done to validate the virtual scanline method. The average spacing of joint sets A and B in structural Zone 3 was estimated using the virtual scanline technique on point clouds 2/1, 6/1, 7/1, 7/2 (Table 3.2). The average spacing of joint set A ranges from 4 to 7 meters. The average spacing of joint set B was considered at two different scales. Highly persistent, plane B discontinuity zones exist at the scale of the entire rock slope. The average spacing of these plane B zones ranges from 12 to 15 meters. The spacing of less persistent planes within these zones ranges from 3 to 5 meters (see Appendix D for calculations). The very-highly persistent, extremely-widely spaced plane B zones appear to be more important for overall slope stability. The standard scanline technique with the Terzaghi correction was used to estimate the spacing of joints measured by the scanline surveys. The average spacing of plane A in Zone 2 (scanline survey JS1) is approximately 4 meters. The average spacing of plane A at the base of Zone 3 (scanline survey JS2) is approximately 2 meters. The average spacing of plane C, found only in Zone 2, is approximately 8 meters. However the spacing measured with the traditional scanline survey may not be representative of the entire slope because only a small portion of the slope, not directly involved in the slope failure, was sampled (see Appendix A). 4.2.2.3 Persistence Estimates of persistence and exposed persistence of joint sets A and B in structural Zone 3 were attained by three different methods: (1) measuring trace length on photographs; (2) measuring the maximum dimension of exposed planes in the 3-D point cloud; and (3) 58 relating the exposed persistence to area of patches in the 3-D point cloud. Table 4.2 is a comparison of the mean persistence/exposed persistence and standard deviation obtained by the three methods. Appendix D is a complete record of the persistence calculations. Persistence estimates obtained by method 2 are the most reliable because they are direct measurements of the parameter; method 2 is preferred over method 1 because distance measurements are more accurate on the 3-D point cloud than the photographs. Joint set B did not form prominent traces in the photographs, as it strikes parallel with the rock face; therefore method 1 was not used to estimate persistence of joint set B. Table 4.2 Estimates of persistence and exposed persistence in structural Zone 3 mean mean exposed standard Joint sets persistence persistence deviation (meters) (meters) (meters) Method 1 28 n/a 14 A Method 2 n/a 42 28 Method 3 n/a 34 iv Method 1 ** n/a ** B Method 2 n/a 84 20 Method 3 n/a 50 15 trace of joint undetectable as the joint set strikes sub-parallel to the rock face/photographs. 4.3 Mechanical Properties 4.3.1 Rock Mass Propert ies RocLabl.O software (Rocscience 2002) was used to estimate rock mass strength parameters based on the generalized Hoek-Brown failure criterion. Material properties of the orthogneiss rock mass were estimated from tabulated values of intact rock properties (Kulhawy 1975), and the results of uniaxial compressive strength (UCS) and point load tests performed as part of the URS and Wyllie & Norrish monitoring and investigation program (URS and Wyllie & Norrish Rock Engineers 2004). The intact UCS was corrected for the influence of discontinuities using the GSI that was assigned to each structural domain (Marinos and Hoek 2000). The Mohr-Coulomb friction angle, tensile strength and cohesion parameters were estimated from a best-fit to the Hoek-Brown classification curve (Hoek et al. 2002). 59 The average uniaxial compressive strength of the intact orthogneiss is approximately 90 MPa according to UCS tests performed by GeoTest Unlimited, and from point load correlation (URS and Wyllie & Norrish Rock Engineers 2004). This corresponds to 'R4- strong rock' according to Brown (1981). During the 'field mapping' phase of investigation, the slope was broken into four structural domains called 'Zones'. The GSI within each zone is similar and ranges from 'disintegrated' with 'fair' surface conditions (GSI-30) to 'blocky' with 'good' surface conditions (GSI-65). Table 4.3 shows the range of reasonable values for each property required for numerical modeling of the orthogneiss rock mass. The range of reasonable values was tested in the numerical models. Table 4.3 Estimated range of rock mass mechanical properties in each structural zone. R o c k M a s s P r o p e r t y Z o n e 1 Z o n e 2 Z o n e B Z o n e 3 S o u r c e o f i n f o r m a t i o n U C S ( M P a ) 90 90 90 90 (URS and Wyllie & Norrish Rock Engineers 2004) G S I 45-50 60-65 30-35 60-65 Field estimate I n t a c t r o c k p a r a m e t e r , M i 28 28 13-23 28 Estimated with Roclabl.O D e n s i t y ( kg/m 3 ) 2600 - 2700 2600 -2700 2600 -2700 2600 -2700 (Kulhawy 1975) I n t a c t Y o u n g ' s m o d u l u s , E i ( G P a ) 65 65 65 65 (Kulhawy 1975) R o c k m a s s D e f o r m a t i o n m o d u l u s , E m ( G P a ) 5-9 20-29 1.6-2.6 20-29 Estimated with Roclabl .0 P o i s s o n ' s r a t i o , V 0.2 0.2 0.2 0.2 (Kulhawy 1975) B u l k m o d u l u s , K ( G P a / m ) 3-5 11 - 16 0.9-1.4 11 - 16 Calculated K-  E m 3(1 -2v) S h e a r m o d u l u s , G ( G P a / m ) 2-4 8-12 0.7-1.0 8-12 Calculated G-  E m 2(1 + v) C o h e s i o n ( M P a ) 0.8-2 1.3-2.8 0.5-1.2 1.3-2.8 Estimated with Roclabl .0 F r i c t i o n a n g l e 52°- 62° 55° - 64° 40° - 52° 55° - 64° Estimated with Roclabl.0 T e n s i l e s t r e n g t h ( M P a ) 0.06-0.08 0.18-0.25 0.04-0.05 0.18-0.25 Estimated with Roclabl.0 60 4.3.2 Discontinuity Properties The required mechanical properties for numerical modeling of rock discontinuities describe the stiffness and strength. Laboratory or in-situ testing of discontinuities was not attempted in this study; therefore initial estimates of stiffness and strength were derived from published data of strength tests on similar materials (Barton 1976; Kulhawy 1975). These values were verified and constrained during the numerical analysis. Table 4.4 shows the range of reasonable discontinuity properties required for numerical modeling. The shear stiffness, Ks, was obtained from a set of published data compiled by Kulhawy (1975). Published values of normal stiffness, Kn, for plutonic and meta-plutonic rocks were sparse; therefore the relationship proposed by Duncan and Goodman (1968) was used to calculate the reasonable values of Kn. Duncan and Goodman showed that for an equivalent elastic material, the normal stiffness, Kn, must equal 2Ks to 3Ks. Kulhawy pointed out that laboratory tests in shale and sandstone show that discontinuities do not behave as equivalent elastic materials, however he did not offer a different relationship or recommend a range of values for Kn. Hart (1993) noted that normal stiffness can be over 100 GPa/m in tight granite joints, but as low as 100 MPa/m in soft clay-filled joints. He cautioned that low values of normal stiffness may cause problems with interpenetration in distinct element models. The discontinuity strength properties required for the Mohr-Coulomb criterion are the joint friction angle and joint cohesion. The joint friction angle required for the Afternoon Creek numerical modeling was an apparent value rather than a material property. In the "dry" numerical models, it incorporated the basic friction angle for the material, the strength- reducing effect of pore-water pressure, and the strength-increasing effect of discontinuity roughness. An estimate of the basic friction angle was taken from laboratory test results published by Barton (1976). The magnitude of the pore-water pressure at the time of failure is unknown. The roughness of the sliding surfaces in the failure scarp was estimated with considerable uncertainty since the actual sliding surfaces were inaccessible. Joint surfaces at the base of Afternoon Creek, which were logged in the scanline survey, typically were planar and smooth or undulating and smooth with Joint Roughness Coefficient (JRC) of about 7. However it is important to note that none of these surfaces were involved in the recent slope failures. Many of the large boulders in the debris have at least one smooth, polished side 61 with JRC of about 2. These surfaces may be a better indication of the roughness of sliding surfaces in the failure zone. Due to these uncertainties, a wide range of joint friction angles was considered reasonable. To account for minor roughness, and drained conditions near the surface, the estimated reasonable range was a few degrees larger than the range of basic friction angles reported by Barton (1976). There is typically no cohesion in a fully-persistent discontinuity with no infilling. However at the scale of the Afternoon Creek rock slope, many discontinuities are not fully persistent. As Terzaghi (1962) first pointed out, discontinuous joints cannot connect with other adjacent joints without cutting across intact rock bridges. The intact rock bridges add a component of shearing resistance called effective cohesion. The magnitude of effective cohesion decreases as the joint propagates leading up to failure. The true value of the effective cohesion is unknown and impossible to measure at Afternoon Creek; therefore no initial estimate was made, and each numerical model was subjected to a wide range of effective cohesion values. To simulate the propagation of joints in the numerical models, the effective cohesion was decreased in increments from the maximum value to zero. Wyllie and Mah (2004) recommend that shear zones infilled with clay and angular fragments have cohesion ranging from 0 to 100 KPa. Table 4.4 Estimated range of discontinuity strength and stiffness parameters for numerical modeling. D i s c o n t i n u i t y P r o p e r t y J o i n t s (Set A , B , C ) Z o n e 1,2,3 J o i n t s (Set A , B , C ) Z o n e B S h e a r z o n e s S o u r c e o f i n f o r m a t i o n C o m m e n t s J o i n t n o r m a l s t i f f ness , K n ( G P a / m ) 1-2.5 1-2.5 0.25 - 1 (Hart 1993; Kulhawy 1975) Low values may cause 'contact overlap' error J o i n t s h e a r s t i f f ness , K s ( G P a / m ) 0.5-1 0 .5-1 0.1-0.5 (Hart 1993; Kulhawy 1975) Low values may cause 'contact overlap' error J o i n t c o h e s i o n ( M P a ) 0 - 1 0 - 1 0-0 .1 (Wyllie and Mah 2004) Clay infilling with angular fragments in shear zone J o i n t f r i c t i o n a n g l e 28° -33° 23° -26° 23° -26° (Barton 1976) 62 5 B A C K A N A L Y S I S O F T H E 2 0 0 3 A F T E R N O O N C R E E K S L O P E F A I L U R E The Afternoon Creek rock slope poses a hazard to the SR 20 highway at two locations due to the morphology of the slope. One hazard is a rock avalanche that originates from the Afternoon Creek side of the ridge and travels down Afternoon Creek, potentially impacting the highway. The second more likely threat is from rock debris that originates from near the top of the ridge and travels down Falls Creek, before landing on the highway. The purpose of the slope stability back analysis was to increase our understanding of the failure mechanisms and controls that acted in the recent slope failures. This understanding can be used to anticipate the character, location, and size of future slope failures at Afternoon Creek. To meet these objectives, two central questions were considered in the slope stability analysis of the November, 2003 rock slope failure: 1. What were the mechanisms and controls of the November rock slope failure? 2. Why did a portion of the Afternoon Creek rock avalanche travel down the Falls Creek side of the ridge? 5.1 Failure Initiation in Afternoon Creek The back analysis performed for the failed Afternoon Creek rock mass provided understanding of the failure mechanism, and constraints on the mechanical properties of the rock mass and discontinuities, and location and orientation of discontinuities and weak zones. These constrained values and new understanding of mechanisms guided the present day hazard assessment (chapter 6) and runout analysis. The objectives of the instability back analysis of the November, 2003 failure in Afternoon Creek were the following: 1. Constrain estimates of the slope geometry and physical properties, including: a. Orientation and spacing of discontinuities; b. Mechanical parameters of the rock mass and discontinuities. 2. Identify and understand the failure mechanism: a. Determine why failure occurred at this location in the slope; 63 b. Understand the interaction of Zone B with the massive zone above; c. Determine how the operative failure mechanism affects the volume of failed material; d. Determine i f failure occurred as a single event or as a series of multiple events. e. Determine how the mechanism affects the post-failure movement of the material. Several analysis techniques were used, including: (1) kinematic analysis; (2) limit equilibrium analysis; (3) 2-D distinct element numerical modeling (UDEC); and (4) 3-D distinct element numerical modeling (3DEC). Multiple methods were used to exploit the advantages and benefits of each technique. Results of the multiple analysis methods built on one another; the more simple analysis methods aided in the design and interpretation of the more complex 2-D and 3-D distinct element numerical models. Pore-water pressures were not considered in any of the analyses because they would have added considerable complexity to the models and were not necessary for studying the structural and topographic controls of the failure. The magnitude of pore-water pressure could not be adequately constrained due to the discontinuous nature of the fluid pathways in the impervious orthogneiss rock, and lack of hydrogeology data for the Afternoon Creek slope. However it should be noted that elevated pore-water pressures probably triggered the November, 2003 failure. Because the driving force associated with water-filled tension cracks and strength-reducing effect of pore-water pressures were neglected, the back- calculated discontinuity strength parameters were actually 'effective' strength parameters, which incorporated the effects of the elevated pore-water pressures. 5.1.1 Kinematic Ana lys i s A kinematic analysis compares the orientation of the important discontinuity sets with the orientation of the slope to determine feasible structurally-controlled failure modes. For the Afternoon Creek slope, the kinematic analysis aided in the interpretation of the failure mechanism. In this case, two important joint sets exist - joint sets A and B. The orientation of these two sets was measured on the LiDAR-derived point clouds. 64 A topographic map of Afternoon Creek shows the variation in slope trend (Figure 5.1). The strike of the slope varies due to a contrast in the weathering resistance of the rock in the different structural domains. The highly fractured Zone B domain weathers more easily than the surrounding massive, competent Zone 2 and Zone 3 domains. Weathering of the wedge of Zone B material created a notch in the slope that may have been an important control of the slope failure (figure 4.1). As a result, near the interface of structural domains Zone B and Zone 3, the local slope direction is 100°, in contrast to the average slope direction of approximately 075°. In both cases, the dip of the slope averages 66° to the east. The kinematic analysis compared the varied orientation of the slope with joint sets A and B. An equal area (Schmidt), lower-hemisphere stereonet was used for the analysis. 700 600- 500 400 300 200 100 111 bip^66^'E ^ \ \ ) ) \ X<Wx y Wectipn: 1 0 0 M '"WW Figure 5.1 Topographic map of Afternoon Creek after the November, 2003 failure, showing the average and local slope orientation used in the kinematic analysis. 65 When the average Afternoon Creek slope was considered, the two possible failure mechanisms were wedge failure along the intersection of joint sets A and B or planar failure along joint set B (Figure 5.2). However based on field observations it does not appear that planar failure along joint set B was an important failure mechanism. The highly persistent discontinuities that bound the failed mass are steeper than the average inclination and steeper than the Afternoon Creek slope; therefore these discontinuities do not allow kinematic freedom for sliding (i.e., non-daylighting). Most of the joint set B surfaces that are inclined at an angle shallower than the slope face are relatively small and terminate at intersections with joint set A surfaces. When the local Afternoon Creek slope is considered, there is kinematic freedom for wedge failure along the intersection of joint set A and joint set B as well as planar failure along joint set A (Figure 5.3). Field observations of the slope indicates that planar failure along the smooth, highly persistent joint set A planes was probably the dominant failure mechanism in the Zone 3 structural domain. The relative stability of these two failure mechanisms was considered in the limit equilibrium analysis considered below. Axial N = 77 Figure 5.3 Stereographic projection of the important discontinuity sets compared with the local orientation of the Afternoon Creek slope at the interface of Zone B and Zone 3. 5.1.2 Limit Equi l ibr ium Ana lys is A limit equilibrium analysis was undertaken to compare the three failure modes found in the kinematic analysis above: (1) wedge failure on the average Afternoon Creek slope orientation, (2) wedge failure on the local slope orientation at the interface o f Zone B and Zone 3, and (3) planar failure of joint set A on the local slope orientation at the interface of Zone B and Zone 3. The purpose of the analysis was to determine which failure mode was the least stable, given the same strength parameters. The least stable failure mode was probably the dominant mode during the November, 2003 slope failure. It was not necessary to include pore pressures or to include the actual material strength properties because the analysis was meant only to determine the relative stability of the three different geometric slope configurations. The geometric configuration and strength parameters were input into the Rocscience programs Swedge (Rocscience 2002), for the wedge failure modes, and Rocplane (Rocscience 2001), for the planar failure mode (Figure 5.4). These programs calculated the 67 Factor of Safety, which is the ratio of the resisting forces to the driving forces, for the given slope orientation, joint orientation, joint friction, and joint cohesion. The relative stability of the three failure modes was determined by comparing the joint cohesion necessary to achieve a Factor of Safety of 1.0 for a given joint friction angle. The friction angle was varied from 23° to 35°. Equivalent joint strength parameters were always used for both joint sets during Figure 5.4. Illustration of the simplified limit equilibrium analysis models. The Rocscience program Swedge was used for wedge stability analysis; Rocplane was used for planar stability analysis. A second purpose of the limit equilibrium analysis was to test the sensitivity of the slope problem to the strength parameters, including the joint cohesion, and joint friction angle. There is considerable uncertainty in the actual value of these parameters; therefore it is important to understand how a deviation from the true value affects the slope stability and modeling results. This analysis was based on the solution for planar failure described in Hoek and Bray (1991). The value of each parameter was systematically varied across a reasonable range and the effect on computed factor of safety observed. The sensitivity of each parameter relative to the other inputs was noted and helped to control the subsequent development of the more complex distinct element models. The comparison of the cohesion necessary for stability over a range of friction angles showed that the planar failure of joint set A on the local Afternoon Creek slope was the least stable, and the wedge failure on the average Afternoon Creek slope was the most stable of the feasible modes (Figure 5.5). The planar failure mode required more than twice the joint 68 cohesion of the most stable failure mode to achieve stability. Also it is interesting to note that the wedge failure mode is less stable at the local slope orientation at the interface of Zone B and Zone 3, than at the average slope orientation. This may explain why the failure occurred here as opposed to some other location along the slope. _ 8 E c o 5) o j= o O 2 3 Limit Equilibrium Analysis Factor of Safety = 1.0 l o c a l A f t e r n o o n C r e e k s l o p e : w e d g e f a i l u r e 5: L o c a l A f t e r n o o n C r e e k s l o p e : p l a n a r f a i l u r e a l o n g j o i n t s e t A A v e r a g e A f t e r n o o n " C r e e k s l o p e : w e d g e f a i l u r e 2 5 27 29 Friction Angle 31 33 3 5 - local (planar) - • - local (wedge) • a v e r a g e (wedge) Figure 5.5 Comparison of the joint strength properties necessary to achieve stability for the three failure modes. The results of the parametric study are summarized in a plot of Percent Change (%) versus Factor of Safety (Figure 5.6). The Percent Change is the relative difference between the minimum value of a parameter (0 percent) and the maximum value of a parameter (100 percent). The study shows that stability of the planar failure mechanism is most sensitive to changes in joint cohesion, and less sensitive to changes in joint friction angle. 69 Percent Change (%) vs. Factor of Safety Min imum Average Maximum Cohesion (t/m 2) 2.8 7.8 12.8 Friction angle (°) 25 30 35 0 10 20 30 40 50 60 70 80 90 100 Percent Change (%) -*- Cohesion — Friction Angle Figure 5.6 Summary of the parametric study - Percent Change versus Factor of Safety. 70 5.1.3 Two-Dimensional Numerical Model ing 5.1.3.1 2-D Representation The widely established 2-D distinct element numerical code, UDEC (Itasca 2000), was used for numerical modeling, as opposed to a continuum code, because it was clear from the field observations that most of the mass failed along clearly defined, pre-existing discontinuities. Numerical models were developed to constrain the mechanical properties and geometric configuration of the slope, and were used to investigate how changes in these parameters affected the failure mechanism and volume. One assumption of a 2-D analysis is that the slope of interest extends infinitely without change in the third dimension; generally the 2-D cross-section that is modeled is parallel to the direction of mass movement (usually in the direction of steepest slope). However the Afternoon Creek failure does not meet this key assumption. Firstly, the shape of the failed volume is different in the northern part of the failed zone compared to the southern part (Figure 2.7). In the southern half of the area, the failed volume is a distinct ridge, while in the northern half it is tabular-shaped. Secondly, it was hypothesized (see chapter 2), based on the pre-failure topography and wedge-shaped failure scarp, that the northern and southern failed masses moved in two different, oblique directions (Figure 5.7). If the southern and northern sections moved in different directions, then the total failure may have occurred in two stages, with each stage involving a wedge of material that was over 60 meters in thickness. It was hypothesized that initial collapse occurred in the southern portion of the failed zone. This material moved directly down slope into Afternoon Creek, possibly along the line of intersection of joint sets A and B. The event unloaded the toe, and provided lateral release for the northern section of the slope. The northern blocks slid on highly persistent plane A discontinuities, initially oblique to Afternoon Creek, forming the second phase of failure. 71 Figure 5.7 Two-stage failure hypothesis showing failure masses and movement directions. Therefore, in consideration of the different failure shapes, and possible bi-directional movement of the failed rock mass, two different cross-sections were concurrently modeled parallel to the presumed movement direction of the two failure stages (Figure 5.8): 1. Cross-section A - A ' : Approximately parallel to the dip direction of plane A. Hypothesized mass movement direction of the northern portion (Figure 5.9). 2. Cross-section B - B ' : Approximately parallel to the slope direction in Zone B. Hypothesized mass movement direction of the southern portion (Figure 5.10). Zone 3 100 m Figure 5.8 Map of Afternoon Creek (after the November, 2003 failure) showing location of cross-sections and source volume (contour units = meters). Figure 5.9 Cross-section A - A ' before and after the November, 2003 failure. The post-glacial topographic profile is inferred. 750 700 650 600 550 500 450 400 450 m Figure 5.10 Cross-section B-B' before and after the November, 2003 failure. The post-glacial topographic profile is inferred. 73 5.1.3.2 Modeling Methodology The modeling procedure was the same for cross-sections A - A ' and B - B ' (Figure 5.11). First, a baseline model was created that incorporated the information acquired during the data collection phase of the project. The input parameter values used in these models are shown in Table 5.1. The model was run to completion, and the simulated volume of displaced material and shape of the failure surface were compared to the actual event. The baseline models became the benchmarks to which all subsequent models were compared. Due to the large uncertainty associated with some of the input parameters, trial models were then designed that tested the effects of varying the input parameters. Each parameter was systematically varied across the range of reasonable values established during the data collection phase (Chapters 3 and 4). Each model was run to a steady-state for each variation of the parameter. The results of each trial run were compared to the actual failure surface, and the results of the baseline models. If the trial model closely reproduced the actual post-failure cross-section, the tested parameter value was considered reasonable. However i f the trial model did not reproduce the actual failure surface, then that parameter value was considered unreasonable. In this way, the range of reasonable values for each parameter was constrained. Additionally, this method provided insight to the relative influence of each parameter on the failure mechanism. >y in cross-section A \ Baseline Model Trial Models cross-section B best-guess ) Parametric studies-vary the input parameters 'nput parameters • I Z o n e B Irt-situ G e o m e t r i c M e c h a n i c a l e q u i v a l e n t s t ress p a r a m e t e r s ' p r o p e r t i e s c o n t i n u u m rat io /̂ T̂ ~~~. //''*^\ o r i e n t a t i o n ' p e r s i s t e n c e r o c k m a s s j o i n t f r i c t i o n s p a c i n g s t r e n g t h a n g l e Figure 5.11 Flow chart of modeling procedure The baseline model consisted of two material types: a massive orthogneiss unit, and a wedge of highly-fractured orthogneiss that represented Zone B. Zone B was separated from the rest of the rock mass by two, bounding shear zones. The slope model was divided into blocks using discontinuity sets. Three discontinuity regions were created, with different discontinuity sets in each region. One region is the upper half of the model, above the Base 74 Shear Zone. It corresponds to the Zone 3 structural domain. The failed volume of the November, 2003 events originated from this upper region. Two joint sets, representing joint sets A and B were defined in this zone. The second discontinuity region was the Zone B wedge. To approximate the highly fractured nature of this wedge, Zone B was divided into randomly-sized polygonal blocks, called Voronoi blocks by UDEC. A third discontinuity region was defined below the Base Shear Zone. One joint set was defined here that represents the average dip of joint set B, measured in the structural domain Zone 2 (Figure 5.12). The discrete blocks were made deformable by dividing the block into a finite difference mesh. To increase accuracy in the zone of interest, smaller mesh elements were assigned to areas where tensile yielding was expected to occur. Figure 5.12 Block outline for cross sections A - A ' and B - B ' . In-situ boundary stresses were assigned so that vertical, horizontal, and out-of-plane stresses were initially equal. To prevent yielding during initial loading, the discontinuity strength parameters (cohesion and friction angle) were set to artificially high values, and the elastic constitutive model was used to represent the rock mass. The model was run until equilibrium was reached to initialize the stresses in the rock mass (Figure 5.13). 75 Figure 5.13 Horizontal (Sxx) and vertical (Syy) stress contours for baseline models A and B. After stress initialization, all zones were changed to the 'Mohr-Coulomb' elasto- plastic constitutive model. The highly-fractured, Zone B wedge was assigned weaker rock mass and discontinuity strength parameters than the surrounding competent rock. A requirement of the distinct element method is to include fully persistent, interconnected discontinuities. In reality, joints are discontinuous and separated by bridges of intact rock. To account for this, Terzaghi's (1962) effective cohesion concept was used and the joint cohesion values in UDEC set accordingly. To simulate weathering and progressive failure of the rock slope with time, the effective cohesion was incrementally decreased in a series of 76 damage states (Table 5.1), following the example of Eberhardt et al. (2004). The model was run for 10,000 cycles at each damage state or until a steady-state was reached (Appendix E). The model response was monitored with plots of unbalanced forces, displacement, velocity, and plastic state of elements. For the constitutive model used, the plastic state of an individual element can be one of the following: 'elastic', 'at yield', 'yield in past', or 'tensile failure'. 'Yield in past' indicates that a particular element has reached the yield state in an earlier time step, but is in the elastic state under the current stress conditions, at the current time step. Table 5.1 Geometric and mechanical parameters for baseline UDEC models. Geometric Parameters Parameter Joint set A Joint set B Tower shear Base shear Dip 50° 61° -85° E -20° W Spacing (m) 10 25 na na Mechanical Properties Parameter Massive orthogneiss (Zone 3) Highly fractured (Zone B) Young's modulus (GPa) 24 2 Poisson's ratio 0.2 0.2 Density (kg/m ) 2650 2650 Rock mass cohesion (MPa) 2.0 0.7 Rock mass friction (deg) 60 46 Rock mass tensile strength (KPa) 220 44 Joint normal stiffness (GPa/m) 10 10 Joint shear stiffness (GPa/m) 5 5 Initial Damage Damage Damage Damage state state 1 state 2 state 3 state 4 Joint cohesion (KPa) 1000 500 100 10 0 ** Joint friction (deg) 33 33 33 33 33 ** Joint friction in Zone 2 & 3 only. Joint friction in Zone B was 26°. 77 5.1.3.3 Baseline Model Results Baseline model A - A ' bisects the failure volume in the northern section of the slope. This material may have failed due to loss of lateral and toe support immediately following failure of the southern slope. The direction of cross-section A - A ' is parallel to the slope of the failure scarp, which is approximately parallel to the average dip of joint set A . In the lower section of the failed volume, the cross section bisects the hinge of a large 3-D wedge in the slope topography (Figure 5.7, 5.8). Baseline model B - B ' is parallel to the presumed movement direction of the southern portion of the failed rock mass. The failure volume in this section of the slope formed a distinct 2-D ridge that rose more than 60 meters above the average slope (Figure 5.10). This volume may have been part of a first stage of failure at Afternoon Creek. The material properties used for each baseline model are listed in Table 5.1. Displacement vectors, joint shear indicators, and contoured plots of horizontal displacement show the boundaries of the failure mass in baseline models A - A ' (Figure 5.14) and model B - B ' (Figure 5.15). The failure mode derived was planar sliding on the joint set A and B surfaces, which is consistent with the interpretation of the actual November, 2003 event. The sliding surface was bi-linear; the upper portion of the mass slid on a joint set B surface, while the toe of the mass slid on a shallower joint set A surface. Plastic state indicators showed that many of the rock mass elements above the failure plane failed in tension in both baseline models (Figure 5.16; Figure 5.17); shear failure was insignificant. In all damage states, total displacement and horizontal displacement were greatest at the toe of the failure mass and decreased towards the top of the failure mass. The additional displacement at the toe was the culmination of tensile failure and subsequent deformation of the rock mass above. In both baseline models, the failure surface did not develop through the Zone B material. Instead, the Zone B wedge buttressed the lower portion of the steep slope, forcing the slide surface to daylight above. Although Zone B is composed of a weaker rock mass and discontinuities, it was able to buttress the slope because the critical joint planes (joint set A and B) did not persist through the zone. Zone B did not fail in shear under the modeled stress conditions. In baseline model A - A ' , the failure surface did not fully develop until damage state 2, 78 when the effective cohesion was reduced to 100 KPa. Failure continued on the same planar surfaces after the joint effective cohesion was reduced in damage state 3 and 4. In baseline model B - B ' , the entire protruding ridge was removed by the modeled slope failure, smoothing this part of the slope to the average topographic gradient. The failure volume did not fully develop until damage state 3, when the effective cohesion was reduced to 10 KPa, although extensive tensile failure of the volume began at damage state 2. Planar sliding began in damage state 3 and continued after the joint effective cohesion was reduced in damage state 4. A concentration of tensile failure indicators did develop at the top of the slope, above the main failure volume, in damage state 3 (noted in Figure 5.17), although this material did not noticeably displace. This agrees with field observations of isolated columns, separated from the main rock mass by highly persistent tension cracks (Figure 2.3), which are evidence of high tensile stresses that acted on this region following the actual November, 2003 failure event. The failure surface in both baseline models adequately reproduced the actual November, 2003 sliding surface. Additionally the volume of material displaced and mechanism of failure in the models closely matched the interpretation of the actual event. The bi-linear shape of the modeled sliding surface is similar to the steep head, and shallower toe of the actual slide surface. Two notable differences between the baseline models and the actual sliding surface were apparent. Firstly, the baseline models showed the sliding surfaces extending to the base of the over-steepened slope section, near the top of Zone B. In the actual event, the toe of the sliding surface daylighted approximately 50 meters above Zone B, half-way up the over- steepened slope. This discrepancy may be caused by the required assumption of fully- persistent joints in the U D E C model. Since the steeper joint set was fully-persistent in the model, the steep failure plane at the top of model was connected to shallow joints at the base of the over-steepened slope. In reality, the location of the sliding plane is controlled by the connectivity of natural, discontinuous joints. The steep and shallow joint planes may only have connected in one spot, 50 meters above Zone B, half-way up the over-steepened section of slope. Additionally, the discontinuities higher in the slope may be weaker due to increased susceptibility to weathering. Secondly, the failure surfaces produced by the baseline models is limited to two planes - a steep rear release surface and a shallower toe 79 surface - while the actual sliding surface is stepped, connecting four or five planar surfaces. This discrepancy is, once again, probably due to the assumption of fully persistent joints in UDEC model. In reality, the discontinuities are not fully persistent, and therefore structurally controlled failures are limited to joint planes that connect (i.e., step-pathed). 80 Figure 5.14 Baseline model A - A ' results, damage states 2 and 4 relative to November, 2003 slide surface; horizontal displacement contours (top); total displacement vectors and joint shear indicators (red) (bottom). 81 Figure 5.15 Baseline model B - B ' results, damage states 3 and 4 relative to November, 2003 slide surface; horizontal displacement contours (top); total displacement vectors and joint shear indicators (red) (bottom). 82 JOB TITLE : Damage State 2 (jcoh=1e5) UDEC (Version 4.00) L E G E N D 31-Mar-06 10:41 cycle 43400 no. zones : total 15359 at yield surface (*) 0 yielded in past (X) 1708 tensile failure (o) 474 block plot Alex Strouth University of British Columbia yielded in past (X) tensile failure (o) I r- I -3.000 -1 000 1.000 C10«2) Damage State 2 Jcohesion = 1 OOKPa outline of slide surface Damage State 4 Jcohesion = OKPa Figure 5.16 Baseline model A - A ' results, damage state 2 and 4 relative to the November, 2003 slide surface; plasticity indicators. 8 3 JOB TITLE: Damage State 3 (jcoh=1e4) UDEC (Version 4.00) LEGEND 31-Mar-06 10:23 cycle 62940 no. zones : total 12468 at yield surface (*) 0 yielded in past (X) 562 tensile failure (o) 301 block plot Alex Strouth University of British Columbia T 1 r 0 0 0 0 1 . 0 0 0 2 . 0 0 0 C 1 0 » 2 ) i r 3.000 4.000 Damage State 2 Jcohesion = 100KPa outline of slide surface Damage State 4 Jcohesion = OKPa ine of slide surface Figure 5.17 Baseline model B - B ' results, damage state 2 and 4 relative to the November, 2003 slide surface; plasticity indicators. 84 5.1.3.4 Trial Model Results Only a few of the input parameters, including the topographic profile and location of the shear zones at the surface, were known with a high level of confidence. The value of many of the other parameters could be limited to a reasonable range; however the true value remained uncertain. Therefore test models were run to reduce some of this uncertainty and to investigate the effects that parameter changes had on the failure mechanism and volume. Multiple changes were often made in a single trial model. For example, the rock mass cohesion, friction angle, and tensile strength were all lowered to a minimum value simultaneously to determine the effects of a weaker rock mass. Parameter changes that caused significant changes in the model results are discussed in the following sections. Zone B as an equivalent continuum Zone B was divided into Voronoi polygons in the baseline models to simulate the highly fractured nature of the material. An alternative way to represent the weaker Zone B rock mass is as an equivalent continuum. This was tested in the first trial model. When the mechanical properties shown in Table 5.2 below were used, the trial model results were basically equivalent to the baseline model results. Zone B acted as a buttress to the lower portion of the slope and was not otherwise involved in the failure (Figure 5.18). Limited shear and tensile yielding occurred in Zone B. Representing Zone B as an equivalent continuum significantly reduced the model runtime; therefore it was represented this way for the remainder of the trial models. Table 5.2 Mechanical parameters of Zone B modeled as an equivalent continuum Zone B - Equivalent continuum Parameter Initial value Young's modulus (GPa) 2 Poisson's ratio 0.2 Density (kg/m3) 2650 Rock mass cohesion (MPa) 0.5 Rock mass friction (deg) 40 Rock mass tensile strength (KPa) 35 85 Cross-section A-A' Cross-section A-A' Cross-section B-B' Cross-section B-B' Figure 5.18 Zone B modeled as an equivalent continuum, damage state 4 showing horizontal displacement contours and plasticity indicators; cross-section A - A ' (top); cross-section B - B ' (bottom). 8 6 In-situ stress ratio Stresses were initialized in the baseline models assuming a horizontal to vertical stress ratio of 1.0 (i.e., K = oH/av =1.0). Eberhardt et al. (2004) and Stead et al. (1995) demonstrated that the K ratio is an important input parameter for numerical modeling of rock slopes. Therefore trial models were run with K=0.5 and K=2.0 to test the influence of the in - situ stress regime on the failure mechanism at Afternoon Creek. In these trial models, the failure mechanism and volume remained the same as in the baseline models. The reason that the failure mechanism and volume is insensitive to changes in the horizontal in-situ stress may be because of the ridge morphology of the failure mass, and its elevation above the valley floor. Mechanical properties The strength of the Zone 3 rock mass and discontinuities was varied to determine how changes in these parameters affected the failure mechanism and volume. Each parameter was varied across the reasonable range of values (Table 5.3). One set of trial models tested the rock mass strength parameters. A second set of trial models tested the joint friction angle. Table 5.3 Range of reasonable mechanical parameters for zone Mechanical Properties (Zone 3) Parameter Initial R a n o e tested Effect on model D id not change volume or mechanism D id not change volume or mechanism Limits tensile failure of elements Joint friction (deg) 33 28 - 40 Controls sliding on shallow planes Rock mass cohesion (MPa) Rock mass friction (deg) Rock mass tensile strength (KPa) 2.0 1 .3 -2 .8 60 55 - 64 220 2 2 0 - 1000 The rock mass cohesion, friction angle, and tensile strength were varied across the reasonable range of values (Table 5.3). Changes to rock mass cohesion and friction angle did not affect the failure mechanism or failure volume. This was expected since the failure mechanism in the initial model involved only tensile yielding and sliding on discontinuities. Variations in the rock mass tensile strength significantly affected the trial model results. In the baseline model (tensile strength = 220 KPa) , nearly every element in the 87 failure mass yielded in tension. When the tensile strength was increased to 500 KPa, corresponding to a gneissic rock mass with GSI-75 (Rocscience 2002), elements near the surface and at the head of the failure volume failed in tension. When the tensile strength was increased to 1 MPa, corresponding to a gneissic rock mass with GSI-85 (Rocscience 2002), no elements were at the tensile yield state at the completion of damage state 4 (Figure 5.19). The increased tensile strength also limited the rate of deformation of the failure volume. Figure 5.19 Comparison of tensile yielding for 3 different rock mass tensile strength values. Damage state 4, cross-section B-B' models. The effective joint friction angle was varied from 28° to 40° in several of the trial models. This parameter incorporates the influences of pore-water pressures and surface roughness in addition to the basic friction angle for gneissic material. Pore-water pressures decrease the value, while surface roughness increases the bulk value. The magnitude of pore pressures at the time of failure is entirely unknown, and it was not possible to accurately estimate the surface roughness because the failure scarp was inaccessible. Therefore the joint friction angle was poorly constrained prior to modeling. The joint friction angle was set to 33° in the baseline analysis. This value is several degrees above the basic friction angle for gneiss according to Barton (Barton 1976). At the baseline value, and when the joint friction angle was increased (between 33° and 40°) in trial models, minor sliding occurred on joint set A surfaces, but was primarily limited to the steeper joint set B discontinuity surfaces. These results are contrary to November, 2003 sliding surfaces, seen in the cross-sections, which are composed primarily of shallow dipping surfaces similar to joint set A. Sliding on the shallower joint set A surfaces was encouraged when the joint friction angle was reduced to 28° (Figure 5.21). The results of these trial 88 models (i.e., joint friction angle = 28°) more closely matched the November, 2003 sliding surfaces. Geometric parameters The following trial models tested how changes in the geometric parameters affected the failure mechanism and volume. The value of each parameter in the trial models remained within the reasonable range determined during the data collection phase of the project (Table 5.4): 1. Reduce spacing of both j oint sets 2. Joint set B variable orientation and persistence 3. Steeper joint set B combined with shallower joint set A 4. Single joint set Table 5.4 Range of geometric parameters used in the trial models Geometric Parameters Parameter Initial value Reasonable range Joint Dip O 50 40-55 set A Spacing (m) 10 7 - 2 0 Joint Dip(°) 61 55-70 setB Spacing (m) 25 15 -50 Tower Shear Dip(°) -85° E 70° E - 90° Base Shear Dip (°) ~20W 0° - 40° W 1. Reduce spacing of both joint sets: The spacing of joint set A was reduced to 7 meters and the spacing of joint set B was reduced to 15 meters. These correspond to the spacings measured in the L i D A R derived point clouds. The purpose was to determine i f the size of the failure volume would change i f smaller blocks were used to represent the slope. These smaller spacings were not used in the baseline models because using a greater number o f smaller blocks significantly increased the model runtime. Two important differences were noted when smaller blocks were used: (1) the sliding surface was shallower (i.e., failure volume decreased), and (2) sliding occurred almost entirely on joint set B (Figure 5.20). These results can, once again, be explained by the 89 assumption of fully persistent joint set B discontinuities. Sliding occurred on the deepest joint set B discontinuity that daylighted at the top and bottom of the slope. In the baseline model, due to the wider set spacing, and in the actual slope, due to limited joint persistence, the sliding surface was forced to step through interconnected shallowly (joint set A) and steeply (joint set B) dipping discontinuities. Incorporation of the shallower dipping joint segments resulted in the sliding surface stepping deeper into the slope; however when the spacing of joint set B is decreased (as in the trial models), the sliding surface did not incorporate the shallower dipping joint set. The sliding surface in the trial models was much shallower than the November, 2003 sliding surface in cross-section B - B ' , but very similar in the lower portion of cross-section A- Figure 5.20 Trial models of reduced joint set spacing, damage state 4, showing horizontal displacement. 2. Joint set B variable orientation and persistence: Joint set B was assigned a limited persistence and variable orientation to more closely replicate the natural slope. It was necessary for joint set A to be fully persistent so that discrete blocks were formed and connected to the surface. The orientation of joint set B was 61° with a standard deviation of 8°. The persistence was varied from 65 meters to 150 meters with a standard deviation of 20 meters. The joint set spacings were kept at the reduced values (i.e., joint set A = 7 m, joint set B = 15 m) to increase the number of intersecting discontinuities. 90 The main sliding surface in these trial models was stepped, similar to the actual November, 2003 sliding surface. The cross-section A - A ' trial model failure volume was similar to the actual failed volume; however the sliding surface was vertically offset by approximately 20 meters. Additionally, although the model sliding surface was stepped similar to the actual surface, most of the movement occurred along joint set B contrary to the actual event. The cross-section B - B ' trial model did not match the actual sliding surface well when the joint friction angle was kept at 33°; however when the joint friction angle was reduced to 28°, movement occurred more readily on joint set A discontinuities and the sliding surfaces match more closely (Figure 5.21). These results confirm that the assumption of fully persistent joints is one factor that causes discrepancies between the actual sliding surface and the modeled sliding surface. 91 Cross-section A Cross-section B Cross-section B Figure 5.21 Trial models with variable joint set B persistence and orientation, damage state 4 showing horizontal displacement. 92 3. Steeper joint set B combined with shallower joint set A: This trial tested the end limits of the range of orientations measured during the data collection. The dip of joint set A was decreased to its lower limit (40°) and the dip of joint set B was increased to its upper limit (70°). The joint set spacing and persistence were equivalent to the parameters used in the baseline model. The joint friction angle was reduced to 28° to encourage movement on joint set A. The sliding surface in these trial models did not reproduce the shape or location of the actual sliding surface (Figure 5.22). Therefore this is probably not the geometric configuration of the actual Afternoon Creek slope. Cross-section A Cross-section B Figure 5.22 Trial models with joint set B dip = 70°, joint set A dip = 40°, damage state 4 showing horizontal displacement. Joint friction angle = 28°. 4. Single joint set: Only one joint set, that represented both joint set A and B was used in these trial models. The set was inclined at 55° and spaced 10 meters. The joint set inclination corresponded to the upper limit for joint set A and the lower limit for joint set B. It tested the opposite end limits of geometric trial model 3, above. A stepped failure surface developed in these models that is similar to the actual sliding surface (Figure 5.23). Sliding on the single joint set was made kinematically possible 93 due to tensile yielding of elements at the head and rear of the failure volume. This concentration of tensile stresses may have caused existing joint set B fractures to rapidly propagate and connect the joint set A sliding surfaces, forming the stepped surface that exists in the actual November, 2003 failure scarp. Cross-section A Cross-section B Figure 5.23 Trial models with one joint set, dip = 50°, damage state 4, showing horizontal displacement and plasticity indicators. 94 5.1.4 Three-Dimensional Numerical Model ing Several issues related to the slope failure process were left unresolved by the 2-D analysis, including: (1) the interaction of the northern and southern failure volumes; (2) the number of failure stages; and (3) the movement direction of the northern and southern failure volumes. To examine these further, the three-dimensional (3-D) distinct element numerical code, 3DEC (Itasca 2003), was used. The results of the 3-D analysis were compared with the 2-D UDEC results. The comparison aided the interpretation of the results, and was used to evaluate the usefulness of 3-D numerical modeling for this situation. The benefits, in terms of increased understanding, were compared to the costs of 3-D modeling, in terms of computing time and effort. 5.1.4.1 Modeling Methodology A single model was created that followed the methodology and design of the 2-D UDEC models. The model was centered on the November, 2003 source zone. The 3-D model was generated from the pre-failure digital elevation model using the Itasca pre- processor program, P G E N (Itasca 2003). PGEN is able to create complex topography from a series of user-defined planar sections. In this case, the planar sections were the topographic contours (50 meter spacing) of the pre-failure slope. P G E N created polyhedra between the topographic levels. The polyhedra were joined to form the 3-D block model. Although the interfaces between polyhedra, called construction joints, can not be hidden from view, the joined polyhedra behave as a single continuous block (Figure 5.24). 3DEC requires that all polyhedra are entirely convex. Concave shapes are constructed by joining two or more convex polyhedra. Numerous polyhedra were required to recreate the complex, undulating Afternoon Creek slope. The balance between accuracy (in terms of reproducing the true topography) and model complexity/feasibility was a primary consideration during model generation. An accurate representation of the topography was required to investigate the topographic controls on slope failure and deformation; however irregularities in the topography required numerous, small, and wedge-shaped polyhedra. These small polyhedra were problematic because they increased model run-time and limited joint set cuts and finite- difference mesh generation. 95 The slope model was divided into three regions by the three major shear zones observed in the field. Material between the Tower Shear Zone and Base Shear Zone represented the Zone B wedge of highly-fractured orthogneiss. Material between the Tower Shear Zone and the North Shear Zone represented the massive, blocky Zone 3 orthogneiss. Material outside of the shear zones, called the 'surrounding region', was not involved in the November, 2003 failure, however was necessary to reduce boundary effects on the regions of interest. Zone 3 was divided into blocks using two joint sets - joint set A and B. To reduce model runtime and decrease complexity, the surrounding region and Zone B were not cut by discontinuities; instead both were modeled as equivalent continuums, with Zone B being significantly weaker than the surrounding region. The slope model was made deformable by dividing the discrete blocks into a finite difference mesh. To increase accuracy in the zone of interest, smaller mesh elements were assigned to Zone B and Zone 3, where plastic yielding was expected to occur. Figure 5.24 The 3-D block model of Afternoon Creek, colored by region. 9 6 In-situ stresses were assigned so that the vertical and two horizontal principal stresses were initially equal. To prevent yielding during initial loading, the elastic constitutive model was used to represent the rock mass and discontinuities. Gravity was turned on, and the artificial model boundaries were given a zero displacement boundary condition in the direction normal to the boundary. The model was run until equilibrium was reached to initialize the stress state in the rock mass. After stress initialization, the Zone B and Zone 3 regions, and discontinuities were changed to the 'Mohr-Coulomb' elasto-plastic constitutive model. Material properties for the rock mass and discontinuities came directly from the UDEC analysis and parametric study (Table 5.5). The highly-fractured Zone B wedge was assigned weaker strength parameters than the surrounding rock mass. As in UDEC, fully persistent, interconnected discontinuities were required in the 3DEC model. In reality joints are discontinuous and separated by intact rock bridges. Joint cohesion was applied to simulate the shearing resistance caused by the rock bridges. The progressive failure of the rock bridges which occurs in nature through weathering and loading was simulated by incrementally decreasing the joint cohesion in a series of damage states. The model was run until a steady state was reached for each damage state (Appendix E). The model response was evaluated using 3-D and cross-section plots. The model could be displayed as a solid block or wire-frame mesh. Displacement and velocity vectors, and plastic yield indicators could be viewed in the wire frame mode. Filled contours of displacement, velocity, and stresses could be viewed in the solid block mode. The location and orientation of cross-sections are unrestricted by 3DEC; therefore they were the most effective way to view the results. Overall though, the post-processing was cumbersome due to limited plotting options and numerous program errors. 97 Table 5.5 Geometric and mechanical parameters for the 3DEC model. Geometric Parameters Parameter Joint set A Joint set B Tower shear Base shear Dip 51° 71° 85° SE 15°NW Dip Direction 116° 57° 108° 250° Spacing (m) 10 25 na na Mechanical Properties Parameter Massive orthogneiss Highly fractured (Zone B) Young's modulus (GPa) 24 2 Poisson's ratio 0.2 0.2 Density (kg/m3) 2650 2650 Rock mass cohesion (MPa) 2.0 0.5 Rock mass friction (deg) 60 40 Rock mass tensile strength (KPa) 220 35 Joint normal stiffness (GPa/m) 10 10 Joint shear stiffness (GPa/m) 5 5 Initial Damage Damage Damage Damage state state 1 state 2 state 3 state 4 Joint cohesion .. A A A 500 100 10 0 (KPa)** Joint friction (deg) 33 33 33 33 33 ** Joint friction and cohesion of joint set A and B only. Shear zone friction was held constant at 26°, cohesion was set to 0 KPa. 5.1.4.2 3-D Model Results The failure mode, extent, and volume in the 3DEC model were similar to the 2-D UDEC model results and the prior interpretation of the actual November, 2003 event. The failure mechanism of the 3DEC model was sliding on discontinuity surfaces that daylight above Zone B in a locally concave section of the slope. It is hypothesized that this concavity formed gradually since deglaciation of the valley due to weathering and raveling of the highly-fractured Zone B wedge. The failure surface was stepped, incorporating both joint set A and B surfaces. Sliding occurred primarily in the dip direction of joint set A ; joint set B provided lateral release for the sliding blocks (Figure 5.25). In all damage states, total displacement was greatest at the toe of the failure mass and decreased towards the top of the failure mass. 98 3-D v i e w o f t h e fa i led mass f r o m ' i n s i de ' t he s l o p e Figure 5.25 3-D model results, (a) 3-D view of the failed mass from 'inside' the slope. 9 9 Total displacement 3DEC (Version 3.00) Cross section ptot: 6-May-06 23:44 dip= 90.00 above dd = 130.00 center 1.802E+02 6.35OE+02 4.095E+02 cut-pl. 1.2O0E+02 mag = 16 00 cycle 25000 displacement contours interval = 4 .000E+00 from to m 2.300E+01 1.900E+01 m I.900E+01 1.500E+01 Mi 1.500E+01 1.100E+01 m 1.100E+01 7.0O0E+00 • i 7.000E+00 3.000E+00 m 3 .000E+00 -1 .000E+00 University of British Columbia Plasticity Indicators ( c ) £~ J elev. 600 m l i e * 500 m 3DEC (Version 3.00) Cross section plot: 6-May-06 23:44 dip= 90.00 above dd = 130.00 center 1.802E+02 6.350E*02 4.095E+02 cut-pl. 1.2O0E*02 mag= 16.00 cycle 25000 Failure indicators • matrix shear matrix tension University of British Columbia Figure 5.25 continued 3-D model results, (b) Cross-section of failed volume showing f i l led contours of total displacement. Primary direction of sliding is out of the page, (c) Indicators o f plastic yielding o f the rock mass, showing the approximate location o f the November, 2003 sliding surface. 100 Tensile yielding occurred extensively in the rock mass above the sliding planes; therefore, in this case, the plastic yielding indicators clearly show the extent of the failure volume at the surface (Figure 5.26) and in cross-section (Figure 5.25; 5.27). The failure volume extended longitudinally from the top of Zone B to the North Shear Zone, and laterally from the ridge crest to the northern extent of the concave slope section. The extent of the failure volume in the 3DEC model closely matched the extent of the unstable zone in the actual slope. This indicates that the location of failure in the November, 2003 event was strongly controlled by the slope topography and the two identified joint sets. As in the U D E C models, the 3 D E C model adequately reproduced the depth, and shape of the November, 2003 sliding surface (seen in cross section); however with the same notable differences. The primary difference is that the sliding surface in the 3 D E C model is deeper than the actual sliding surface. The model showed the sliding surface extending to the base of the over-steepened slope section, near the top of Zone B. In the actual event, the toe of the sliding surface daylighted approximately 50 meters above Zone B, half-way up the over- steepened slope. Plasticity Indicators Figure 5.26 3-D model plot colored by plastic yielding state of surface zones. 101 102 Z o n e B a c t e d as a bu t t r e s s at the toe o f the s l o p e , b e c a u s e d i s c o n t i n u i t i e s d i d not pe r s i s t t h r o u g h the z o n e . T h e t h i n , u p p e r e d g e o f Z o n e B w a s d i s p l a c e d b y the f o r c e o f the s l i d i n g b l o c k s a b o v e . T h e d i s p l a c e m e n t w a s a c c o m m o d a t e d b y s h e a r a n d t e n s i l e d e f o r m a t i o n o f the Z o n e B f i n i t e - e l e m e n t m e s h . T h e r e g i o n o f d e f o r m a t i o n w a s r e s t r i c t e d to the top c o r n e r a n d s u r f a c e o f Z o n e B ( F i g u r e 5.28) . F i g u r e 5 .28 i s a s e c t i o n a c r o s s o n e o f the m o s t e x t e n s i v e d e f o r m a t i o n r e g i o n s i n Z o n e B ; o the r c r o s s - s e c t i o n s i n Z o n e B s h o w e d c o n s i d e r a b l y l ess y i e l d i n g . P l a s t i c y i e l d i n d i c a t o r s Figure 5.28 Cross-section showing plastic yielding in Zone B. T h e r e d i d n o t a p p e a r to b e a c a u s e a n d e f f e c t r e l a t i o n s h i p b e t w e e n the n o r t h e r n a n d s o u t h e r n f a i l u r e v o l u m e s i n the 3 D E C m o d e l . B e f o r e the 3 D E C a n a l y s i s b e g a n , a two-s tage f a i l u r e s c e n a r i o w a s h y p o t h e s i z e d that p r o p o s e d i n i t i a l c o l l a p s e o f the s o u t h e r n h a l f o f the f a i l u r e z o n e . T h i s e v e n t w a s b e l i e v e d to h a v e u n l o a d e d the toe a n d p r o v i d e d l a t e r a l re lease f o r a s e c o n d f a i l u r e s t age i n v o l v i n g the n o r t h e r n h a l f o f the f a i l u r e v o l u m e . T h i s m e c h a n i s m , a n d the o r i e n t a t i o n o f t he c u r r e n t s l o p e , s u g g e s t e d that the s l i d i n g d i r e c t i o n s o f the t w o f a i l u r e s tages w e r e o b l i q u e . H o w e v e r the 3 D E C r e su l t s d i d no t s u p p o r t t h i s h y p o t h e s i s . In the s i m p l i f i e d 3 D E C m o d e l , a l l u n s t a b l e p o r t i o n s o f the s l o p e d i s p l a c e d i n the s a m e d i r e c t i o n a n d 103 began moving at the same damage state. The direction of sliding for all parts of the failure volume (i.e., northern and southern section) was 105° to 115°, approximately coincident with the dip direction of joint set A . There was no indication of bi-directional or oblique movement of the failure volume. Although parts of the rock mass on the Falls Creek side of the ridge were involved in the failure, no blocks in the 3DEC model displaced towards Falls Creek or entered the Falls Creek travel path. Displacement of the entire failure volume (i.e., both sections) began in damage state 1, when the joint cohesion was reduced to 500 KPa, and fully developed in damage state 2, when the joint cohesion was reduced to 100 KPa. The velocity and total displacement of the northern section of the failure volume were greater than those of the southern portion in all damage states. This indicates, contrary to the original hypothesis, that the northern failure section was the least stable portion of the slope. The northern half of the failure volume 'pushed' the southern half to failure in a single stage failure event. Prior to the November, 2003 failure, the slope of the northern half of the failure volume was at the less-stable, concave orientation, while the slope of the southern half was at the more-stable, average orientation. 104 5.2 Runout Analysis along the Afternoon Creek Travel Path The runout analysis was a quantitative assessment of the travel path and distance of the November, 2003 rock avalanche in Afternoon Creek. The mobile rock avalanche was one of two hazards that threatened the SR 20 highway. Although the leading edge of the rock avalanche flowed more than 500 meters in horizontal distance from the source area, it did not reach the highway in 2003. The runout back-analysis was important because the information gained was used to anticipate the effects of future rock avalanches in Afternoon Creek. The numerical dynamic/rheological flow model DAN3D (McDougall and Hungr 2004) was the primary tool used for the analysis. The 3-D model was used (instead of a 2-D model) because it is able to simulate complex, multi-directional movement. The objectives of the back-analysis of the November, 2003 runout were the following: 1. Determine a rheology that approximates the behavior of the flowing mass 2. Obtain reasonable estimates of relevant material parameters (depending on the rheology) a. Basal bulk friction angle b. Internal bulk friction angle 5.2.1 Runout Ana lys i s Methodology The information required for the DAN3D analysis was a digital elevation model (DEM) of the runout path, the volume and location of the initial sliding mass, an appropriate rheological model, material properties of the runout path, and the material properties of the runout material. The runout path and source volume were derived by comparing the topography of Afternoon Creek before and after the event. The material properties were initially estimated, then calibrated by trial-and-error during the analysis. The source zone and rock avalanche deposit were identified and delineated on aerial photographs and during field mapping. The program Surfer 8.0 was used to quantify the thickness, area, and volume of the source zone and deposit by comparing the DEMs from before and after the rock avalanche. The before D E M was subtracted from the after D E M to create a difference map (Figure 5.29). Areas with decreased mass were part of the source zone, while areas with increased mass were part of the deposit. Several zones of increased or decreased volume appear in the difference map that were not involved in the rock avalanche. 105 These zones are evidence of the uncertainty associated with estimation of the source and deposit volume. The uncertainty was caused by a combination of errors in each D E M . Although the uncertainty was not quantified, it appears to be small enough that it does not limit the usefulness of the results. The source area and deposit area were outlined and the volume of each calculated with Surfer 8.0. The volume of the source area was approximately 641,000 m 3 , while the volume of the deposit was approximately 868,000 m 3 , corresponding to a volume increase of 135%. The volume increase, called bulking, was caused by fragmentation of the source volume (Figure 5.30). The necessary input files, including a D E M of the source and a D E M of the runout path, were prepared with Surfer 8.0. The source volume was created by outlining the source area on the difference map and multiplying by the 135% bulking factor. The source volume (before the bulking factor was applied) was subtracted from the before D E M to create the runout path topography. 106 Figure 5.29 Difference map overlying the before D E M . Increased mass shown in blue, decreased mass shown in red. Contour interval = 20 m. A dry-factional rheology was assumed for this analysis based on the previous experience of Scott McDougall (personal communication), and Hungr and Evans (1996). As such, the only required material parameters for the rheological model were the internal friction angle and basal friction angle. These parameters, once calibrated, are considered apparent, rather than actual, material parameters (McDougall and Hungr 2004). The internal bulk friction angle, used to derive the tangential stress coefficients (Hungr 1995), can be related to the angle of repose, which was measured in the field. The basal bulk friction angle is the average friction angle along the entire avalanche path at the interface between the path and the moving mass. This value depends on the hard-to-quantify effects of several parameters including composition of the path and moving mass, and the velocity of the moving mass. The value was initially estimated and then constrained by an iterative trial- and-error method suggested by McDougall and Hungr (2004). Each friction angle value was systematically varied in a series of model runs, until the results of the model most closely reproduced the runout distance, shape of the deposit, distribution of mass, and path extent of the actual avalanche. A hypothesis suggested during the slope stability analysis is that failure occurred in two sequential stages. To test this hypothesis, in a second series of models, the source volume was divided into two failure stages and the runout of each stage modeled sequentially. The total failed volume of the two-stage analysis was equivalent to the total failed volume of the single-stage analysis. The first failure stage was allowed to runout until the entire mass came to rest. Then the second stage was allowed to runout over the deposit of the first failure stage material. The results of the two-stage analysis were compared to the single-stage analysis. 5.2.2 Runout A n a l y s i s Results Aerial photographs that pre-date the events show that the Afternoon Creek valley was a bedrock channel filled with boulders before the avalanche. The rock avalanche debris is primarily composed of blocky orthogneiss boulders ranging in size from <1 meter to 20 meters in diameter. There was no silt or clay and negligible sand-sized particles observed in the deposit. Cobble-sized material dominates near the Afternoon Creek valley wall 107 immediately below the highly-fractured structural Zone B. The average angle of repose of the deposit is approximately 37°. This was the initial estimate for both the basal and internal friction angles. Single-stage analysis In the single-stage analysis, the entire source volume began post-failure motion at the same time. After several calibration runs, the frictional strength parameters that most closely reproduced the physical characteristics of the actual runout event (Figure 5.31) were the following: Basal Bulk Friction Angle: 37° Internal Bulk Friction Angle: 40° 108 0 100 200 300 400 500 600 Figure 5.31 Calibrated DAN3D analysis results. Single-stage analysis results. The results of the analysis resembled the actual event in a number of ways: the shape of the deposits and reach of the leading edge of the debris are similar, and the model trimline closely matched the actual trimline. In addition, the bulk of the simulated and actual avalanche debris were deposited in the narrow section of Afternoon Creek. However, there are also several differences between the calibrated model and the actual event that should be noted, including the following: (1) the center of mass traveled too far; (2) too much spreading after the debris exited the narrow Afternoon Creek canyon; and (3) run-up on the opposite valley wall is too high. One reason for these differences is the assumption (required by DAN3D) that the entire source volume fragmented immediately when the model simulation 109 was started. Therefore the internal pressures of the source volume were much larger in the model then they would be in reality. These high pressures increase the momentum of the entire flowing mass causing the excessive run-up, excessive spreading, and excessive runout of the center of mass. A second explanation for these differences is that a single average value was used for both the basal and internal friction angles along the entire path because it is not possible to vary the friction angle along the path in the current version of DAN3D. In reality, the parameters vary along the runout path. For example, the basal friction angle at the top of the runout path, in the failure zone, may have equaled the joint friction angle. The basal friction angle may have increased when the debris impacted, and traveled over the boulder-filled Afternoon Creek canyon, and then dramatically decreased when parts of the debris reached the finer-grained Afternoon Creek fan, at the end of the runout path. Small differences in the shape of the trimlines can be attributed to irregularities in the runout path and the bouldery composition of the runout debris. Individual features in the debris were large enough compared to the depth and total volume to influence the direction and behavior of the flow. One such 20-meter diameter boulder caused the notch in the trimline on the distal end of the actual rock avalanche deposit. . The effects of these individual, localized features can not be simulated by the runout model. Two-stage analysis Differences between the simulated and actual runout may have been caused by the failure initiation sequence assumed in the single-stage analysis. In the single-stage analysis it was assumed that the entire source volume began post-failure motion simultaneously, as indicated by the failure initiation back-analysis; however to test the two-stage failure hypothesis, the analysis was repeated using equivalent input parameters and dividing the source volume in half. Runout of the second event began after the first half came to rest in the Afternoon Creek valley. When the following input parameters, equivalent to the single- stage analysis, were used, the results of. the simulation more closely resembled the actual event (Figure 5.32): Basal Bulk Friction Angle: 37° Internal Bulk Friction Angle: 40° 110 I 1 1 1 1 1 I - 0 100 200 300 400 500 600 Figure 5.32 Calibrated DAN3D analysis results. Two-stage analysis. There are many aspects of the two-phase model that match the actual event very well. In general, the differences between the calibrated model and actual event (i.e., excessive spreading, run-up, center of mass travel distance) still exist in the two-phase simulation, but to a lesser degree. The center of mass of the model debris traveled slightly farther than the actual November, 2003 avalanche, and the debris ran-up the opposite valley wall farther in the model than in the actual event. The two-stage model significantly reduced the excessive spreading that occurred in the single-stage model when the debris exited the narrow confines of the Afternoon Creek canyon. I l l 5.3 Falls Creek Travel Path Although the volume of rockslide debris from the November, 2003 event that traveled down Afternoon Creek was significantly larger, the small volume, (less than 10%) that fell the opposite direction down Falls Creek actually impacted the highway. Therefore it is important to understand the mechanisms and controls that caused portions of the debris to travel down Falls Creek concurrently with failure in Afternoon Creek. Four mechanisms were proposed and tested. The purpose was to determine which mechanism was the most important in causing material to travel down Falls Creek. Each mechanism considered material that originated at the crest of the ridge that divides Afternoon Creek and Falls Creek. Photographs, numerical models, and field observations were used to evaluate each mechanism. Mechanism 1 and 2 propose that structurally controlled failure occurred concurrently with failure in Afternoon Creek; the failure mechanism caused displacement towards Falls Creek. Mechanism 3 and 4 propose that the ridge topography of the runout path caused fragmented rock to enter the Falls Creek travel path. These assume that the debris was fragmented before reaching the ridge, and that some of the debris landed on or was pushed to the Falls Creek side of the ridge. Structural control during failure: 1. Translational failure: Planar or wedge failure on discontinuities that are sub-parallel to the Falls Creek topography. 2. Toppling failure: Toppling failure towards Falls Creek, including failure of individual columns, and global, block-toppling failure of the slope. Topographic control during runout: 3. Spreading of a flowing mass: Fragmented rock avalanche debris flows and spreads over the ridge, away from the primary direction of movement. 4. Rockfall travel path: Isolated blocks that originate from the slope above the ridge land on the Falls Creek side of the ridge. 112 5.3.1 Mechan ism 1: Translat ional Failure Translational failure refers to blocks that may have slid towards Falls Creek on planar or wedge-shaped discontinuity surfaces. Sliding would occur on persistent slope-parallel discontinuities that daylight due to abrupt changes in the topographic surface, creating relatively large, thin slabs of rock. This failure mechanism requires lateral and head release which may have been provided by brittle, tensile failure that occurred when the main Afternoon Creek rockslide mass moved away from Falls Creek (Figure 5.33) Figure 5.33 Illustration of the translational failure mechanism in Falls Creek. Slope-parallel discontinuities are common to the Afternoon Creek area. Many of these are probably exfoliation or stress relief joints that were caused by rapid unloading due to erosion and glacial ice retreat during and after glaciation. Although a joint set parallel to the Falls Creek side of the slope was not found in lidar-derived point clouds of Afternoon Creek, indications of slope parallel discontinuities can be seen in photographs of the Falls Creek side of the ridge. Lidar scans were attempted of the Falls Creek side of the slope, but the nearest set-up stations were outside the effective range of the instrument, resulting in unusable data. A few small smooth, planar surfaces that slope towards Falls Creek can be seen in photographs of the top portion of the Afternoon Creek/ Falls Creek ridge (Figure 5.34). However most of the ridge crest appears to be composed of highly-fractured, crushed rock, at least partly caused by the shear zone that transects the ridge. The crushed rock may have 113 been the sliding surface for a translational failure, however a single highly persistent surface does not exist, as would be expected i f this were the primary mechanism. Figure 5.34 Photograph of the crest of the Afternoon Creek/ Falls Creek ridge from the Falls Creek side. One hypothesis was that slabs of rock dipping towards Falls Creek were supported by the tensile strength of the rock mass, and that translational failure towards Falls Creek would occur due to removal of the Afternoon Creek rock mass. The hypothesis was tested by building a model of the ridge in UDEC. The stability of the Falls Creek slabs before and after the Afternoon Creek rockslide was compared to determine the plausibility of this translational failure mechanism. A cross-section parallel to the direction of movement down Falls Creek (cross-section C - C ) was modeled with UDEC (Figure 5.35). The idealized slope consisted of one material type and three joint sets. The two main joint sets (joint set A and B) were inclined at their apparent dip (with respect to the strike of the cross-section line), and the third joint set was shallow and sub-parallel to the Falls Creek slope inclination. Because the apparent dip was used, joint set A discontinuities were inclined at only 6°. In-situ stresses were initialized under gravitational loading and elastic conditions. A Mohr-Coulomb elasto-plastic constitutive model was then used together with realistic rock mass and discontinuity strength 114 properties to model the resulting slope behavior. The factor of safety, estimated with the strength reduction technique (Itasca 2000), was used as a means to compare the stability of different slope geometries, given the same arbitrary input parameters. As required by UDEC, all discontinuities were fully persistent. The steeply-dipping joint set B discontinuities provided rear release for sliding towards Falls Creek. In reality these discontinuities are not fully persistent; therefore these joints were given a tensile strength that approximated the tensile strength of the rock mass. The strength reduction technique was applied only to the joint cohesion, joint tensile strength, and joint friction parameters and excluded the rock mass strength properties. The models were run several times to determine the influence of joint tensile strength and horizontal to vertical stress ratio. The tensile strength of the joints, intended to replicate the strength of rock bridges, was systematically varied from 0 to 10 MPa (approximately the tensile strength of intact orthogneiss). The stress ratio, varied from purely gravitational loading (i.e., K=0.33) to K=1.0, had a negligible effect on the model results. — i s ° r x Q \ $ ^ \ . 60° SOO . 11 •' O # 300 / $ 4* . . . . . . . . . . ..... , 0 100 200 300 400 500 600 Figure 5.35 Map of pre-failure topography showing location of cross-section C - C . Filled red contours indicate location and thickness of the failed zone. 115 The model was sensitive to changes in joint tensile strength; however, regardless of the joint tensile strength, the Falls Creek slabs were more stable after removal of the Afternoon Creek rockslide material. The values and figures reported below are for 65 KPa of joint tensile strength. The Factor of Safety before failure in Afternoon Creek was 1.4. The rock slope failure in Afternoon Creek was then simulated by deleting all of the mass that would have been involved in the failure. This removed the blocks that were adjacent to the Falls Creek slabs, thereby removing the support provided by the tensile strength of the joints. Removal of the Afternoon Creek material also reduced the total stress acting in the Falls Creek blocks. The Factor of Safety after failure in Afternoon Creek was 2.3 - significantly higher than before the material in Afternoon Creek was removed (Figure 5.36). This shows that the driving influence of the total stress was greater than the failure-resisting effect of the rock mass tensile strength. A direct comparison is possible using the strength-reduction factor of safety algorithm because the failure surface and mechanism were equivalent in each situation. Figure 5.36 U D E C models of the translational failure mechanism before and after removal of Afternoon Creek material. (A) and (B) show Factor of Safety and horizontal velocity contours at failure. (C) and (D) show contours of the major principal stress. 116 5.3.2 Mechan i sm 2: Topp l ing Failure Toppling failure refers to columns that rotate about a fixed base towards Falls Creek. Two categories of the toppling mechanism exist: (1) single column failure caused by tension crack propagation, and (2) global block toppling involving many interacting columns. The first mechanism involves a single column of rock, leaning towards Falls Creek, separated from the main rock mass by a tension crack that is open at the top and terminates into intact rock at the base of the column. Toppling failure towards Falls Creek is caused by propagation of the tension crack. The second category is global failure of many interacting columns on the Falls Creek side of the slope. This mechanism requires a discontinuity set that dips steeply into the slope, satisfied by joint set B, and orthogonal cross joints at the base of the toppling blocks to allow kinematic freedom. The shorter columns at the toe of the slope are pushed forward by the weight of the longer columns behind, allowing toppling failure to progress higher up the slope. Evidence for category 1 toppling towards Falls Creek is based on examples of toppling in Afternoon creek. Toppling of individual columns, towards Afternoon Creek appears to be an active mechanism today. Columns exist that are separated from the main rock mass by tension cracks that are wider at the top than at the base (Figure 5.37). Similar columns may have existed, leaning towards Falls Creek, prior to the November, 2003 events. Deformation of the rock mass towards Afternoon Creek during the rockslide may have triggered rapid propagation of any tension cracks that existed, causing the column to topple towards Falls Creek at the same time as failure in Afternoon Creek. This mechanism requires a several meter tall, near-vertical cliff on the Falls Creek side of the ridge. Due to the scale of the available aerial photographs, it is not clear whether a vertical cliff existed in the failed zone on the Falls Creek side of the ridge before November, 2003; however given the morphology of the surrounding slope, it certainly is a possibility. 117 Figure 5.37 Column toppling towards Afternoon Creek. Photograph date December, 2004; provided by URS Corporation. To explore the feasibility of the global toppling failure mechanism (category 2) a UDEC model was created along the path of steepest descent in Falls Creek (cross-section C- C ) - the same cross-section used to model the translational failure mechanism (Figure 5.35). For the topography and joint set spacing measured in the data collection phase of the analysis, block toppling is not possible because the blocks formed by joint set A and B do not meet the shape requirement for toppling described by Wyllie and Mah (2004), with respect to the base to height ratio and inclination of the block. Therefore the numerical analysis was used to see i f block toppling towards Falls Creek could occur i f small changes in the geometry of the slope (i.e., topography, joint set orientation, joint set spacing) were made. For all model runs, the idealized slope consisted of one material and 2 joint sets (modified 118 joint set A and B). To allow the blocks to meet the shape criterion for toppling, the spacing of joint set A (cross-column set) was increased, while the spacing of joint set B was decreased to create tall, thin columns. For the first model run, the orientation of joint set A and B as measured in the data collection, and adjusted to the apparent dip for the cross section, were used (Figure 5.38). For the second model run, joint set A was ignored, and a hypothetical cross-column set was used that dipped out of the slope at 30°, orthogonal to joint set B (Figure 5.39). This created ideal conditions for block toppling to occur, however there is no evidence that the orthogonal joint set exists in the actual Afternoon Creek slope. Figure 5.38 U D E C model with the measured joint orientations (run 1) showing potential for toppling towards Falls Creek.(A) and (B) show total displacement vectors and plasticity indicators. (C) and (D) show contours of x-displacement. 119 For both model runs, the modeling procedure remained the same as that described for the translational failure mechanism (section 5.3.1). Joint friction was set to 35° to prevent sliding on the cross-column set; joint cohesion and tensile strength were set to zero. The factor of safety, estimated with the strength reduction technique (Itasca 2000), was used to compare the stability of the Falls Creek columns under different slope geometries, given the same arbitrary material parameters. Figure 5.39 U D E C model with cross-joints orthogonal to set B (run 2) showing potential for toppling towards Falls Creek.(A) and (B) show total displacement vectors and plasticity indicators. (C) and (D) show contours of x-displacement. For the first model run, when the apparent dip of joint set A was used to define the cross-column joints, toppling towards Falls Creek developed poorly. Displacement vectors and x-displacement contours showed that toppling towards Falls Creek occurred (Figure 5.38), but an equally important failure mechanism appeared to be sliding towards Afternoon Creek on joint set A . The Factor of Safety, as determined by the strength reduction 120 technique, was 1.76. For the second model run, when the cross-joints dipped out of the slope, orthogonal to set B, block toppling of the entire slope towards Falls Creek was the primary failure mechanism (Figure 5.39). The Factor of Safety to toppling was 1.07. Next, to test how removal of the Afternoon Creek rock mass would affect the stability with respect to toppling towards Falls Creek, a portion of the slope representing the Afternoon Creek rockslide volume was deleted for each model run. This removed the blocks that were adjacent to the Falls Creek columns, reducing the total stress acting on the columns. For the first model run (using the measured joint set A and B orientations), block toppling towards Falls Creek stopped when the Afternoon Creek mass was removed and reverse toppling towards Afternoon Creek became the primary failure mode (Figure 5.38). The Factor of Safety for this situation was 1.96. For the second model run (adding a hypothetical orthogonal cross-column joint set), block toppling towards Falls Creek remained the primary failure mode; however the Factor of Safety to toppling increased to 1.16, significantly higher than before removal of the Afternoon Creek material. Therefore the analysis showed that removal of the Afternoon Creek rockslide mass actually helped to stabilize any global, block toppling failure towards Falls Creek. 5.3.3 Mechan ism 3: Spreading of a Flowing Mass According to the topography of the failure scarp, the main body of the rock mass slid towards Afternoon Creek, away from the ridge; however some debris may have been pushed over the crest of the ridge towards Falls Creek by the dispersive forces that cause spreading in a fragmented, flowing rock avalanche. However it is unlikely that the rock mass was heavily fragmented at the onset of failure, and therefore should not have behaved as a flowing rock avalanche at the top of the failure zone near the ridge. The coarse, blocky nature of the debris found in Afternoon Creek and Falls Creek proves that the failed mass fragmented at some point between initial failure and coming to rest in the channel. The failed mass did not travel as a single, coherent block, but instead broke into fragmented blocks of a few meters in diameter. It is easy to imagine that a majority of this fragmentation occurred during runout of the debris (due to impacts along the 121 base and within the sliding mass), but some fracturing must have occurred during initial deformation and break-up of the brittle source block. The topography of the failure scarp should have caused the debris to flow away from the ridge towards Afternoon Creek. It is common knowledge that rock avalanche debris spreads horizontally during movement. Therefore at Afternoon Creek, i f parts of the rock mass fragmented enough to begin spreading before they fell below the ridge, then some of the debris may have been pushed over the ridge towards Falls Creek in the opposite direction of the main failed mass. Davies and McSaveney (2002) and Davies et al. (1999) endorse the hypothesis that continuous fragmentation of clasts creates an isotropic dispersive stress within the flowing rock avalanche. They claim that this fragmentation is responsible for the extraordinarily long runout of large rock avalanches. This dispersive stress could be responsible for pushing some material over the Falls Creek-Afternoon Creek ridge. However, Davies et al. (1999) clearly point out that the fragmentation theory does not apply to initial break-up of the source block along pre-existing defects, it only applies to the inside of a shearing mass where the intact clasts break. In the Dynamic Analysis code DAN3D, the dispersive stress applied is simply a fluid pressure adjusted by lateral earth-pressure coefficients (Hungr 1995); (McDougall and Hungr 2004). When the Afternoon Creek rock avalanche is modeled in DAN3D, a portion of the debris travels down the Falls Creek runout path due to this spreading (Figure 5.40). The rock avalanche debris is modeled as an "equivalent fluid" that is governed by a simple rheology, and a few resistance parameters. One assumption of the code is that the entire source volume fragments, and behaves as the "equivalent fluid" immediately when the model simulation is started. Although the fluid assumptions are valid further down the runout path, they are probably not valid at the moment of failure onset, before the source area has fully fragmented. In the Afternoon Creek slope, part of the fragmentation occurred as the rock mass deformed, but it is likely that a majority of the fragmenting occurred in the subsequent moments of runout. Since the rock mass initially moved away from Falls Creek towards Afternoon Creek on basically planar surfaces, it is likely that the rock mass initially behaved as a cohesive mass, and did not begin to behave as an 'equivalent fluid' until it reached the 122 base of Afternoon Creek. Therefore, although part of the debris traveled down Falls Creek in the DAN3D simulations, the simulations can not be used to prove that mechanism 3 was the primary mechanism that caused debris to enter the Falls Creek travel path. Figure 5.40 DAN3D simulations during runout. Red contours indicate source zone. Filled blue contours indicate deposit thickness. Dotted black line indicates the trimline mapped on air photos. 123 5.3.4 Mechan ism 4: Rockfal l caused by dilation of the failed mass Regardless of the mechanism that initiated the movement of rock blocks into Falls Creek, all blocks eventually became rockfall at some point during the descent. However, mechanism 4 refers only to individual blocks that fell as a result of the large-scale failure towards Afternoon Creek, and were required by the slope topography to travel down the Falls Creek runout path. Due to the massive, brittle nature of the rock mass that composed the ridge, fracturing, fragmentation, and spalling occurred during rapid deformation of the ridge. This deformation may have increased the occurrence of rockfall in Falls Creek during the catastrophic November, 2003 failure event. Numerical back-analysis of the Afternoon Creek failure (section 5.1.3) showed that high tensile stresses caused tensile fracturing of elements in the failure volume. Dilation of the source rock mass, including opening of existing discontinuities and brittle fracturing of intact rock, would have increased the occurrence of rockfall down the Falls Creek runout path. Photographs of the ridge crest show fresh, scarred surfaces on the top of surface undulations that appear to be caused by rockfall (Figure 5.41). It is expected that sliding (such as Mechanism 1- translational failure) would have produced more uniform striations. Also the location of the scarring indicates that the debris fell from above rather than slid from the direction of Afternoon Creek. Figure 5.41 The crest of the Aftenoon Creek-Falls Creek ridge showing scarring caused by rockfall. 124 Davies and McSaveney (2002) and Davies et al. (Davies et al. 1999) studies on the isotropic dispersive forces within rock avalanches suggests that intact rock clasts fragment explosively, as in laboratory unconfmed compression tests, or in a mine rock burst. At each fragmentation episode, elastic potential energy is cycled to kinetic energy and equal and opposite components of momentum are input to the clast fragments. This kinetic energy is responsible for dislodging blocks from the rock mass causing them to fall. Fragments that break free on the Falls Creek side of the ridge will fall down the Falls Creek runout path. Accounts of brittle rock fracture at Afternoon Creek several months after the rockslide support the hypothesis that fragmentation of intact rock was an important component of the November, 2003 failure. It also gives an indication of the energy released when intact rock breaks. One account was reported in Chapter 2 of this thesis: "Large scale fractures were recognized in the intact bedrock near the failed face following these events. On several occasions workers heard loud explosion-like booms and felt the ground vibrate beneath their feet. Although the sounds emanated from the slide area, they were not followed by rolling rocks or dust clouds. These booms may have been the result of intact brittle rock fracturing in the slope." URS Corporation and Wyllie & Norrish Rock Engineers used the Colorado Rockfall Simulation Program (CRSP) (Jones et al. 2000), a widely accepted algorithm for simulating rockfall, to predict the trajectory and energy of falling rock blocks at points of interest along a profile of Falls Creek. The purpose of the study was to design a rockfall embayment at SR 20 to provide a catchment area for rocks originating at the top of the Afternoon Creek - Falls Creek ridge (URS and Wyllie & Norrish Rock Engineers 2004). Construction of the embayment was completed in February, 2006. In the study, the calibrated CRSP simulation was able to reproduce the following observations of the actual event: (1) few rock fragments came to rest beyond the center of the Skagit River Channel; (2) the majority of the rockfall fragments came to rest on SR 20; and (3) multiple fragments on SR 20 were in the three to five-foot size range (URS and Wyllie & Norrish Rock Engineers 2004). These observations in combination with the successful rockfall simulation support the conclusion that the material in Falls Creek fell as individual 125 blocks rather than a rock avalanche. More importantly this study showed that rocks that enter the Falls Creek travel path are very likely to impact the SR 20 embayment area. 126 5.4 Total Slope Back Analysis Summary 5.4.1 Afternoon Creek Travel Path The Total Slope Analysis of the Afternoon Creek travel path involved an investigation of the failure initiation with limit equilibrium analysis tools and the numerical modeling tools UDEC and 3DEC, coupled with an investigation of the rock avalanche runout using DAN3D. The purpose of the slope instability back analysis was to gain a more complete understanding of the operative slope failure mechanisms and to constrain estimates of the slope geometry and physical properties. Knowledge of the failure mechanisms guided parts of the runout back-analysis as well as the analysis of future events at Afternoon Creek (Chapter 6). The constrained slope geometry and physical property values provided input parameters for all subsequent analyses. Table 5.5 displays input parameters that were constrained during the U D E C analysis. The 3DEC analysis, using these constrained parameters, adequately replicated the interpreted failure mechanism and sliding surface, showing that these parameters sufficiently describe the slope. The primary failure mechanism at Afternoon Creek was wedge sliding on joint set A and joint set B. Primary sliding occurred on joint set A surfaces, while joint set B provided lateral release. Tensile failure of the rock mass provided rear release for sliding and caused disintegration of the failure volume. In such brittle rock, this failure mechanism is typically extremely rapid and catastrophic (Hungr and Evans 2004). The volume of failed material included the entire rock mass above the critical sliding plane, extending to the lateral and rear release surfaces. Failure occurred at this particular location in the Afternoon Creek slope due to the local slope concavity created by differential weathering of the highly fractured Zone B. It is hypothesized that Zone B is highly fractured due to the intersection of the Base and Tower Shear Zones. Differential weathering caused a bench to form in Zone B and steepening of the slope above (Figures 5.9, 5.10). Joint set A surfaces daylighted in the over-steepened slope concavity. The slope direction in this concavity allowed kinematic freedom for sliding along the highly persistent joint set A surfaces. 127 The 2-D numerical modeling analysis with UDEC consisted of baseline models and a series of trial models used for parametric studies. Two cross-sections were considered because it was suspected that the northern and southern portions of the slope moved in different, oblique directions, and that failure occurred in two or more distinct stages. One cross-section bisected the northern, higher-elevation portion of the slope (cross-section A- A' ) , while the second cross-section bisected the southern portion of the slope (cross-section B-B') . The trial models were most sensitive to changes in the orientation and spacing of the critical joint sets, and in the joint friction angle and rock mass tensile strength. The geometric configurations for which the models most closely reproduced the actual failure surface were the baseline model configuration (Figure 5.14; Table 5.1) and when joint set B had a variable orientation and limited persistence (Figure 5.21; Table 5.3). When a single joint set inclined at 55° was used, a stepped failure surface, formed by the joint set and steeper tensile yielding surfaces, developed (Figure 5.23). This concentration of tensile stresses shows how discontinuous joint set B surfaces may have rapidly propagated to initiate the November, 2003 failure event. Discrepancies between the model results and the actual failure surface can be attributed, in part, to the required assumption of fully-persistent joints in UDEC. In reality, the location of the sliding plane is controlled by the connectivity of natural, discontinuous joints, resulting in a stepped failure plane. The trial models showed that the effective joint friction angle was probably between 28° and 33° to allow planar sliding on the shallow joint set A surfaces. The rock mass tensile strength was probably less than 500 KPa. In models of cross-section A - A ' (hypothesized second failure stage), the failure volume developed at damage state 2, when the effective joint cohesion was reduced to 100 KPa. However in models of cross-section B - B ' (hypothesized first failure stage), the failure did not fully develop until damage state 3, when the effective joint cohesion was reduced to 10 KPa. This indicates that the cross-section A - A ' material was less stable than the lower- elevation, cross-section B - B ' , mass. Therefore, it is likely that the cross-section A - A ' mass was supported laterally and at the toe by the cross-section B material. When the cross- section B - B ' material failed, it is likely that cross-section A - A ' material followed immediately, in a single failure stage. 128 Three-dimensional modeling with 3DEC was used to investigate several issues that were left unresolved by the 2-D models, including (1) the interaction of the northern and southern failure volumes; (2) the number of failure stages; and (3) the movement direction of the entire failure mass. The 3DEC model provided additional understanding of the structural and topographic controls to failure at Afternoon Creek because it considered the 3-D relationship of these rock slope features. The 3DEC model was able to closely replicate the results of the 2-D U D E C models and actual shape of failure surface. Discrepancies between the model and the actual event were caused by uncertainties related to the slope geometry and material properties, simplifying assumptions, and the required assumption of fully- persistent joints. The 3DEC analysis indicated a single-stage failure sequence, which agrees with the interpretation of the U D E C results. It showed that the higher elevation, northern failure section was less stable than the southern half of the failure volume, indicating that the northern half of the volume drove the entire slope to failure in a single stage. Secondly, the 3DEC analysis showed that both sections of failure volume displaced in the dip direction of joint set A , rather than in oblique directions as otherwise hypothesized. Overall the 3DEC analysis was beneficial as it provided unique insight into the problem. However the analysis was costly in terms of time and effort. The 3DEC software and pre-processor program, PGEN, were difficult to use and plagued by bugs and program errors. Post-processing was time consuming and difficult due to limited graphics options and program bugs; therefore it was extremely beneficial to use input parameters that were pre- constrained during the U D E C analysis. The runout analysis with DAN3D was based on the results of the failure initiation study, which showed that failure occurred in a single, extremely-rapid stage involving the entire volume determined by comparing the before and after digital elevation models. After several calibration runs, the simulation adequately reproduced the actual event using the frictional rheology with a Basal Bulk Friction Angle of 37° and Internal Bulk Friction Angle of 40°; however there were several notable differences between the simulated and actual event, including the following: (1) the center of mass traveled too far; (2) too much spreading of the debris onto the Afternoon Creek fan; and (3) excessive run-up on the opposite valley wall. These discrepancies were slightly reduced when a two-stage failure sequence was 129 used; however these results do not supersede the failure initiation analyses which indicate a single failure stage. Possible causes for the discrepancies include the required assumption that the source volume fluidizes immediately when the model simulation is started, and the assumption of a single, average friction value for the entire path. Additionally individual, large features in the actual avalanche, such as 20-meter diameter boulders, may have significantly influenced the direction and behavior of the flow. 5.4.2 Falls Creek Travel Path It is important to understand the mechanisms and controls that caused portions of the debris to travel down the Falls Creek travel path because this debris actually impacted the highway. Four mechanisms, including (1) translational failure, (2) toppling, (3) spreading of a flowing mass, and (4) rockfall, were proposed and tested to determine which was the most important. Small translational failures (mechanism 1) on the Falls Creek side of the ridge may have contributed to the debris in Falls Creek as there may have been failure of small, thin slabs caused by ground shaking, and impact of falling rocks. However models show that pervasive planar or wedge sliding failure was unlikely. U D E C models show that the stability of slabs increased with removal of the Afternoon Creek rock mass by reducing the driving forces necessary for translational failure. Additionally, a smooth persistent sliding plane is not seen in photographs of the ridge crest. Small isolated slabs may have failed by sliding, triggered by seismic shaking, or rockfall. Toppling of individual columns into Falls Creek (mechanism 2) may have contributed to the debris in Falls Creek. Columns that can be seen in Afternoon Creek today are an example of this mechanism (Figure 5.37). Similar columns may have existed at the Afternoon Creek - Falls Creek ridge before the failure, and this mechanism could account for a significant portion of the total mass that fell down the Falls Creek travel path. However, block toppling of the entire Afternoon Creek-Falls Creek ridge towards the Falls Creek runout path (mechanism 2) probably did not occur. The orientation and spacing of the existing joint sets does not create blocks which meet the shape criteria for toppling. UDEC 130 modeling showed that even when the block shape was adjusted to be ideal for global block toppling, the stability to toppling increased with removal of the Afternoon Creek rock mass. DAN3D runout simulations show that debris originating in the upper portion of the source zone was directed away from the ridge, towards Afternoon Creek, so no opportunity existed for spreading over the ridge (mechanism 3) to occur. Additionally, it is unlikely that the entire rock mass was fragmented immediately at the onset of failure at the top of the source area. Therefore the rock mass was not subject to the isotropic dispersive stresses caused by fragmentation of clasts, nor did it to behave as an "equivalent fluid" at the top of the runout path, near the ridge. Although the DAN3D simulations show a small portion of the mass flowing down Falls Creek, the fluid-pressure assumptions which caused this are not valid at the onset of failure. The rock mass probably behaved as a cohesive block near the top of the source zone. Spreading of the flowing mass over the ridge into the Falls Creek travel path was probably not an important mechanism in the November, 2003 failure event. Rockfall (mechanism 4) is most likely to be responsible for a majority of the debris found in Falls Creek. This mechanism refers to individual blocks which broke free from the main rock mass due to brittle fracturing, dilation and deformation of the rock mass immediately prior to, or during catastrophic failure. Even i f isolated cases of toppling or translational failure occurred they could be classified as rockfall for all practical purposes. URS Corporation and Wyllie & Norrish Rock Engineers used CRSP to predict the trajectory and energy of falling rock blocks at points of interest along a profile of the Falls Creek travel path. The successful rockfall simulation supported the conclusion that the material in Falls Creek fell as individual blocks rather than a rock avalanche. Their study showed that rocks that enter the Falls Creek travel path are very likely to fall all the way to the SR 20 highway level. None of the proposed mechanisms could be definitively rejected or accepted. Each could conceivably be responsible for a portion of the debris that was found in Falls Creek, and many of the mechanisms probably occurred to some degree. Therefore, based on the analyses, the mechanisms were subjectively ranked in order of importance in Table 5.6. The most important mechanism is considered to be responsible for a majority of the debris in Falls Creek, while the mechanisms at the bottom of the list were insignificant or probably did not occur. 131 Table 5.6 Mechanisms that allow debris to enter Falls Creek - Ranked in order of importance. Rank Mechanism Importance Probably occurred to some extent 1 Mechanism 4: Rockfall travel path Most Important 2 Mechanism 2: Toppling failure (individual columns) 3 Mechanism 1: Translational failure 4 Mechanism 3: Spreading of a Probably did not 1 flowing mass r occur £ Mechanism 2: Toppling failure Least 3 (global block toppling) Important 132 6 ANALYSIS OF FUTURE EVENTS AT AFTERNOON CREEK Back analysis of the slope instability and failure initiation (chapter 5.1, 5.3) provided understanding of the mechanisms and controls that created the hazard at Afternoon Creek, and constrained estimates of the slope geometry and rock mass properties. Back analysis of the post-failure motion (chapter 5.2) provided a calibrated DAN3D runout model. The analysis described in Chapter 6 uses these results in a preliminary assessment of the rock hazards that currently exist at Afternoon Creek with respect to the SR 20 highway. It uses the knowledge and calibrated models acquired during the back-analysis to estimate the location, volume, and mechanism of future rock slope failures, and the runout path, distance, and velocity of post-failure motion. It must be stressed that these preliminary estimates only refer to the spatial aspects of failure. The equally important question of when failure will occur was not the focus of this study. 6.1 Failure Volume Assessment Runout analysis is perhaps the most important component of the hazard assessment. It can be used to predict the effects of a future rockslide on the SR 20 highway, including the impact area, velocity, and deposit depth. However, before an accurate runout analysis can be completed it is necessary to locate, and accurately estimate the volume of the rock avalanche source (McDougall and Hungr 2004). The constrained estimates of the Afternoon Creek slope geometry, mechanical properties, and understanding of operative failure and deformation mechanisms in the past event guided the assessment and provided input to simple numerical models of the current slope configuration. The results of numerical modeling were combined with simple judgment (based on observations and understanding of past events) to estimate the location, volume, and failure mechanism of future rock slope failures at Afternoon Creek. Specifically the objectives of the failure volume assessment included the following: 133 1. Estimate the source location and volume of future rock failures. 2. Identify operative failure mechanisms and controls: a. Determine how the mechanisms and controls affect the volume of failed material, b. Determine how the mechanisms and controls affect the runout of failed material. 6.1.1 Methodology Modified versions of the A - A ' and B - B ' cross-sections used in earlier UDEC back analyses (see section 5.1.3) were used to forward model the failure mechanism, location and volume of potential future rock avalanches at Afternoon Creek. The model design and modeling methodology used in the predictive analysis were also the same, except that the topographic surfaces were adjusted to match the post-failure topographic profiles (Figure 6.1). Figure 6.1 Post-failure block model, cross-section A - A ' and B - B ' . 134 Three models were run, for both cross-sections, to test the geometric configurations that produced the best results in the back analysis (Table 6.1). Mechanical properties that had been calibrated during the back analysis were used (Table 5.1), except joint friction angle which was reduced to 28° to encourage sliding on joint set A . The minimum rock mass tensile strength tested in the back analysis (220 KPa) was used in the predictive models, as it produced the largest failure volumes in the back-analysis and therefore represents the "worst- case scenario." Table 6.1 Geometric configurations of the forward analysis U D E C models, Afternoon Creek Joint set A Joint set B # Joint geometric Dip Spacing Persistence Dip Spacing Persistence configuration (°) (m) (m) (°) (m) (m) 1 Baseline model 50 10 Fully 61 25 Fully Joint set B variable 50 7 Fully 61 15 150 z orientation & persistence ± 8 ± 2 0 3 Single joint set 55 10 Fully na na na The modeling procedure was the same for all three models. After stresses were initialized using the elastic constitutive model for the rock mass and excessive joint strength, all zones were changed to the 'Mohr-Coulomb' elasto-plastic constitutive model to allow yielding. The joint cohesion (i.e., discontinuity strength) was reduced by increments to simulate weathering and discontinuity propagation with time. The increments, called 'damage states', are described in Table 5.1. The model was run to equilibrium and the response monitored for each damage state. The numerical models showed the likely mechanism of failure, and gave an indication of failed thickness. The area of potential failure was approximated by comparing the failure location in the modeled cross-sections with the topography. The failure area was extrapolated from the modeled cross-sections to adjacent regions with similar topographic relief. Failure volume was estimated by interpolating and extrapolating the thickness of the failed mass from the cross-section A - A ' and B - B ' modeling results to the remainder of the failure area. 135 6.1.2 Locat ions of S lope Instabilities Despite the data collection program and the amount of back analysis performed, the physical properties, location and orientation of individual discontinuities, etc., in the Afternoon Creek slope can never be known exactly. Therefore, no model, regardless of complexity, is able to definitely predict the location and volume of future failures at Afternoon Creek. Instead, the three models in this analysis were used to identify unstable sections of the slope and place boundaries on potential failure volumes. The results of the models were compared with mechanisms and controls of the past event, and observations of the current slope, to constrain estimates of the future failure locations, volumes, and mechanisms. Due to the dissimilar topography of the northern (cross-section A - A ' ) and southern (cross-section B-B') slope regions, the numerical modeling results for the two cross-sections (Figure 6.2 and Figure 6.3, respectively) differ. Two areas along the northern, cross-section A - A ' , region failed in the numerical models, including: (1) the over-steepened failure scarp crest; and (2) the material that remains near the middle of the November, 2003 failure scarp. In the southern cross-section B - B ' models, failure was limited to the failure scarp crest. Numerical modeling results of the three geometric configurations show that the steeper joint set B limited the size of the failure volume. In geometric configuration 1 and 2, the joint set B discontinuities provide rear release for the failure mass. However, in geometric configuration 3, when joint set B was excluded, the failure volume incorporated all of the material above a single, fully-persistent joint plane. The joint plane daylighted at the top of Zone B and extended to the top of the slope, several hundred meters above the November, 2003 failure scarp. Although the failure scarp crest was not accessed during the field survey, it is likely, according to field observations and the numerical back analysis, that steeper cross-cutting discontinuities exist, either as tension cracks or as a complete joint set. Therefore it is expected that the extent of the failure volume will be limited by these steeper discontinuities, similar to the November, 2003 rockslide, and the results of forward analysis model configurations 1 and 2. 136 JOB TITLE : Geometric Configuration 1 - Initial model UDEC (Version 4.00) 18-Apr-06 15:21 cycle X displacement contours contour intesvai= 2 OOOCtOO O.OOOE+OOto I.000E+01 0.000E+00 20006*00 4.0006*00 e.ooo6*oo 8.Q00E+00 1.000E+01 JOB TIT LI: C.uumt'in̂  Configurntio_n 2 - Joint set B variable orientation & persistence UDEC (Version 4.00) 18-Apr-06 18:51 cycle 79220 X displacement contours contour inlmvat- 2.0QOE+00 OOOOE+OOLo 1 0OOE*01 0.0006+00 2.0006*00 4.0006+00 6.000E+00 8.000E+00 1.0006*01 JOB TITLE : Geometric Configuration 3 - Sim UDEC (Version 4.00) LEGEND 18-Apr-06 18:42 cycle 66771 X displacement contours contour Interval 2 OOOE+00 O.OOOE+OOto 1.000E+01 0.00OE+00 2.000E + 00 4.000E*00 6.0006*00 8.0006.00 10006*01 Alex Strouth University of British Columbia Figure 6.2 Cross-section A - A ' failure volume assessment for future events; horizontal displacement contours. Geometric configurations 1, 2, and 3. 137 JOB TITLE : Geometric configuration 1 - initial model JOB TITLE : Geometric configuration 2 - Joint set B variable orientation & persistence UDEC (Version 4.00) 1B-Apr-06 18:51 cycle 79220 DO zones. total at yield surface l*i yielded in past (X! tensile failure to displace niacin 826 238 J 0 1E 2 block piot shear displacement on joint max sliear disp = 1628E+Q1 each line thick = 3.256E+00 Alex Strouih University of British Columbia Figure 6.2 continued. Cross-section A - A ' failure volume assessment for future events; plastic yield indicators. Geometric configurations 1, 2, and 3. 138 JOB TITLE : Geometric configuration 1 - initial model UDEC (Version 4.00) LEGEND 18-Apr-06 9:33 cycle 72740 X disrHMcenxinl con touts contour intervals 5 000L-01 O.OOOE+OOtO SQOOE-tQO O.OOOE+OO 5.OO0E-01 1.000E-00 1.500E+0O 2.000E+0O 2.500E+0O 3.0OOE*0O Alex Strouth University of British Columbia Figure 6.3 Cross-section B-B' failure volume assessment for future events; horizontal displacement contours. Geometric configurations 1, 2, and 3. 139 JOB TITLE : Geometric configuration 1 - initial model UDEC (Version 4.00) LEGEND 18-Apr-06 9:33 cycle 72740 no m m total 1331 al vield surface (*) 0 yielded in past (X) 130 tensile failure lp) 37 ô piacemem vectors maximum = 4,51 T:: + 00 L. 0 2E 1 block plot shear displacement on joint max shear disp - 4.267E+00 each line thick *. 8.533E-01 Alex Strouth University of British Columbia JOB TITLE : Geometric configuration 2 - Joint set B variable orientation & persistence UDEC (Version 4.00) LEGEND 18-Apr-06 10:13 cycle 80440 no sines • MMM t<i0l at yield surface {*) 0 v̂ ldeci in past O0 236 tensile failure (o) 123 displacement vectors maximum = 1.569E*0t I I I 1 I I 0 5E 1 block plot shear displacement on joint max shear disp = 1.1S4E+01 each line thick • 2.387E+00 Alex Strouth University of British Columbia .300 1.&0O 1.700 JOB TITLE : Geometric configuration 3 - single joint set UDEC (Version 4.00) LEGEND 18-Apr-06 18:39 cycle 70650 no s n M . total 1353 Si yield surface <") 0 yielded in past jX) 29 lamto failure (o) 33 tifeptSQtftwH vectors 2.0786+00 0 1E 1 block plot shear displacement on joint max shear disp - 1.807E+00 each line thick 3.B15E-01 Alex Slrouth University of British Columbia Figure 6.3 continued. Cross-section B-B' failure volume assessment for future events; plastic yield indicators. Geometric configurations 1,2, and 3. 140 The over-steepened crest of the November, 2003 failure scarp is a potential source zone for two different types of future slope failures (Figure 6.4; Figure 6.5). Firstly, all of the numerical modes show potential for sliding to occur on any discontinuities that daylight in the scarp, resulting in retrogression of the failure crest. The distance of retrogression appears to be limited by the presence of steeper joint set B surfaces. In numerical models 2 and 3, due to the assumption of fully-persistent joint set A surfaces, and lack of joint set B surfaces, the failure crest retrogressed up to 70 meters. However it is likely, due to the expected spacing of joint set B surfaces (modeled in geometric configuration 1), that retrogression would be limited to 10-25 meters in reality. Although in the two-dimensional numerical analysis the failure mechanism appears to be planar sliding on joint set A , the kinematic analysis (see section 5.1.1) showed that at this slope orientation the only feasible failure mechanisms are planar sliding on joint set B or wedge failure. This highlights a limitation of using a 2-D numerical model to simulate a complex 3-D slope, and is an example of the understanding and judgment required to interpret the results of the numerical model. Large scale translational failures are possible in the over-steepened failure crest, however they are considered unlikely due to the orientation of the slope. Any translational failure that does occur is expected to be extremely rapid, similar to the November 2003 event, due to the brittle nature of the intact rock. Large-scale wedge or planar failure could result in a rock avalanche, with potential to flow down both the Afternoon Creek and Falls Creek runout path. The maximum volume of a translational failure in the failure scarp crest is expected to be on the order of 100,000 m 3 ; however it is unlikely that the entire volume would fail simultaneously due to the orientation of the crest, and the wedge failure mechanism. If one part of the crest does fail, it will not necessarily destabilize the remainder of the crest. Evidence of an imminent large-scale planar or wedge failure in the slope crest has not been found in the actual Afternoon Creek slope. 141 Figure 6.4 Two potentially unstable areas of the Afternoon Creek slope. Photograph by Erik Eberhardt (August 2005) Secondly, the numerical models show high tensile stresses and tensile yielding of elements near the failure scarp crest. In reality, the high tensile stresses can cause existing fractures to open and new fractures to form. Highly persistent, near vertical tension cracks observed in the slope today are evidence that the high tensile stresses do exist (Figure 6.6). These cracks separate columns, with the potential to topple, from the main rock mass. Toppling of an individual column will occur when the hinge at the base of the column fails in a brittle manner. The volume of the existing column is approximately 1000 m 3 . Toppling will most likely occur towards Afternoon Creek, however it may be possible for parts of the toppled column or associated rockfall to land in the Falls Creek runout path, because of the position of the failure crest above the ridge. Another hazard, and a source for future rock avalanches in Afternoon Creek, is the material that remains in the northern section of the current failure scarp (Figures 6.4 and 6.5). This zone is potentially unstable because joint set A discontinuities daylight in the over-steepened slope section above Zone B. A l l of the cross-section A - A ' numerical model configurations, which bisect the northern slope region, 142 show that this material has potential to fail by planar sliding on joint set A , with extensive tensile yielding of the failure mass. Joint set B provides rear release for the sliding blocks in model configurations 1 and 2. The failed material forms a 10-20 meter thick slab that parallels the current topography. Planar sliding below this slab is buttressed by Zone B, because the critical discontinuities do not persist through the zone. The actual failure volume is additionally supported by the adjacent rock mass in the third-dimension. Lateral release along the plane B surfaces is required for failure to occur. The maximum failure volume for this hazard is expected to be on the order of 300,000 m 3 . The entire volume has potential to fail catastrophically, as a single event because the failure volume is elongated in the anticipated direction of movement. Failure at the toe of the failure mass would likely cause immediate failure of the rock mass above. This mechanism is equivalent to the failure mechanism of the November, 2003 event. Therefore the failure is expected to be extremely rapid, resulting in a rock avalanche towards Afternoon Creek. This failed volume originates far below the Afternoon Creek-Falls Creek ridge, so it does not have potential to travel down the Falls Creek runout path. 143 Figure 6.5 Map and schematic cross-section D-D' showing estimated location, thickness, and volume of the slope hazard sources. 144 Figure 6.6 Example of an unstable column that has potential to cause rockfall in Falls Creek. August ,2005. 6.1.3 Summary of Potential Rock S lope Hazard Sources In summary of the analysis above, three potential rock slope hazard sources exist at Afternoon Creek (Table 6.2). They are ranked from most likely to occur (rank #1) to least likely to occur (rank #3). Table 6.2 Summary of the three hazard sources at Afternoon Creek. Rank Location Failure mechanism Approximate maximum volume (m3) Anticipated travel path j Failure scarp crest - individual columns Toppling 1,000 per column Afternoon Creek Falls Creek 2 Middle of existing failure scarp Extremely rapid translational 300,000 Afternoon Creek 3 Failure scarp crest Extremely rapid translational 100,000 Afternoon Creek Falls Creek 145 Debris from the middle of the existing failure scarp wil l most likely be restricted to the Afternoon Creek travel path. Translational failure of this part of the rock mass is essentially a continuation of the November, 2003 event, except with a considerably smaller source volume. Sources originating at the failure scarp crest have potential to send debris down both the Falls Creek and Afternoon Creek runout paths. Toppling of columns in the failure scarp crest, is considered the most likely to occur, based on observation of a characteristic, V-shaped, tension crack that exists today. Translational failure of the failure scarp crest, is considered least likely to occur, due to the favorable orientation of the slope. 6.2 Analytical Runout Assessment Runout assessment was used to predict the effects of future rockslides on the SR 20 highway, including the impact area, velocity, and deposit depth. Each of the three hazard source zones (Table 6.2) was considered separately. The results of section 6.1 provided an estimate of the failure mechanism, location, and volume of potential rock debris sources. Back-analysis of the November, 2003 rock avalanche runout (section 5.2) with DAN3D provided understanding of the rheology and estimates of apparent material properties of the runout path and failed mass. According to the recommendation of Hungr (1995), these back- calculated properties can be applied to the forward prediction of future events with reasonable confidence. Specifically, the objectives of the runout prediction analysis were the following: 1. Estimate the effects of future rock avalanche/rockfalls at Afternoon Creek on the SR 20 highway, including the following: a. Runout path, b. Flow depth, c. Deposit depth, d. Runout velocity. 146 6.2.1 Source Rank 1: Topp l ing and Rockfal l at S lope Crest Material that originates from the failure scarp crest has potential to enter the Falls Creek travel path due to the ridge topography of the slope. It is likely that any material that enters the Falls Creek path will travel as rockfall down to the elevation of SR 20 at the bottom of the slope. Construction of an embayment to protect the highway from future rockfall events was completed in February, 2006 (Figure 6.7). A thorough study of rockfall trajectory and energy at the base of Falls Creek was completed to design the embayment (summarized in section 5.2.2). To build on this, an investigation of the boundary of the Falls Creek rockfall source zone was performed using the 3-D rockfall simulation model PIR3D (developed by Mag-Informatique, France). This analysis was undertaken by Amandine Brosse from the Ecole Nationale des Travaux Publiques de l'Etat. PIR3D is a 3-D, lumped-mass rockfall simulation program. Relevant input parameters required by PIR3D for this simulation were topography, coefficient of restitution, variation angle, and initial fall height. The current slope topography was defined by the post- failure D E M at 5-meter resolution. The coefficient of restitution describes the energy loss at each bounce. The value for this parameter was chosen based on recommendations by the user manual for "hard bedrock", and by comparison with the calibrated CRSP model used for the embayment design. The variation angle describes the angle of rebound. It adds a random element to the simulation to approximate the behavior of an irregularly shaped block bouncing across uneven terrain. For this analysis the maximum variation angle was 10°. The angle of rebound for the bouncing point mass was equal to the angle of incidence plus or minus the variation angle. The variation angle at each bounce was selected by the program randomly between 0° and the maximum angle (10°). The initial fall height was assumed to be one meter for all blocks. For each simulation, the rockfall source was defined by a single line. A specified number of 'rocks' originated from evenly distributed points along the line. Rockfall simulations were run for 34 different source lines that transected the failure scarp crest. 100 rocks originated from each source line. During the simulations, some of the rocks traveled down the east side of the ridge into Afternoon Creek, while others traveled west of the ridge into Falls Creek. The boundary of the Falls Creek rockfall source zone (called the "boundary point") is the point which divides these two classes of simulated blocks (Figure 6.8). A l l of 147 the simulated rocks that originated east of the boundary fell into Afternoon Creek. These blocks did not reach SR 20. The results of the analysis are shown as a map indicating the Falls Creek rockfall source zone boundary line (Figure 6.9). The ridge topography was the primary control on the source zone boundary line. The results were insensitive to changes in the restitution coefficients. West of the boundary line is the Falls Creek rockfall source zone. Rocks that originate in this zone have potential to enter the Falls Creek travel path. The analysis showed that all rocks that enter the Falls Creek travel path will travel all the way to the embayment at the elevation of SR 20, which is consistent with the CRSP analysis performed by URS Corporation and Wyllie & Norrish Rock Engineers for the design of a rockfall protection embayment at SR 20 (2004). Figure 6.7 Completed embayment on Washington State Route 20 at Falls Creek in February, 2006. Photograph provided by WSDOT. 148 Figure 6 .8 Rockfall simulation with P I R 3 D , showing plan view and 3 -D view. Figure 6 . 9 Map showing the Falls Creek rockfall source zone boundary line. 149 6.2.2 Source Rank 2: Mid-Slope Failure Scarp A translational failure from the middle of the November, 2003 failure scarp can cause a rock avalanche similar to the November, 2003 event. However, due to the location of mid- slope failure volume, runout would be limited to the Afternoon Creek travel path. Therefore, the DAN3D model, calibrated by back-analysis (section 5.2), was used to assess the likely path, depth, and velocity of future rock avalanche debris. A baseline forward assessment was first made that incorporated the findings (in terms of the friction parameters and bulking ratio) of the Afternoon Creek travel path runout back-analysis (section 5.2). Then trial simulations were run to determine the sensitivity to increases in the source volume and reduction of the bulk basal friction angle. Input files for the DAN3D runout simulations were prepared from the post- November, 2003 D E M and the estimated source volumes derived from UDEC modeling (discussed in section 6.1). The source volume was created by gridding the thicknesses of the potential failure area with a triangulation with linear interpolation function using Surfer 8.0. Due to volumetric increase of the debris during fragmentation, a bulking ratio was applied to the estimated source volume. The bulking ratio estimated for the November, 2003 event was 1.35. Therefore, this same value was applied to the initial runout assessments. The runout path was created by subtracting the source volume (before the bulking ratio was applied) from the current topographic surface (i.e., the post-November 2003 DEM). A l l runout simulations used the frictional rheological model. The baseline runout assessment used the input parameter that produced the best results in the back analysis: Basal Bulk Friction Angle = 37°, and Internal Bulk Friction Angle = 40°. The simulations were run until the velocity of all particles approached zero. Three trial models were run. In Trial 1, the Basal Bulk Friction Angle was reduced to 34° - an arbitrary value. The purpose of the trial was to test i f a small reduction in the friction angle could cause the debris runout to intersect SR 20. Due to house-sized boulders that are common in Afternoon Creek today, but did not exist in the runout path prior to the November, 2003 events, it is likely that the Basal Bulk Friction Angle is significantly greater than the back analysis result (37°). It is unlikely that the Basal Bulk Friction Angle would actually be reduced in future runout events, however the reduced value was tested as a 'worst-case scenario'. 150 In Trial 2, the source volume was increased. The frictional parameters were returned to the initial values and the bulking ratio applied to the source volume was increased from 135% to 200%. The purpose was to test how a significant increase in the source volume affected the runout simulation results. The 200% bulking ratio meant that the maximum volume estimated for each source in the Failure Volume Assessment (section 6.1.3) was doubled for the runout simulation, once again testing a 'worst-case scenario'. In Trial 3, the Basal Bulk Friction Angle was reduced and the source volume increased simultaneously. The Basal Bulk Friction Angle was reduced to 34°. The source volume was doubled by applying the 200% bulking ratio. Table 6.3 summarizes the baseline and trial runout models for material originating at the middle of the existing failure scarp. In all models, no material entered the Falls Creek runout path because the entire source volume was below the Falls Creek - Afternoon Creek ridge. The material in the Afternoon Creek travel path did not reach the SR 20 highway in any of the rock avalanche runout simulations (Figure 6.10). Table 6.3 Summary of runout models of the source volume at the middle of the failure scarp. Hazard Volume #2 failure scarp midc lie Approx. volume reaching SR 20 Model basal friction angle (°) internal friction angle (°) Bulked source volume (*1000 m3) Falls Cr. Travel path (m3) Afternoon Cr. Travel path (m3) Baseline 37 40 407 0 0 Trial 1 34 40 407 0 0 Trial 2 37 40 603 0 0 Trial 3 34 40 603 0 0 In the baseline model, when the 135% bulking ratio and calibrated frictional parameters were used, the runout path was confined within the narrow Afternoon Creek valley walls. The bulk of the debris was deposited in the upper reaches of Afternoon Creek, at the base of the source zone. A thin layer of debris (less than 5m thick) ran out onto the Afternoon Creek fan. The leading edge of the deposit traveled approximately 550 meters in horizontal distance from the top of the source zone, and stopped more than 200 meters short of the SR 20 highway. The leading edge of the material came to an abrupt stop, decelerating over a very short distance. The maximum velocity reached at most areas of the travel path 151 was between 15 m/s and 25 m/s. The overall maximum velocity was slightly greater than 45 m/s (Figure 6.10). In the worst case scenario, trial model 3, when the 200% bulking ratio and the reduced Basal Bulk Friction Angle (34°) were used, the leading edge of the material traveled more than 100 meters farther than the leading edge of the baseline model. However this was still over 100 meters short of the SR 20 highway. The shape of the deposit in trial model 3 was basically the same as the baseline model, except a few meters thicker in all locations. The maximum velocity reached at many parts of the runout path increased to over 25 m/s. Such a large volume is unlikely to originate from this location, and the actual Basal Bulk Friction Angle is probably several degrees larger than assumed, yet the debris still did not reach the highway. The runout of trials 1 and 2 was greater than the baseline model, but less than trial 3. Therefore it is concluded that debris from the middle of the November, 2003 failure scarp source is unlikely to reach SR 20 via Afternoon Creek or Falls Creek, during extremely rapid rock avalanche runout (Figure 6.10). 152 700 600 500 400 300 200 100 5 m • Potent ia l source vo l ume — Mode l depos i t — Mode l t r iml ine —> SR 20 h i ghway S R 2 0 -100 -100 100 200 300 400 500 600 700 600 500 400 300 200 100 • . Potent ia l ' — ' source vo l ume M o d e l depos i t * • - Mode l t r iml ine SR 2 0 h ighway S R 2 0 700 600 500 400 300 200 100 M a x i m u m ' ve loc i ty (m/s) - M o d e l depos i t Mode l t r im l ine SR 2 0 h i ghway S R 2 0 -100 -100 700 600 500 400 300 200 100 ^ _ M a x i m u m ^ ™ ve loc i ty (m/s) Mode l depos i t * * - Mode l t r im l ine SR 2 0 h i ghway S R 2 0 -100 -100 -100 0 100 200 300 400 500 600 -100 100 200 300 400 500 600 Figure 6.10 DAN3D runout assessment of an Afternoon Creek rock avalanche originating from middle of the failure scarp, showing source volume depth (meters), deposit depth (meters), and maximum velocity. Benchmark model results (top); "worst-case" trial model 3 results (bottom). 153 6.2.3 Source Rank 3: Translat ional Failure at S lope Crest The DAN3D forward runout analysis used to assess the mid-slope failure scarp source was repeated for the failure scarp crest source zone. The analysis procedures and model configurations are equivalent to those used in the previous section (section 6.2.2). Table 6.4 summarizes the baseline and trial runout models for material originating at the crest of the failure scarp. Because this material forms the Falls Creek - Afternoon Creek ridge, there is potential for material to enter both runout paths. Table 6.4 Summary of runout models of the failure scarp crest source volume. Hazard Volume #3 failure scarp crest Approx. volume reaching SR 20? Model basal internal Bulked source Falls Cr. travel Afternoon Cr. friction friction volume path travel path angle (°) angle (°) (*1000 m3) (*1000 m 3) (m3) baseline 37 40 123 2 0 Trial 1 34 40 123 3 0 Trial 2 37 40 182 5 0 Trial 3 34 40 182 6 0 Material in the Afternoon Creek travel path did not reach the SR 20 highway in any of the rock avalanche runout simulations (Figure 6.11). In the baseline model, when the 135% bulking ratio and calibrated frictional parameters were used, the runout path was confined within the narrow Afternoon Creek valley walls. The bulk of the debris was deposited directly below the source zone near the middle of Afternoon Creek. There was a distinct ridge in the runout path which the debris flowed around. This ridge does not exist today, but did when the post-failure aerial L i D A R was collected in December, 2003; however most of the ridge was removed during a slope failure soon after the D E M data was collected. Although this ridge affects the model runout in the upper sections of Afternoon Creek, the effort to remove the ridge from the D E M was not made because it is not expected to change the effects of the debris runout on SR 20. In most locations of the Afternoon Creek channel, the debris is spread to less than 5 meters thickness. The leading edge of the debris traveled to the mouth of the steep-walled Afternoon Creek valley, approximately 450 meters in horizontal distance from the middle of the source zone, and over 200 meters short 154 of the SR 20 highway. The debris was able to accelerate as it fell more than 200 meters down the steep failure scarp. The maximum velocity reached in many parts of the failure scarp was over 35 m/s. The overall maximum velocity was greater than 45 m/s (Figure 6.11). In the worst case scenario, trial model 3, when the 200% bulking ratio and the reduced Basal Bulk Friction Angle were used, the leading edge of the material traveled nearly 100 meters farther than the leading edge of the baseline model. However this was still approximately 150 meters short of the SR 20 highway. The shape of the deposit in trial model 3 was basically the same as the baseline model, except due to the lower friction angle the front of the deposit was able to completely split from the rear of the deposit. The maximum velocity reached over much of the runout path was similar or slightly increased. Such a large volume is unlikely to originate from this location, and the actual Basal Bulk Friction Angle is probably several degrees larger than assumed, yet the debris still did not reach the highway. The runout of trials 1 and 2 was greater than the baseline model, but less than trial 3. Therefore it is concluded that debris from the described source is unlikely to reach SR 20, via Afternoon Creek, during extremely rapid rock avalanche runout (Figure 6.11). In the DAN3D analyses summarized above, 2% to 3% of the total source volume entered the Falls Creek runout path. A back analysis of rockfall in Falls Creek (URS and Wyllie & Norrish Rock Engineers 2004) showed that this debris is likely to impact the highway embayment. The potential for future rockfall in Falls Creek is discussed in section 6.2.1. 155 7 0 0 6 0 0 5 0 0 4 0 0 2 0 0 1 0 0 - 1 0 0 Potent ia l source v o l u m e • Mode l c o l u m n Mode l depos i t M o d e l t r im l ine mm SR 20 h i ghway SR20 7 0 0 6 0 0 5 0 0 4 0 0 3 0 0 2 0 0 100 M a x i m u m ve loc i ty (m/s) M o d e l depos i t • • • M o d e l t r im l ine SR 20 h i ghway SR20 - 1 0 0 - 1 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 - 1 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 6 0 0 5 0 0 4 0 0 3 0 0 2 0 0 1 0 0 Potent ia l source v o l u m e | • Mode l c o l u m n M o d e l depos i t Mode l t r im l ine | mm SR 20 h i ghway SR20 7 0 0 6 0 0 5 0 0 4 0 0 3 0 0 2 0 0 1 0 0 - 1 0 0 M a x i m u m ve loc i ty (m/s) M o d e l depos i t • • - M o d e l t r im l ine mm SR 20 h i ghway SR20 - 1 0 0 - 1 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 - 1 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 Figure 6.11 DAN3D runout assessment of an Afternoon Creek rock avalanche originating from the failure scarp crest, showing source volume depth (meters), deposit depth (meters), and maximum velocity. Baseline model results (top); 'worst case scenario' trial model 3 results (bottom). 156 6.4 Hazard Assessment Summary The runout models show that it is unlikely for rock avalanche debris originating from the crest or middle of the failure scarp to reach the SR 20 highway via the Afternoon Creek travel path during a rapid runout event. However, the loose debris deposited in Afternoon Creek does increase the possibility of debris flows which have closed the SR 20 highway in the past. Each failure in Afternoon Creek replenishes the supply of loose materials which may be entrained in future debris flows. Since debris flows from Afternoon Creek have affected the highway several times in the past (see section 2.3), an investigation of past debris flows and the relationship between the coarse rock avalanche deposit and debris flows is recommended for future work. There is potential for the crest of the failure scarp to retrogress either due to toppling of individual columns or by planar or wedge sliding on the joint set A and B surfaces. Any failure of the upper scarp crest will send debris towards the Afternoon Creek travel path. However any material that originates west of the Falls Creek rockfall source zone boundary line (Figure 6.10) has potential to enter the Falls Creek travel path. Any rock that enters the Falls Creek travel path is likely to reach the SR 20 embayment because the slope is steep and the travel path is composed of hard, smooth bedrock. 157 7 DISCUSSION OF TOOLS AND METHODS This thesis was primarily an analysis of slope instability and runout at Afternoon Creek, Washington. The secondary purpose was to provide guidelines for a total slope analysis linking data collection using LiDAR, with initiation analysis, using UDEC and 3DEC, and runout analysis, using DAN3D. Chapter 7 is a discussion of the benefits and limitations of the utilized tools. It is meant to provide practical recommendations based on experience gained during this project. 7.1 Terrestrial Laser Scanning (LiDAR) An accurate characterization of the discontinuity orientation, spacing, and persistence at the Afternoon Creek slope was required input to the numerical analysis programs. Traditional methods of collecting rock mass characterization data, such as scan-line surveying or outcrop mapping, were not feasible because the critical slope exposure was steep, high, hazardous, and inaccessible. Therefore a terrestrial-based L i D A R scanner was used to collect the rock mass characterization data remotely. 7.1.1 L i D A R Benefits 1. The only feasible method: When the rock slope of interest is inaccessible or dangerous, remote sensing methods provide the only feasible means to collect rock mass and discontinuity data. This was the case for the Afternoon Creek rock slope, for which the orientation of key joint sets was obtained through L i D A R point cloud analysis. It was more accurate to measure joint spacing and persistence from point clouds than single, oblique photographs. 2. Time savings: A complete data set can be collected and processed significantly quicker with the L i D A R survey and point cloud analysis as opposed to traditional scan-line and window surveys. A typical laser scan of the Afternoon Creek slope, including setup, took less than 60 minutes. A simple analysis of the point cloud also took less than 60 minutes. A full mapping survey of the slope, consisting of several scanlines, generally takes an entire day. Additional time would then be required to manually process and plot the data. 1 5 8 3. Objective, reproducible data record: A L i D A R survey creates a digital 3-D model of the target. This model is a data record that can be revisited months or years later. Questions about the geometry of the slope surface, long after the field work has been completed, can be answered by inspection of the digital point cloud rather than requiring an additional site visit. 4. Automation of analysis procedures: Some of the point cloud analysis steps, such as recognizing joint planes, calculating orientations, and calculating exposed persistence, can be fully or partially completed automatically by the analysis software. These automated procedures save time and can find joint surfaces that would otherwise be overlooked by the analyst. The potential exists to automate other aspects of the analysis as well. It should be stressed though that the automatic procedures should only be used as tools; they can not replace the judgment or expertise of the analyst. Each step involving an automated procedure should be carefully inspected and modified as necessary. 7.1.2 L iDAR Limitations 1. Survey locations: Survey stations must be positioned at a suitable distance from the target, and the line-of-sight between the survey station and target must be unobstructed. The effective range of the Optech ILRIS-3D scanner for the case study presented above was about 600-m; however we suspect that useable data can be collected from distances of up to 800-m if scans are attempted at the highest possible resolution, and the atmosphere between the scanner and target is clear. This maximum range also depends on the target reflectivity; the relationship is described by Optech (2002). Also it is important that the survey stations are not too close to the target. This was a problem for many of the scans where a close proximity of the instrument to the rock face was forced by the narrow confines of Afternoon Creek, which then did not allow for the point clouds to overlapped, and therefore a single slope model could not be formed. 2. LiDAR scanning can not replace actual observation of a rock slope: A 3-D point cloud is useful for making orientation and distance measurements; however it can not be used to 159 describe the rock strength, roughness, weathering characteristics, seepage conditions, or infilling/aperture of joints. Manual field observations are required in addition to L i D A R scanning to determine the nature and origin of the discontinuities, and to adequately characterize the rock mass. 3. Sample Bias: Discontinuities which do not form sufficiently large exposed surfaces are not sampled with the L i D A R methods used in this study. This may include highly-persistent, important cracks that would easily be recognized and sampled with traditional techniques, however are invisible in the L i D A R point cloud. Exposed surfaces that are small relative to the point spacing and mesh size are not sampled. Additionally, surfaces that strike sub-parallel to the line-of-sight of the scanner tend to reflect few laser strikes; therefore joint sets that strike parallel are poorly sampled, while sets that strike perpendicular are well sampled, introducing a bias to the final data. This point is illustrated by Figure 3.6, showing rose diagrams of automatically-generated patches found in four different point clouds of the failure scarp. Joint set A surfaces were preferentially recognized in the scan from survey station 2 because this joint set was orthogonal to the scanner line-of-sight. Joint set B surfaces were preferentially recognized in the scans from survey stations 6 and 7. This bias can be removed by scanning the target from all possible angles and analyzing all of the scans (either individually or by aligning the scans into a single point cloud). 7.1.3 L iDAR Recommendat ions 1. Accurately measure the orientation of the LiDAR scanner for each survey: For point cloud analysis, the orientation of surfaces in the point cloud is based on the line-of-sight and tilt of the scanner. Any error in the recorded scanner orientation will be projected to all of the joint orientation measurements derived from the point cloud. 2. Check the digital photograph quality: The Optech ILRIS-3D scanner records a digital photograph with the point cloud. The included camera does not automatically adjust to 160 optimum settings for photograph quality. The photographs are important for visualization of the rock slope, as well as for estimating joint persistence and spacing. 3. Consider the weight and bulk of the instrument when designing a LiDAR survey: The Optech ILRIS-3D laser scanner can be carried by one person on foot; although due to its weight and bulk, carrying the scanner across rough terrain (e.g. bouldery, rock avalanche debris) can be very challenging as was the case with the Afternoon Creek rock slope survey. In such cases, it may require two people and longer setup times to move equipment and perform the survey. 4. Consider the weather when designing a LiDAR survey: Clouds, fog, smoke, or haze in the air between the instrument and the survey target do affect the results of the survey. This effect depends on the survey distance, and density of the cloud. During the Afternoon Creek survey, only one survey was completed during a period of light rain; however as the setup point was only 100-m from the rock face, little to no affect was detected in the survey results. 5. The area of interest should be scanned from several different angles: The laser scanner is a line-of-sight instrument, meaning that the position of the first object in the laser's path is recorded. Therefore small (and large) variations in topography, and vegetation create shadows - i.e., areas where no data is collected - in the resulting point cloud. Additionally, surfaces that strike sub-parallel to the line-of-sight of the scanner tend to serve as poor reflectors; therefore joint sets that strike parallel are poorly sampled, while sets that strike perpendicular are well sampled introducing a bias to the final data. The area of interest should be scanned from several different angles to remove this bias and to collect data that would otherwise be masked by shadows. 6. Overlapping point clouds should be aligned to form a single model of the slope: The data analysis phase of the investigation is simplified and more comprehensive when a single model of the entire slope is used. Joint surfaces are truncated at the boundary of each point cloud scan. The number of truncated discontinuity surfaces can be minimized by aligning overlapping point clouds. Estimates of discontinuity persistence and spacing are most 161 accurate when the entire rock slope is considered. Note that several scans from different angles are required to align overlapping point clouds. 7. Inspect and edit the automatic discontinuity characterization results: Automatic routines for identifying joint surfaces and estimating persistence are quick, objective, repeatable and capable of finding joint surfaces that otherwise are not obvious; however they can not replace the expertise of a trained engineer or geologist. The 'patches' that are automatically generated by programs like SplitFX™ must be inspected and edited as necessary. Some patches may need to be altered, others will need to be deleted or added. 7.2 Numerical Modeling Tools The models used in this research were not intended to recreate the exact conditions that exist in the Afternoon Creek slope. Instead they were used as a virtual laboratory where hypotheses concerning the slope geometry or operative failure and runout mechanisms could be tested. The total slope analysis procedure, using these tools, was completed once for back analysis of the November, 2003 event, and then repeated for a forward analysis of future events at Afternoon Creek. The failure initiation processes were investigated with UDEC and 3DEC, resulting in increased understanding of the failure volumes, mechanisms, and controlling features. These guided the analyses of rock avalanche runout with DAN3D. 7.2.1 U D E C UDEC was the most extensively used modeling tool for this study. An idealized, simple model was calibrated by back-analysis of the November, 2003 event and then used to assess the possible location and volume of future slope failures at Afternoon Creek. The simple models used in this study were not used to make confident predictions of future events. True predictions would require a more complex, highly calibrated model including accurate estimates of the spatially variable material parameters, pore water pressures, and structures. In this case, the only controls used to calibrate the back-analysis model were the shape and location of the sliding surface, and the existence of tension cracks at the failure 162 scarp crest. Material and geometric parameters were roughly calibrated so that the failure mechanism and sliding surface geometry qualitatively matched the interpretation of the actual event. The U D E C models were most useful for comparing competing hypotheses of the failure mechanism, material parameters, or geometric parameters. The understanding gained by these numerical experiments aided the assessment of future rock avalanche source volumes. The knowledge and understanding gained through the U D E C modeling either provided secondary insights or confirmed hypothesis developed by studying the cross- sections used to build the models. The UDEC modeling also provided an advantage over limit equilibrium methods, in that the failure surface did not need to be predefined. However this was only partially true in this study as the location and shape of the sliding surface was largely pre-determined by the location of user defined discontinuities. Fracturing of the rock mass can not be simulated by UDEC; therefore fractures must be input prior to running the model, and fully persistent discontinuities must be used. A hybrid finite/ discrete element numerical code could be used to investigate the propagation of fractures, however with considerable additional complexity. The two-dimensional U D E C code was inadequate for exploring inherently three- dimensional questions, such as those concerning the movement direction and interaction of slope features. One large source of uncertainty was the selection of a cross-section that represented the entire slope. It was challenging to determine the optimum cross-section location and orientation when the failure mechanism and mass movement direction were unknown. This problem was compounded by the variable shape of the failure volume. This limitation of U D E C was addressed by using 3DEC to answer the three-dimensional questions. 7.2.2 3 D E C Although it was difficult and time consuming to build the initial 3-D geometry, the 3DEC study was worthwhile because it was uniquely able to simulate the 3-D movement and interaction of the slope. The modeling procedure and commands, for 3DEC are basically equivalent to those of UDEC. Therefore with an adequate understanding of UDEC, 3DEC 163 can be quickly learned. Although there is no reason to be intimidated by 3DEC it does have some additional complexities and deficiencies which make it cumbersome to use. Of course, model runtime is significantly longer with 3DEC than with the UDEC. The most difficult and time consuming steps of 3DEC modeling were the generation of the initial 3-D geometry and interpretation of the results. 3DEC does not seem to be designed for generating models of natural topography. Simple shapes can be quickly constructed, but there is no good way to make complex surfaces from a digital elevation model. The pre-processor program PGEN was used in this study, but it was difficult to use, and it created poorly shaped blocks. Many of the blocks created by P G E N were long and wedge-shaped; these were problematic to cut and divide with a finite difference mesh. Since the interfaces between these construction blocks can not be hidden, they interfere with plots of the results. There is no way to distinguish the construction joints from the important discontinuities in 3-D plots or cross-sections. Interpretation of the results was difficult because of poor graphics, limited display options, and numerous program bugs. 3DEC was most efficiently used in conjunction with the U D E C models. Since 3DEC is more time consuming to build, run, and interpret it was beneficial to use input parameters that were pre-calibrated during the UDEC analysis. However, in instances where all of the objectives of a project can be achieved using UDEC, then the time and effort required to build a related 3DEC model is not really justified. 7.2.3 DAN3D A DAN3D model was calibrated by back analysis of the November, 2003 event. The calibrated model was then used in a forward analysis to estimate the probable runout path of future rock avalanches at Afternoon Creek. The most important benefit of using a 3-D runout analysis tool in this study was that the runout path did not need to be pre-determined. This facilitated the study of the topographic control of the Afternoon Creek-Falls Creek ridge on runout. It was especially important in this case since a small amount of the rock debris traveled down the back side of the ridge and actually impacted the highway. This complex, multi-directional flow could not be adequately simulated with a similar 2-D model. DAN3D 164 was an important tool for determining the likelihood of future rock avalanche debris falling on the Falls Creek side of the ridge. DAN3D is designed to simulate rapid flows. The Afternoon Creek rock avalanche probably behaved as a rapid flow in sections of the runout path; however at the onset of failure, before the jointed orthogneiss fully fragmented, it probably behaved as a cohesive mass. Therefore in the source area, near the ridge, the material behavior did not match the fluid flow assumptions used by DAN3D. As such, DAN3D may not have accurately simulated the rock mass movement at the ridge. DAN3D simulations showed that the source rock mass spread over the ridge, towards Falls Creek, immediately when the simulation was started. In contrast, analysis of the failure initiation with UDEC and 3DEC showed that all of the rock mass slid away from the ridge towards Afternoon Creek. More confidence was given to the U D E C and 3DEC results since these codes are designed to model failure initiation, and the limitations of DAN3D are understood. It served as an example of how the limitations of one analysis tools can be overcome by using several different tools in combination. 7.3 Total Slope Analysis Method The Total Slope Analysis methodology facilitated the hazard characterization study by providing a framework for coupled assessment of slope failure initiation and runout, using several different numerical modeling codes. Each step provided input to subsequent parts of the analysis. The data collection program was optimized for the numerical modeling application. The failure initiation analysis provided volume estimates and described the character of failure, which guided the runout analysis. The primary benefit of the coupled analysis was that the result of each step (in terms of constrained parameters, and insight) built on previous results as well as provided input to subsequent parts of the analysis. Three different numerical modeling tools were used to complete the analysis, each with its own uncertainties, assumptions and limitations. The iterative Total Slope Analysis provided a way to overcome the limitations and uncertainties of each individual program and maximize our understanding of complex slope processes. 165 In general, limitations of the method were caused by combining three different numerical codes to complete the analysis. The results of the U D E C and 3DEC models, such as failure stages, and volumes, could not be directly input into DAN3D. Considerable interpretation of the failure initiation results and manipulation of DAN3D input files was necessary before running DAN3D simulations. Additionally, it took considerable time to become familiar with each code and set-up each model. A single hybrid code which is able to consider the entire slope process from failure to runout could overcome this limitation. Completing the Total Slope Analysis using three numerical modeling tools required considerably more time and effort than doing a single analysis and assuming input parameters; however it also produced more certain results. The Total Slope Analysis is best suited for cases which warrant extra time and effort. The Total Slope Analysis procedure is not necessary in simple cases where assumptions can confidently replace the results of expensive, time-intensive numerical models. 166 8 C O N C L U S I O N S A N D R E C O M M E N D A T I O N S 8.1 Conclusions The 750,000 m 3 rock mass that failed in November 9, 2003 composed a ridge that divided the Afternoon Creek and Falls Creek watershed. The primary failure mechanism of this mass was planar sliding on a highly persistent joint set that dips to the east-southeast at approximately 50° (called joint set A in this study). The failure mass displaced directly down-dip into Afternoon Creek. A second highly persistent joint set that dips steeply to the northeast (joint set B) provided lateral release for the massive sliding blocks. It was originally hypothesized that failure occurred in two stages of approximately equal volume; however subsequent analyses suggested that the failure occurred as a single extremely-rapid stage involving the entire volume. The small percentage of material that traveled down the Falls Creek travel path and damaged the roadway can be classified as rockfall. Dilation of the failure mass prior to and during failure, and loss of lateral support at the failure scarp following failure increased the occurrence of rockfall. Some of this rockfall entered the Falls Creek travel path due to the ridge topography of the slope. No evidence was found of structurally-controlled failure that displaced towards Falls Creek. A concavity in the slope topography allowed joint set A discontinuities to daylight and provided kinematic freedom for planar sliding. The concavity was formed by gradual differential weathering since deglaciation of a highly-fractured region of the slope (called Zone B). The highly-fractured region exists near the intersection of two important shear zones (called the Tower and Base shear zones). Although water pressures were not considered in the investigation, it is likely that the slope failure was triggered by high discontinuity water pressures caused by record rainfall during the month before the failure. The runout of the November 9, 2003 rock avalanche debris in Afternoon Creek was simulated with the dynamic analysis model, DAN3D (McDougall and Hungr 2004). The calibrated model was able to adequately reproduce the runout distance, travel path and deposit morphology using the frictional rheology. Differences between the actual and simulated events were caused, in part, by (1) individual features in the debris that were able to influence the direction and behavior of the flow, (2) the use of single bulk friction 167 parameters for the entire flow path, and (3) immediate application of full dispersive earth pressures. Future rock slope failures have the potential to intersect the SR 20 highway at Afternoon Creek as a rock avalanche, or at Falls Creek as rockfall. Numerical models and observation of existing slope features suggest that there are two potential source zones. The most threatening source zone is located at the crest of the existing failure scarp. The failure mechanism here may be toppling of individual columns (approximate volume of 1,000 m 3 per column) or sliding towards the failure scarp on existing discontinuities (maximum volume of 100,000 m3). Either failure mechanism has potential to cause rockfall to enter the Falls Creek travel path. It is likely that any rock that enters the travel path will fall all the way to the SR 20 embayment at the base of the slope. The second source zone includes the massive blocks that remain in the middle of the existing failure scarp (maximum volume of 300,000 m3). These have the potential to fail similar to that of the November, 2003 event, i.e., by sliding on joint set A discontinuities. Debris from this source zone would be restricted to the Afternoon Creek travel path. Runout assessment with the calibrated DAN3D model shows that rock avalanche debris in Afternoon Creek from either of these sources is unlikely to intersect SR 20 during extremely rapid motion. A secondary purpose of the research was to evaluate the state-of-the-art data collection and numerical modeling tools utilized during the investigation as part of the Total Slope Analysis. Rock mass characterization data was collected using a Light Detection and Ranging (LiDAR) scanner because traditional methods were limited by the steep, high, hazardous, and inaccessible slope exposure. This method was invaluable for providing the rock mass description data necessary for numerical modeling. Benefits of using L i D A R included: (1) it allowed measurement of discontinuity orientation, spacing, and persistence at an inaccessible slope exposure; (2) it allowed time savings for both data collection and processing compared to traditional methods; (3) it produced an objective, and reproducible data record; and (4) it allowed some of the data analysis procedures to be automated. However, L i D A R is not able to replace traditional field observations because many important rock mass characteristics, such as roughness, seepage, infilling, rock strength, and weathering, can not be extracted from the point clouds. Additionally, L i D A R scanning is limited by the location of survey stations. The stations must be positioned a suitable distance 168 from the target, and the line-of-sight between the survey station and target must be unobstructed. Hypotheses concerning the slope geometry and operative failure mechanisms were tested with numerical model simulations using the distinct element codes U D E C and 3DEC. The model results helped provide additional understanding to that which was gained from critical inspection of cross-sections and rock mass characterization data during model construction. The models also allowed for competing hypotheses to be compared, and for the mechanism and volume of future slope failures at Afternoon Creek to be assessed. 3DEC was uniquely able to simulate the 3-D movement and interaction of the slope, and answer questions that were left unresolved by the 2-D analysis. With an adequate understanding of UDEC, 3DEC can be quickly learned; however it must be noted that 3DEC is significantly more cumbersome to use than its 2-D partner. Model generation, especially for natural slope topography, with 3DEC can be exceedingly time-consuming, and post-processing is made difficult by poor graphics, limited display options, and numerous program bugs. 8.2 Recommendations for Further Work Several questions concerning the Afternoon Creek case study remain open and provide the grounds for additional work. These include the following: 1. The Afternoon Creek rockslide was preceded by record-setting rainfall. Therefore, naturally, it has been hypothesized that uncommonly high water pressures triggered the collapse of the slope; however this hypothesis was never tested. This study focused on structural and topographic controls to failure, and therefore largely ignored the influence of water pressures on slope stability and failure. Further study of the Afternoon Creek slope should investigate the role of water pressures as a triggering mechanism in a fractured crystalline rock mass environment. 2. The 'virtual scanline technique' was used to estimate the average joint set spacing from the 3-D LiDAR-derived point clouds; however the method was not extensively tested. The method was used out of necessity because the joint set spacing is an important input to the 2- 169 D and 3-D distinct element models. This method (or a different automatic or semi-automatic method) should be further developed and validated for extracting joint set spacing information from a 3-D point cloud model. Additionally, automatic or semi-automatic methods for estimating the discontinuity persistence from a 3-D point cloud model should be developed. 3. The Three-Dimensional Distinct Element Code (3DEC) could be far more useful for rock slope studies i f the 3-D model generation capabilities were improved. At its current state, it is time consuming and cumbersome to build a 3-D model of natural slope topography with 3DEC. The pre-processor program PGEN, designed to build irregular 3-D shapes, creates thin wedge-shaped blocks that are difficult to cut and zone. A pre-processor program that is capable of building irregular topography and optimizing the shape of construction blocks should be developed. 4. This study attempted to investigate the probable location and size of future slope failures at Afternoon Creek. The equally important question of when failure will occur was not considered. A detailed study to the slope failure triggering mechanisms would be useful for making temporal predictions for future failures at Afternoon Creek. 5. A long history of debris flows covering the highway at Afternoon Creek and Falls Creek Chute shows that debris flows are more likely to impact SR 20 in the future than a highly mobile rock avalanche originating from Afternoon Creek. Although the November, 2003 rock avalanche in Afternoon Creek did not directly affect the highway, it is hazardous because it replenished the supply of loose debris in Afternoon Creek. Re-mobilization of the debris may be possible. Therefore further study should investigate past debris flows and the relationship between the coarse rock avalanche debris and debris flows at Afternoon Creek. 6. Rockfall in the Falls Creek travel path is the most likely hazard; therefore the source zone at the failure scarp crest should be studied in detail. This study should include detailed characterization of the rock mass in the source zone and investigation of toppling and sliding at the failure scarp crest. 170 7. A regional scale study that compares Afternoon Creek to surrounding slopes may be useful for anticipating the location of future rock slope failures in the SR 20 corridor. 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University of Toronto Press, Vol.3. 178 A P P E N D I X A : S C A N L I N E S U R V E Y D A T A Scanline survey 1: Zone 2 from the Afternoon Creek debris (location shown In figure 3.4) i Orientation Position (cm) Strike Dip DDR Persistence (ft) Aperture (mm) Filling Roughness JRC Seepage Joint set Comments Tape 1 orientation plunge: 32; trend: 135 180 25 48 115 30 1,2 US 7 290 50 52 142 20 1,2 us 7 A 400 0 60 90 20 <1 US 7 530j 220 46 130 20 <1 us/ss 10 A 640, 300 36] 30 20 <1 US/SS 10 670 301 37 31 20 <1 SS 680 302 35 32 20 <1 ss 820 44 47] 134 10 <1 soil A 1230 15 87 105 80 10 soil P S 2 dip change to 60 higher in slope 1470 55 70 145 20 soil S R 12 A open, jagged 1650 292 34 22 10 1 soil PS 4 2270 19 73 289 UR 12 C 2420 194 90 NA 150 50 soil/moss P S 5 Stepped on 3 meter scale 2520 220 80 130 15 wide soil us A 2620 340 73 70 100 wide usoil 5 2770 205 80 115 100 wide 2985 40 85 130 100 PS 4 3020 41 80 131 10 PS 4 A 3080 50 71 140 100 >25 soil/veg PS 5 A obvious joint set 3330 51 69 141 100 PS 7 A obvious loint set "3460 49 70 1391 10 PS 7 A obvious joint set 3760i 62 56 1521 100 2 soil/moss PS yes A 1 obvious joint set 3820 4080 4080 61 63 37 57 55 73 151 153 127 10 50 20 2 3 PS US " A A - 5270 30 69 120 150 150 angular RR fresh fracture, no surface weathering 5880 200 76 290 200 150 RR/ sand Pslick 1 C Rake is 32deg with strike 5880 51 61 141 250 A 5990 50 61 140 250 10 angular RR PS 4 A Apparently like plane A 6060 49 61 139 100 150 angular RR PS 4 A 6130 52 61 142 100 150 angular RR PS 4 A 6380 51 61 141 100 150 angular RR PS 4 A Tape 2 orientation olunae: 39: trend 135 6380 192 72 282 100 2 (RR/ sand PS 3 C fractured zone, offset bv station 6420 fracture 6420 82 50 172 50 75 RR LPR 8 6770 200 57 290 10 1 none RR PS 2 C 6830 201 57 291 100 75 PS 6 C ^ a n y small dilated fractures here, not included 6890 80 70 170 10 75 RR UR 8 7210 205 65 295 150 75 RR P S 2 C 7700 209 50 299 150 75 RR PS 2 Joint set properties - Scanline survey 1 Set Napp-1 N-1 L(m) Average Persistence (m) spacing (m) X (degree s) Using Terzaghi correction: N = Napp/sinx. Where x = (angle b/w rock face and strike of joint) A(avg. strike= 54) C(avg7 strike= 201) 17 5 17 61 22 3.6 81 66 6 49 34 8.2 Scanline survey 2: Zone 3 from the Afternoon Creek debris Orientation Position (cm) Strike Dip DDR Persistence (ft) Aperture (mm) Filling Roughness JRC Seepage Comments Tape 1 orientation plunge: 26; trend 160 360 60 46 150 200 20 soil S R 11 ves A moss covered 840 54 46 144 100 10 soil 11 ves A 930 50 79 140 10 10 gtz, none Pslick 3 A rake - 3 0 in direction of strike 1090 56 60 146 100 30 soil S R 11 A Plane A 1270 57 60 147 10 10 soil SR 11 A Plane A 1430 1 55 42 1 145 50 30 soil ISR 11 IA Plane A Joint set properties - Scanline survey 2 Set Napp-1 N-1 L(m) Average Persistence (m) spacing (m) X (degree s) Using Terzaghi correction: N = Napp/sinx. Where x = (angle b/w rock face and strike of joint) A (avg. strike= 54) 5 5 11 24 2.2 106 Abbreviations: RR-rock rubble; US-undulating smooth; UR-undulating rough; SS-stepped smooth; SR-stepped rough; PS-planar smooth; Pslick-planar slickensided 179 Figure A l . Photograph of scanline 1 from position 0 to 2000 cm. 180 A P P E N D I X B : L i D A R P O I N T C L O U D S A N D P H O T O G R A P H S Four Lidar scans were used in the analysis: 2/1, 6/1, 7/1, 7/2 (see Table 3.2). The subsequent figures show the point clouds and digital images collected during each scan (figure B1.B2, B3, B4). Figure B l . Digital image and 3-D point cloud of scan 2/1. Scan area is outlined in red on the digital photo. Figure B2. Digital image and 3-D point cloud of scan 6/1. Scan area is outlined in red on the digital photo. 181  A P P E N D I X C : V E R I F I C A T I O N O F M E T H O D S C. 1 Discontinuity Orientation The purpose of this section is to compare joint surface orientation estimates derived from the following two techniques: 1. Hand measurements with a brunton compass 2. Joint surface 'patches' in a 3-D point cloud Hand measurements were made during joint survey 1. A portion of scanline 1 that overlaps with a 3-D point cloud was considered in this example (Table CI). Table C. 1 The 10 meter portion of scanline survey 1 considered in this example. Scanline survey 1: Zone 2 from the Afternoon Creek debris (location shown In figure 3.4) Orientation Position (cm) Strike Dip DDR Persistence (ft) Aperture (mm) Filling Roughness J R C Seepage Joint set Comments 3020 41 80 131 10 P S 4 A 3080 50 71 140 100 >25 soll/veg P S 5 A obv ious ioint set 3330 51 69 141 100 P S 7 A obv ious joint set 3460 49 70 139 10 P S 7 A obv ious joint set 3760 62 56 152 100 2 so i l /moss P S ves A obv ious ioint set 3820 61 57 151 10 P S A 4080 63 55 153 50 2 A Point cloud 3/4 overlaps the portion of the scanline survey shown in table CI . Discontinuity dip and dip direction were estimated from this point cloud. The approximate spacing of points in the 3-D point cloud is 3 cm. The following procedure was followed to estimate the orientation of joint surfaces: 1. Orient the point cloud based on scanner position: Trend: 230° Plunge: 22.5° back Tilt: 5° right down. 2. Edit the point cloud by removing points not associated with the rock face. 3. Create the mesh. Mesh spacing = 0.15, corresponding to -33 points/grid cell. 4. Manually Insert patches onto joint surfaces 5. Export the orientation of the patches It was only possible to insert patches onto discontinuities that create broad joint surfaces. Discontinuities that are expressed on the surface by a lineation could not be sampled with this method. Eight patches were inserted onto four joint surfaces. The joint surfaces are roughly parallel and closely spaced (Figure CI). 183 Figure CI . A9 point cloud showing inserted patches. The average orientation of joint set A in Zone 2 estimated by the point cloud patch method was (dip direction/dip) 159/52 compared to 144/60 estimated by the scanline survey- JS1 (Figure C2; C3). The point cloud patch method was applied to only a few joint surfaces in a section of Zone 2 that was sampled by both the scanline survey and the terrestrial laser scanner. The 3-D point cloud was referenced to true north based on the line-of-sight, and tilt of the scanner measured with a Brunton compass. Any error in this measurement was projected to the orientation estimates made from the point cloud. A second source of error may be that the automatically generated patches do not accurately represent the orientation of the joint surface. This is possible because the patch orientation is based on very few point reflections in this example. There are few L i D A R point reflections from the critical joint surfaces because the joint surfaces were approximately parallel to the line-of-sight of the scanner. 184 N = 15 Figure C2. Stereonet showing joint surface orientation estimated from the two methods - point cloud patch method (blue triangles); and hand measurements (black circles) from table B l . Equal Area (Schmidt) I I 18E 16E 14E 12E 10E 8E 6E 4E N=53 Figure C3. Comparison of point cloud 3/4 patches (blue triangles) with entire scanline survey in Zone 2. 185 C.2 Discontinuity Set Spacing The purpose of this section is to compare joint spacing estimates derived from the following two techniques: 1. Virtual scanline technique - estimating spacing from a 3-D point cloud (described in section 3.3.3); 2. Standard scanline technique with the Terzaghi correction (described in section 3.2.3). The virtual scanline technique was used to estimate the average spacing of joint set A discontinuities in point cloud 3/4. This point cloud overlaps a 10-meter section of the Zone 2 scanline survey (Table CI). The following procedure was followed to estimate the spacing using the virtual scanline technique (Figure C4): 1. Find patches a. Mesh spacing = 0.2 b. Min. neighbor angle = 7° c. Min. patch size =10 2. Average orientation of joint set: n = <-0.73, -0.09, 0.68> 3. Insert line: Point 1 (6.6, 35.4, -6.8); Point 2 (-3.2, 44.0, 1.5) 4. Scan line properties 5. Length; L = 15 meters 6. Direction;/=<0.63,-0.56,-0.54> 7. Count the number of patches (N) that intersect the scan line. N=7 8. Calculate the spacing (s): l*n= I n cos0 I = n = 1 ; Therefore \ — COS 0 = 0.78 ; L * cos 6 s = N> s = 1.1 m 186 Figure C5 . Scanline survey JS1. T h e f o l l o w i n g p r o c e d u r e w a s f o l l o w e d to e s t ima te the s p a c i n g f r o m the s c a n l i n e s u r v e y da t a ( F i g u r e C 5 ) u s i n g the T e r z a g h i c o r r e c t i o n : 1. C o u n t the n u m b e r o f j o i n t s that c r o s s the l i n e ( N a p p ) . N a p p = 7 2. C a l c u l a t e the d i s t a n c e from the f i r s t to last i n c l u d e d j o i n t ( L ) : L = 10 .6 m e t e r s 3. D e t e r m i n e the a n g l e (8) b e t w e e n the s t r i k e o f the j o i n t s a n d the s t r i k e o f the r o c k f ace . 0 = 81° 4. P e r f o r m the T e r z a g h i c o r r e c t i o n : ^ _ Napp_ N = 7 sin 9 5. C a l c u l a t e the a v e r a g e s p a c i n g : S = — — S=1.8 meters N-\ 187 A P P E N D I X D : S P A C I N G A N D P E R S I S T E N C E C A L C U L A T I O N S D. 1 Average Joint Set Spacing Joint set spacing was estimated from the point clouds with the virtual scanline technique. Each section of Table D . l shows the joint set spacing calculations for a particular joint set on an individual, virtual scanline. D.2 Average Joint Set Persistence The joint set persistence (or exposed persistence) was estimated with three different methods. Table D.2 is a record of the persistence estimated from photographs. Table D.3 is a record of exposed persistence estimated by direct measurement on the 3-D point cloud. Table D.4 shows the calculations used to estimate exposed persistence based on a relationship with the patch area. 188 Table D. 1 Virtual scanline technique calculation tables. J o i n t s e t A j P o i n t c l o u d 2/1 j P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x < i> y c j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 - 0 . 3 0 . 6 s c a n l i n e p o i n t 1 - 2 8 4 2 7 0 . 8 1 2 8 s c a n l i n e p o i n t 2 - 3 5 8 1 4 1 . 8 2 1 4 . 3 s c a n l i n e d i r e c t i o n < l> - 0 . 4 U _ I 0 7 0 . 5 s c a n l i n e l e n g t h ( L ) 1 7 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 6 I n 0 . 2 3 | A v e r a g e s p a c i n g ( L T n 7 N ) 7 m e t e r s J o i n t s e t A - a t z o n e B P o i n t c l o u d P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n <n> 0 . 7 - 0 . 3 0 . 6 s c a n l i n e p o i n t 1 - 8 2 . 8 - 7 7 . 7 - 1 . 2 1 s c a n l i n e p o i n t 2 - 9 2 . 9 - 1 6 3 0 . 9 1 8 s c a n l i n e d i r e c t i o n <l> 0 . 1 1 0 s c a n l i n e l e n g t h ( L ) 8 5 . 7 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 3 P o o r q u a l i t y I n 0 . 2 3 I A v e r a g e s p a c i n g ( L ' l - n / N ) 7 m e t e r s J o i n t s e t IB P o i n t c l o u d 6 /1 P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 0 . 5 0 . 5 s c a n l i n e p o i n t 1 - 1 4 0 - 2 4 3 - 1 4 . 6 s c a n l i n e p o i n t 2 - 1 2 9 - 3 0 4 - 3 . 5 s c a n l i n e d i r e c t i o n <l> - 0 . 2 1 - 0 . 2 s c a n l i n e l e n g t h ( L ) 6 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 3 I n 0 . 2 6 A v e r a g e s p a c i n g ( L ' l - n / N ) 5 m e t e r s J o i n t s e t B I P o i n t c l o u d 7 / 2 | P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 0 . 5 0 . 5 s c a n l i n e p o i n t 1 - 6 1 - 1 3 - 1 3 . 9 s c a n l i n e p o i n t 2 - 1 4 0 - 2 6 3 1 . 3 s c a n l i n e d i r e c t i o n <l> 0 . 9 0 .1 - 0 . 5 s c a n l i n e l e n g t h ( L ) 9 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 3 l -n 0 . 4 3 A v e r a g e s p a c i n g ( L ' l - n / N ) 1 3 m e t e r s J o i n t s e t A P o i n t c l o u d 2/1 P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 0 . 6 s c a n l i n e p o i n t 1 - 3 2 4 2 9 2 1 6 3 . 7 s c a n l i n e p o i n t 2 - 3 5 8 1 4 2 2 1 4 . 3 s c a n l i n e d i r e c t i o n <l> - 0 . 2 - 0 . 9 0 . 3 s c a n l i n e l e n g t h (t_) 1 6 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 7 I n 0 . 3 1 A v e r a g e s p a c i n g ( L T n / N ) 7 m e t e r s J o i n t s e t A P o i n t c l o u d 7 / 2 P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 - 0 . 3 0 . 6 s c a n l i n e p o i n t 1 - 1 0 5 6 . 5 6 - 1 . 0 2 s c a n l i n e p o i n t 2 - 1 5 7 - 4 6 . 1 3 3 . 6 9 s c a n l i n e d i r e c t i o n < l> 0 . 6 0 . 6 - 0 . 4 s c a n l i n e l e n g t h ( L ) 8 1 . 4 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 3 P o o r q u a l i t y ! I n 0 m e t e r ] o k a y A v e r a g e s p a c i n g ( L ' l - n / N ) 0 J o i n t s e t B - s m a l l s c a l e f e a t u r P o i n t c l o u d 7/1 I P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 0 . 5 0 . 5 s c a n l i n e p o i n t 1 - 1 1 5 - 1 4 6 2 0 . 5 s c a n l i n e p o i n t 2 - 1 0 9 - 1 7 8 2 6 . 7 6 s c a n l i n e d i r e c t i o n < l> - 0 . 2 1 - 0 . 2 s c a n l i n e l e n g t h ( L ) 3 3 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 3 I n 0 . 2 6 A v e r a g e s p a c i n g ( L ' l - n / N ) 3 m e t e r s J o i n t s e t B - l a r g e s c a l e P o i n t c l o u d 2/1 | P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 1 0 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 8 0 . 4 0 . 5 s c a n l i n e p o i n t 1 - 2 8 4 2 7 1 1 2 8 s c a n l i n e p o i n t 2 - 3 5 8 1 4 2 2 1 4 . 3 s c a n l i n e d i r e c t i o n < l> - 0 . 4 - 0 . 7 0 . 5 s c a n l i n e l e n g t h ( L ) 1 7 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 4 I n 0 . 3 5 A v e r a g e s p a c i n g ( L ' l - n / N ) 1 5 m e t e r s J o i n t s e t A P o i n t c l o u d 6/1 P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x < i> y < j > z < k > j o i n t s e t o r i e n t a t i o n <n> 0 . 7 - 0 . 4 0 . 6 s c a n l i n e p o i n t 1 - 2 6 . 4 - 7 3 - 3 2 . 7 8 s c a n l i n e p o i n t 2 - 3 1 . 9 - 4 4 - 2 0 . 3 9 s c a n l i n e d i r e c t i o n <l> - 0 . 2 0 . 9 0 . 4 s c a n l i n e l e n g t h ( L ) 3 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 2 I n 0 . 2 6 A v e r a g e s p a c i n g ( L ' l - n / N ) 4 m e t e r s J o i n t s e t A - s h a l l o w , s m a l l s e a P o i n t c l o u d 2/1 | P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 6 - 0 . 3 0 . 7 s c a n l i n e p o i n t 1 - 3 4 7 1 6 0 1 8 4 . 6 s c a n l i n e p o i n t 2 - 1 1 7 3 3 6 2 5 0 . 4 s c a n l i n e d i r e c t i o n < l> 0 . 4 - 0 . 9 - 0 . 3 s c a n l i n e l e n g t h ( L ) 2 0 0 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 6 I n 0 . 3 A v e r a g e s p a c i n g ( L ' l - n / N ) 1 0 m e t e r s J o i n t s e t B - f u l l c l o u d s c a l e P o i n t c l o u d 7/1 I P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 8 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 7 0 . 5 0 . 5 s c a n l i n e p o i n t 1 - 9 1 . 1 - 1 0 6 6 . 6 1 2 s c a n l i n e p o i n t 2 - 1 1 1 - 2 1 1 3 8 . 3 5 s c a n l i n e d i r e c t i o n < l> 0 . 2 0 . 9 - 0 . 3 s c a n l i n e l e n g t h ( L ) 1 1 1 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 4 I n 0 . 4 4 A v e r a g e s p a c i n g ( L ' l - n / N ) 1 2 m e t e r s J o i n t s e t B - l a r g e s c a l e P o i n t c l o u d 2/1 | P a t c h e s ( m i n s i z e / a n g l e ) 1 0 / 1 0 d e g x <i> y < j > z < k > j o i n t s e t o r i e n t a t i o n < n > 0 . 8 0 . 4 0 . 5 s c a n l i n e p o i n t 1 - 3 2 4 2 9 2 1 6 3 . 7 s c a n l i n e p o i n t 2 - 3 5 8 1 4 2 2 1 4 . 3 s c a n l i n e d i r e c t i o n < l> - 0 . 2 - 0 . 9 0 . 3 s c a n l i n e l e n g t h ( L ) 1 6 2 m e t e r s i n t e r s e c t i n g p a t c h e s ( N ) 4 I n 0 . 3 7 A v e r a g e s p a c i n g ( L ' l - n / N ) 1 5 m e t e r s 189 Table D.2 Discontinuity persistence estimated from photograph trace length measurements. Method 1: Measure trace length on photographs Photograph Newhalem079.j pg trace rake angle length Joint set (degrees) (m) A 139 32 A 140 21 A 140 10 A 140 41 A 142 33 A 142 28 A 146 26 A 146 56 A 147 20 A 148 16 A 148 31 A 149 88 A 150 24 A 150 52 A 153 55 A 153 12 A 153 114 A 155 15 A 155 33 A 156 7 A 157 34 A 158 18 A 159 15 A 159 42 A 160 9 A 160 42 A 161 10 A 162 22 A 163 49 A 168 16 A 177 25 mean trace length 32 standard deviation 23 Photograph OLjpg trace rake angle length Joint set (degrees) (m) A 87 25 A 98 58 A 98 15 A 103 19 A 110 43 A 112 23 A 113 20 A 113 17 A 115 13 A 116 41 A 117 9 A 118 57 A 118 10 A 119 21 A 120 7 A 121 13 A 122 16 A 125 15 A 126 29 A 127 16 A 130 22 A 133 29 A 138 14 mean trace length 23 standard deviation 14 Table D.3 Discontinuity exposed persistence estimated by direct measurement on the 3-D point cloud. Method 2: Measure exposed planes on 3D point clouds Point Cloud A3 exposed Point 1 Point 1 Point 1 Point 2 Point 2 Point 2 persistence* Joint set X y z X y z (m) A -310 260 151 -328 283 178 40 A -338 205 167 -384 274 222 100 A -384 291 231 -366 283 216 25 A -345 206 177 -359 218 201 31 A -358 185 191 -334 181 174 30 A -370 188 203 -346 178 182 33 A -354 162 190 -371 173 211 29 A -392 205 222 -371 202 203 28 A -380 236 214 -365 230 202 20 A -322 240 166 -384 294 230 104 A -316 190 153 -291 178 135 33 A -316 190 153 -291 178 135 33 mean exposed p ersistence 42 standard deviation 28 Point Cloud A3 exposed persistence* (m) Joint set Point 1 x Point 1 y Point 1 z Point 2 x Point 2 y Point 2 z B B B B B -313 -336 -338 -346 -426 248 240 204 149 227 151 182 167 184 248 -349 -362 -398 -390 -472 291 278 263 171 257 201 214 226 248 335 mean exposed persistence I standard deviation 76 56 103 81 103 84 20 * exposed persistence is the distance between point 1 and point 2 191 Table D.4 Discontinuity exposed persistence estimated by relating the exposed persistence to patch area in 3-D point cloud. Method 3: relate area of patches to exposed persistence patches (min size/ angle) 50/12, edited by hand Point cloud A3 patch minimum exposed aspect Paten Paten Paten area dimension, x persistence Joint set ratio centroid x centroid y centroid z (mA2) (m) (m) A 4 -290 263 135 239 8 32 A 4 -316 217 144 347 9 38 A 4 -317 236 156 537 12 48 A 4 -320 268 164 309 9 36 A 4 -327 211 157 124 6 23 A 4 -340 186 181 170 7 27 A 4 -435 247 262 166 6 27 A 4 -479 224 309 140 6 24 A 4 -483 233 322 78 4 18 A 4 -351 212 190 107 5 21 A 4 -352 246 194 1395 19 77 me an expose d persistence 34 standard deviation 17 patches edited by hand Point cloud A3 Joint set aspect ratio Paten centroid x Paten centroid y Paten centroid z patch area (mA2) minimum dimension, x (m) exposed persistence (m) B B B 5 4 2 -333 -353 -439 271 223 230 179 188 271 me 852 494 562 an exposec stanc 13 11 17 I persistence ard deviation 67 46 37 50 15 192 A P P E N D I X E : N U M E R I C A L M O D E L I N G I N P U T F I L E S E.1 UDEC Baseline Model A-A' ;;Input File for UDEC Baseline Model A ;;Author: Alex Strouth ;;Date: March, 2006 *********g|0ck*************** New round 0.2 bl -300,0 -300,1100 500,1100 500,0 ***********'pop0grapj1y******** table 1 -300,1020 0,870 30,850 55,825 74,822 & 100,800 132,753 139,718 170,664 193,610 222,527 & 247,507 280,450 330,400 400,365 470,396 490,400 520,350 crack table 1 ;change to air change mat=2 range above table 1 *********** j ^ n o | 0 g i c contacts* ****** ********* ; Tower shear zone crack 200,780 230,430 ;Base shear zone crack -490,240 350,540 change mat=3 range bl 3908 ;zone B = mat 3 j delete * * * * * * * * * * * j g e j g * * * * * * * * * * * * * * * * * * * * * * * * * delete range mat=2 hide mat=3 ;;;zone 3 jregion id 1 -490,240 -490,1100 350,1100 350,540 ;jset A jset -50,0 350,0 0,0 10,0 (170,620) range jregion 1 ;jset B jset -61,0 350,0 0,0 25,0 (30,850) range jregion 1 show mat=3 ;;;zone B jregion id 2 221,492 200,780 350,700 350,540 jdelete vor edge 4 range jregion 2 hide show mat=3 group joint 'zone B' show ;;;zone 2 jregion id 3 -490,240 520,605 520,-1 -490,-1 jset -60,0 350,0 0,0 25,0 range jregion 3 jdelete small blocks jdelete delete range area 2 *************2 o nj ng*******»**»** gen edge 5 range mat 3 gen edge 10 range region 0,450 0,850 300,850 300,450 gen edge 20 ************majerja] p r 0p e rtj e s*********** ;;elastic for stress initialization ;massive gneiss (Em=2.4el0, v=0.2) propmat=l dens=2650 k=1.3el0 g=lel0 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k= 1.1 e9 g=8e8 ;joint sets: zone 2 and 3 change jmat=l prop jmat=l jkn=2e9 jks=5e8 jfric=60 jcoh=2e6 jtens=2e5 ;shear zones & zone B vor joints change jmat=2 range minterface 1 3 ;shear zones = jmat=2 change jmat=2 range group 'zone B' prop jmat=2 jkn=2e9 jks=5e8 jfric=45 jcoh=2e6 jtens=2e5 ***********Q o u n (j a ry conditions** ******** bound xvel=0 range -301 -299 -1 1101 bound xvel=0 range 499 501 -1 1101 bound yvel=0 range-301 501 -1 1 **********gj r e s s e s************** ************ gravity 0-10 insitu str -19.8e6 0 -19.8e6 ygrad 2.65e4 0 2.65e4 insitu szz -19.8e6 zgrad 0 2.65e4 * * * * * * * * * * * * * | n j ^ j a | £Q******** solve save l_init.sav reset hist displ jdispl vel time ********** r c cj cj-j n e jojnt m a j e r j a] s************** change jmat=l hide show mat 3 change jmat=2 show change jmat=2 range minterface 1 3 ;elasto-plastic properties - strong to prevent initial failure ;massive gneiss (Em=2.4el0, v=0.2) change cons=3 range mat= 1 prop mat=l dens=2650 k=1.3el0 g=lel0 prop mat=l fric=60 coh=2.0e6 t=2.2e5 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k=l.le9 g=8e8 prop mat=3 fric=46 coh=7e5 t=4.4e4 ;joint sets prop jmat=l jkn=lel0jks=5e9jfric=33 jcoh=le6 jtens=0 ;shear zones and zone B; shear zones= jmat=2 prop jmat=2 jkn=l el 0 jks=5e9 jfric=26 jcoh=le6 jtens=0 193 prop jmat=3 jkn=l el 0 jks=5e9 jfric=26 jcoh=le6 jtens=0 step 10000 save l_mc.sav t****»***MC material properties*************** ;Damage state 1 ;massive gneiss (Em=2.4el0, v=0.2) change cons=3 range mat=l propmat=l dens=2650 k=1.3el0 g=lel0 prop mat=l fric=60 coh=2.0e6 t=2.2e5 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k=l.le9 g=8e8 prop mat=3 fric=46 coh=7e5 t=4.4e4 Joint sets prop jmat=l jkn=2e9 jks=5e8 jfric=33 jcoh=5e5 jtens=0 ;shear zones and zone B prop jmat=2 jkn=2e9 jks=5e8 jfric=26 jcoh=5e5 jtens=0 prop jmat=3 jkn=2e9 jks=5e8 jfric=26 jcoh=5e5 jtens=0 *********MC material properties*************** ;Damage state 4 ;massive gneiss (Em=2.4el0, v=0.2) change cons=3 range mat= 1 prop mat=l dens=2650 k=1.3el0 g=lel0 prop mat=l fric=60 coh=2.0e6 t=2.2e5 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k=l .le9 g=8e8 prop mat=3 fric=46 coh=7e5 t=4.4e4 ;joint sets prop jmat= 1 jkn=2e9 jks=5e8 jfric=33 jcoh=0 jtens=0 ;shear zones and zone B prop jmat=2 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat=3 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 step 10000 save 1 mc4.sav step 10000 save 1 mcl.sav *********MC material properties************* ;Damage state 2 ;massive gneiss (Em=2.4el0, v=0.2) change cons=3 range mat=l propmat=l dens=2650 k=l .3el0 g=lel0 prop mat=l fric=60 coh=2.0e6 t=2.2e5 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k=l. 1 e9 g=8e8 prop mat=3 fric=46 coh=7e5 t=4.4e4 Joint sets prop jmat=l jkn=2e9 jks=5e8 jfric=33 jcoh=le5 jtens=0 ;shear zones and zone B prop jmat=2 jkn=2e9 jks=5e8 jfric=26 jcoh= 1 e5 jtens=0 prop jmat=3 jkn=2e9 jks=5e8 jfric=26 jcoh= 1 e5 jtens=0 step 10000 save 1 mc2.sav *********MC material properties************* ; Damage state 3 ;massive gneiss (Em=2.4el0, v=0.2) change cons=3 range mat=I prop mat= 1 dens=2650 k= 1.3e 10 g= 1 e 10 prop mat=l fric=60 coh=2.0e6 t=2.2e5 ;zone B (Em=2e9, v=0.2) prop mat=3 dens=2650 k=l.le9 g=8e8 prop mat=3 fric=46 coh=7e5 t=4.4e4 Joint sets prop jmat=l jkn=2e9 jks=5e8 jfric=33 jcoh=l e4 jtens=0 ; shear zones and zone B prop jmat=2 jkn=2e9 jks=5e8 jfric=26 jcoh= 1 e4 jtens=0 prop jmat=3 jkn=2e9 jks=5e8 jfric=26 jcoh=le4 jtens=0 step 10000 save 1 mc3.sav 194 E.2 3DEC Model ;;Input File for 3DEC Model ;;Author: Alex Strouth ;;Date: May, 2006 ;created with PGEN D\3DEC\AC05\PGEN\trial4 ;!ower half of slope t4_l .dat ;upper half of slope t4_2.dat call t4_l.dat call t4_2.dat ;region 1, mat 1 = slope ;region 2, mat 2 = surrounding block delete reg 2 join on hide dd 0 dip 0 or 150 250 500 above mark reg 5 ; base is region 5 seek ***********Joints and shear zones***** set atol 0.1 ;tower shear zone jset dd 108 dip 85 or 184 610 346 id 10000 ;base shear zone jset dd 250 dip 15 or 280 500 330 id 10001 hide dd 108 dip 85 or 184 610 346 below hide dd 250 dip 15 or 280 500 330 below mark reg 4 ; zone B is region 4 seek ;north shear zone (boundary for joint sets) jset dd 115 dip 90 or 67 765 597 id 10002 hide dd 250 dip 15 or 280 500 330 below hide dd 115 dip 90 or 67 765 597 below hide dd 108 dip 85 or 184 610 346 above mark reg 3 ;zone 3 is region 3 seek ;; joint sets in zone 3 hide seek reg 3 del volume 50 Joint set A jsetdd 116 dip 51 num 15 sp 10 or 158 632 504 id 20000 del volume 50 Joint set B jsetdd 57 dip 71 num 10 sp 25 or 148 652 431 id 20001 del volume 50 ******************zonjng*********** seek save 55_block.sav gen reg 3 edge 10 gen reg 4 edge 10 gen reg 1 edge 30 gen reg 5 edge 90 save 55 zone.sav ;A11 materials are elastic - very strong properties - for stress initialization change reg 5 1 mat 1 ;base and surrounding slope change reg 4 mat 2 ;zone B change reg 3 mat 3 ;zone 3 blocks ;base and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=l .3el0 g=lelO ;zone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l.le9 g=8e8 ;zone 3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=l ,3el0 g=lel0 ;;;;;;;;;;Joints;;;;;;;;;;;;; change joint 10000 10001 10002 jmat 1 ;shear zones change joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 7 prop jmat 1 jkn=2e9 jks=5e8 prop jmat 2 jkn=2e9 jks=5e8 • ••••••'(''('•'i* • |̂  Q j-j ̂  2i"y conditions ^"'e!^(!'!3'c''c''t'^!3'£!'!^!i'!!^t''':i'!i't grav 0-10 0 Jnsitu topo kox=l koz=l insitu stress -2.65e7,-2.65e7,-2.65e7 0,0,0 & ygrad 2.65e4,2.65e4,2.65e4 0,0,0 ;east side - at angle bound dip 90 dd 61 org 294 0 522 above xvel=0 zvel=0 ;bottom bound (-155,520) (-1,1) (90,725) yvel=0.0 xvel=0 zvel=0 ;west side bound (-155,-145) (-1,1001) (90,725) xvel=0 ;north side bound (-155,520) (-1,1001) (718,725) zveK) ;south side bound (-155,520) (-1,1001) (95,110) zvel=0 ************Step to initialize stresses************* step 5000 save 55 init.sav **********fjnange to MC model**************** reset vel hide seek mat 1 change cons 1 seek hide seek mat 2 change cons 2 seek hide seek mat 3 change cons 2 seek 195 ************ l̂£ ***************** ;base and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=I.3eI0 g=leI0 ;zone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l. 1 e9 g=8e8 prop mat 2 bfric=40 bcoh=5e5 bt=3.5e4 ;zone 3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=l ,3el0 g=lelO prop mat 3 bfric=60 bcoh=2.0e6 bt=2.2e5 jchange joint 10000 10001 10002 jmat 1 ;shear zones jchange joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 1 prop jmat 1 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat 2 jkn=2e9 jks=5e8 jfric=33 jcoh=le6 jtens=0 step 5000 save 55_mc.sav ************^l£ |***************** ;base and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=l .3el0 g=lel0 ;zone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l. 1 e9 g=8e8 prop mat 2 bfric=40 bcoh=5e5 bt=3.5e4 ;zone 3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=1.3el0 g=lel0 prop mat 3 bfric=60 bcoh=2.0e6 bt=2.2e5 ;change joint 10000 10001 10002 jmat 1 ;shear zones ;change joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 1 prop jmat I jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat 2 jkn=2e9 jks=5e8 jfric=33 jcoh=5e5 jtens=0 step 5000 save 55_mcl.sav * * * * * * * * * * * * ^ j ^ 2***************** ;base and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=l .3el0 g=lel0 ;zone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l.le9 g=8e8 prop mat 2 bfric=40 bcoh=5e5 bt=3.5e4 ;zone3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=1.3el0 g=lel0 prop mat 3 bfric=60 bcoh=2.0e6 bt=2.2e5 jchange joint 10000 10001 10002 jmat 1 ;shear zones ;change joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 1 prop jmat 1 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat 2 jkn=2e9 jks=5e8 jfric=33 jcoh=l e5 jtens=0 step 5000 save 55_mc2.sav ************|yJQ J***************** ;base and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=l ,3el0 g=lel0 ;zone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l. 1 e9 g=8e8 prop mat 2 bfric=40 bcoh=5e5 bt=3.5e4 ;zone 3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=1.3el0 g=lel0 prop mat 3 bfric=60 bcoh=2.0e6 bt=2.2e5 jchange joint 10000 10001 10002 jmat 1 ;shear zones jchange joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 1 prop jmat 1 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat 2 jkn=2e9 jks=5e8 jfric=33 jcoh=le4 jtens=0 step 5000 save 55_mc3.sav ************|yj£ 4***************** jbase and surrounding slope (Em=2.4el0, v=0.2) prop mat 1 dens=2650 k=1.3el0 g=lel0 jzone B (Em=2e9, v=0.2) prop mat 2 dens=2650 k=l. 1 e9 g=8e8 prop mat 2 bfric=40 bcoh=5e5 bt=3.5e4 jzone 3 (Em=2.4el0, v=0.2) prop mat 3 dens=2650 k=1.3el0 g=lel0 prop mat 3 bfric=60 bcoh=2.0e6 bt=2.2e5 jchange joint 10000 10001 10002 jmat jshearzones jchange joint 20000 20001 jmat 2 Joint sets change jmat 1 jmat 2 jcons 1 prop jmat 1 jkn=2e9 jks=5e8 jfric=26 jcoh=0 jtens=0 prop jmat 2 jkn=2e9 jks=5e8 jfric=33 jcoh=0 jtens=0 step 5000 save 55 mc4.sav 196

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