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Slope stability of Nemo and Wee Sandy Creek basins near Slocan Lake, British Columbia 1982

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SLOPE STABILITY OF NEMO AND WEE SANDY CREEK BASINS NEAR SLOCAN LAKE, BRITISH COLUMBIA by ROBERT TAYLOR PACK B.S., Brigham Young U n i v e r s i t y , 1980 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES -(Programme of G e o l o g i c a l Engineering) c We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1982 © Robert T a y l o r Pack, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department Of Geological Sciences The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 30 July 1982 DE - 6 n / s n ABSTRACT In order to determine the p o s s i b l e impacts of f o r e s t e n g i n e e r i n g on l a n d s l i d e occurrence i n four eastward-draining basins near Slocan Lake, southeastern B r i t i s h Columbia, slopes are e v a l u a t e d a c c o r d i n g to a l a n d s l i d e hazard c l a s s i f i c a t i o n scheme based on n a t u r a l t e r r a i n s u b d i v i s i o n s , a s t o c h a s t i c g e o t e c h n i c a l model, and past e n g i n e e r i n g experience. Mass wasting processes c u r r e n t l y at work in the area i n c l u d e shallow d e b r i s avalanches, d e b r i s flows, r o c k s l i d e s and r o c k f a l l s , and i n v o l v e complex g l a c i a l and c o l l u v i a l d e p o s i t s o v e r l y i n g coarse g r a i n e d p l u t o n i c and high grade metamorphic bedrock. Primary f a c t o r s known to i n f l u e n c e l a n d s l i d e occurrence i n the region i n c l u d e slope angle, s o i l shear s t r e n g t h , t r e e root s t r e n g t h , groundwater, and shear plane geometry. A s t o c h a s t i c g e o t e c h n i c a l model can only be a p p l i e d to uniform slopes mantled with s u r f i c i a l m a t e r i a l because of inherent assumptions and r e q u i r e s q u a n t i t a t i v e estimates of parameters fo r broad slope u n i t s . Ranges are estimated f o r values of angle of i n t e r n a l s o i l f r i c t i o n , s o i l cohesion, root cohesion, p i e z o m e t r i c head, depth to shear plane, s o i l bulk d e n s i t y , t r e e surcharge weight, and slope angle. From the model i t i s p o s s i b l e to e x p l a i n the observed d i s t r i b u t i o n of many l a n d s l i d e s i n the study area and surrounding region i n terms of expected f a c t o r of s a f e t y and p r o b a b i l i t y of f a i l u r e . However, the p r o b a b i l i t i e s cannot a c t u a l l y p r e d i c t the number of l a n d s l i d e s l i k e l y to occur on a p a r t i c u l a r s l o p e , nor the l i k e l i h o o d of a l a n d s l i d e o c c u r r i n g i i w i t h i n a c e r t a i n time p e r i o d . A c c u r a t e , q u a n t i t a t i v e p r e d i c t i o n s of l a n d s l i d e occurrence can be made only where model v a r i a b l e s are l e s s s u b j e c t i v e l y determined, and where p r o b a b i l i t i e s are c a l i b r a t e d and compared with observed events. These semi- q u a n t i t a t i v e l y determined i n d i c e s of s t a b i l i t y are best used to compare the s t a b i l i t y of slopes i n the study area with slopes that have responded unfavorably to f o r e s t e n g i n e e r i n g i n other areas. From such comparisons the i n d i c e s can be grouped to form hazard c l a s s e s of use to f o r e s t managers and e n g i n e e r s . In areas where the g e o t e c h n i c a l model does not apply, hazards are assigned according to past e n g i n e e r i n g experience in n a t u r a l t e r r a i n u n i t s s i m i l a r to those in the study area. These u n i t s i n clude c o l l u v i a l fans and aprons, d e b r i s fans, and steep rocky t e r r a i n . Slopes c l a s s e d i n the 'very high hazard' group i n c l u d e those s l o p e s which show s i g n s of a c t i v e l a n d s l i d i n g as i n d i c a t e d by morphology or v e g e t a t i o n , and steep rocky t e r r a i n dominated by g r a v i t y p r o c e s s e s . Slopes c l a s s e d i n the 'high hazard' group i n c l u d e c o l l u v i a l fans, upper p a r t s of d e b r i s fans, and slopes mantled with s u r f i c i a l m a t e r i a l having p r o b a b i l i t i e s of f a i l u r e g reater than 10%. Slopes c l a s s e d i n the 'moderate hazard' group include lower p a r t s of d e b r i s fans and slopes mantled with s u r f i c i a l m a t e r i a l having p r o b a b i l i t i e s of f a i l u r e l e s s than 10% but expected f a c t o r s of s a f e t y l e s s than 1.6. 'Low hazard' s l o p e s i n c l u d e g e n t l y s l o p i n g exposed bedrock and slopes mantled with s u r f i c i a l m a t e r i a l having expected f a c t o r s of s a f e t y g r e a t e r than 1.6. The l a n d s l i d e hazard c l a s s i f i c a t i o n scheme has p r a c t i c a l m e r i t s f o r use i n p l a n n i n g road alignments and l o g g i n g systems. i i i TABLE OF CONTENTS ABSTRACT 1 ACKNOWLEDGEMENTS 9 CHAPTER 1 I n t r o d u c t i o n 1 1.1 The L a n d s l i d e Problem 1 1 .2 Scope Of Study 3 1.3 Previo u s Work In The Study Area 6 CHAPTER 2 Study Area D e s c r i p t i o n 8 2.1 Physiography 8 2.2 Bedrock Geology 10 2.3 S u r f i c i a l Geology 13 2.3.1 M o r a i n a l D e p o s i t s 14 2.3.2 G l a c i o f l u v i a l D e p o s i t s 17 2.3.3 F l u v i a l D e p o s i t s 18 2.3.4 C o l l u v i a l D e p o s i t s 18 2.3.5 Weathering 19 2.4 Geomorphic Processes 20 2.4.1 D e b r i s Avalanche - D e b r i s Flows 20 2.4.2 R o c k s l i d e s 27 2.4.3 R o c k f a l l s 28 2.4.4 E r o s i o n 29 2.4.5 S o i l Creep 29 2.4.6 Snow Avalanching 30 2.5 Climate 31 2.6 V e g e t a t i o n 33 CHAPTER 3 Slope S t a b i l i t y In The Study Area 36 i v 3.1 Approaches To Slope S t a b i l i t y Assessment 36 3.2 The S t o c h a s t i c G e o t e c h n i c a l Model 39 3.3 S o i l Shear Strength 47 3.3.1 E s t i m a t i o n Of S o i l Shear Strength 47 3.3.2 Range Of S o i l Shear S t r e n g t h Values 51 3.4 Root Strength 54 3.4.1 E s t i m a t i o n Of Root S t r e n g t h 55 3.4.2 Range Of Root Cohesion Values 56 3.5 Groundwater 59 3.5.1 E s t i m a t i o n Of P i e z o m e t r i c Pressures 61 3.5.2 Estimated E f f e c t s Of Groundwater In Study Area .. 65 3.6 Slope Angle 69 3.6.1 Measurement Of Slope Angle 70 3.6.2 D i s t r i b u t i o n Of Slope Angles In Study Area 70 3.7 M i s c e l l a n e o u s F a c t o r s 74 3.8 Slope E q u i l i b r i u m In The Study Area 76 CHAPTER 4 L a n d s l i d e Hazard C l a s s i f i c a t i o n 79 4.1 Hazards On Slopes Mantled With S u r f i c i a l M a t e r i a l ... 79 4.1.1 E n g i n e e r i n g Problems Near Study Area 79 4.1.2 Hazard C l a s s e s 83 4.1.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques 87 4.2 Hazards On Steep Rocky Slopes 89 4.2.1 E n g i n e e r i n g Problems Near The Study Area 90 4.2.2 Hazard C l a s s e s 91 4.2.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques 92 4.3 Hazards On C o l l u v i a l Aprons And Fans 93 4.3.1 E n g i n e e r i n g Problems Near The Study Area 93 4.3.2 Hazard C l a s s 94 V 4.3.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques 94 4.4 Hazards On Debris Fans 94 4.4.1 E n g i n e e r i n g Problems In Other Regions 95 4.4.2 Hazard C l a s s 96 4.4.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques 96 4.5 Hazards On Ter r a c e s And G u l l i e s 97 4.5.1 E n g i n e e r i n g Problems Near The Study Area 97 4.5.2 Hazard C l a s s 99 4.5.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques 99 4.6 Summary Of The Hazard C l a s s i f i c a t i o n System 100 4.7 D i s t r i b u t i o n Of Hazard C l a s s e s 101 CHAPTER 5 Road C o r r i d o r Assessments 104 5.1 General 104 5.2 Nemo Creek Road Options 104 5.3 Wee Sandy Creek Road Options 108 CHAPTER 6 Summary And Conclus i o n s 111 BIBLIOGRAPHY 116 APPENDIX A 124 APPENDIX B 1 26 APPENDIX C 130 APPENDIX D 132 APPENDIX E 1 34 APPENDIX F 1 36 v i LIST OF FIGURES 1.1 I ndex Map 3 2.1 Upper Nemo Creek Basin 8 2.2 Geology Of The Study Area 10 2.3 Receeding G l a c i e r In Alask a 13 2.4 G r a i n - s i z e D i s t r i b u t i o n s For SM S o i l s 15 2.5 D e b r i s Avalanche In Lower Nemo Creek Basin 21 2.6 D e b r i s Avalanche - d e b r i s Flow Path In Nemo Creek Ba s i n 22 2.7 D e b r i s Flow System In Lower Wee Sandy Creek B a s i n . ... 24 2.8 B i f u r c a t e d D e bris Flow In Upper Nemo Creek Basin 25 2.9 P r o f i l e Of A Debris Flow Path In Upper Nemo Creek Basin 25 2.10 T o p p l i n g Rock F a i l u r e In Lower Nemo Creek Basin 27 2.11 B u t t r e s s i n g E f f e c t Of Tree Roots R e s i s t i n g S o i l Creep 29 2.12 Index Map Showing Weather S t a t i o n L o c a t i o n s 31 2.13 Mean T o t a l Monthly P r e c i p i t a t i o n Near The Study Area 31 2.14 Twenty-four Hour Extreme P r e c i p i t a t i o n Data 31 2.15 F o u r t y - e i g h t Hour P r e c i p i t a t i o n Extremes For New Denver 32 3.1 D e f i n i t i o n s Of Model Input V a r i a b l e s 40 3.2 S e n s i t i v i t y Of FS To V a r i a t i o n s In The Values Of Model V a r i a b l e s 42 3.3 A l t e r a t i o n s To Slope E q u i l i b r i u m F o l l o w i n g D e g l a c i a t i o n v i i 43 3.4 Range Of 0 Values For V a r i o u s S u r f i c i a l M a t e r i a l s 52 3.5 L a n d s l i d e Analysed For Root Cohesion Determination .... 56 3.6 V a r i a t i o n Of M With Respect To 24 Hr R a i n f a l l 65 3.7 T y p i c a l P r o f i l e Of An I d e a l i z e d H i l l s l o p e 66 3.8 Slope C l a s s I n t e r v a l s Used For The Study Area 71 3.9 Three Maps Showing Slope D e l i n e a t i o n Methodology 72 3.10 Slope P r o f i l e Showing R e l a t i v e S t a b i l i t y Of V a r i o u s Slope Segments 76 4.1 Cutslope F a i l u r e s Caused By Seepage 80 4.2 D e b r i s Avalanche - Debris Flow On Wragge Creek Road ... 82 4.3 F i l l Slope E r o s i o n From C u l v e r t 82 4.4 Rock F a i l u r e On Lower Shannon Creek Road 90 4.5 L a n d s l i d e I n i t i a t e d By Loss Of Root Cohesion 98 4.6 H y p o t h e t i c a l Slope I l l u s t r a t i n g The L a n d s l i d e Hazard C l a s s i f i c a t i o n System 100 5.1 Hazards Traversed By Proposed Road C o r r i d o r s 106 Map A — Foot t r a v e r s e s and sample s i t e s ~~1 Map B -- T e r r a i n Map C — Slope LifU-W Map D — Slope S t a b i l i t y J v i i i LIST OF TABLES 3.1 E f f e c t s Of Slopes F a c t o r s On Model V a r i a b l e s 42 3.2 Estimated <t> Values For C o h e s i o n l e s s S o i l s 49 3.3 D e f i n i t i o n s Of Moisture Regimes 62 3.4 Maximum Values Of M For V a r i o u s Moisture Regimes 69 3.5 Average Bulk D e n s i t i e s For D i f f e r e n t S o i l C l a s s e s 74 3.6 I n - s i t u Bulk D e n s i t i e s Determined In The Study Area ... 74 4.1 S t a b i l i t y I n d i c e s C a l c u l a t e d For Slopes Near E n g i n e e r i n g F a i l u r e s 83 ix ACKNOWLEDEGMENTS The author wishes to thank G. S t i l l and T. Baker of the B r i t i s h Columbia Ministry of Forests for f i r s t suggesting the need for th i s study and arranging f i n a n c i a l assistance; H.T. Smith for providing f a i t h f u l assistance in the f i e l d , sometimes in adverse conditions; G. Utzig for assistance with f i e l d mapping and f i e l d l o g i s t i c s ; R.E. Kucera, M.J. Bovis, W.H. Mathews, and L.M. Lavkulich for continual advice and c r i t i c a l reviews; and f i n a l l y he wishes to thank his wife Shelley whose patience and encouragment helped sustain him during the course of t h i s study. Financial support for l i v i n g expenses was provided by the Science Council of B r i t i s h Columbia in the form of a Graduate Research in Engineering and Technology Award. Support for f i e l d and research expenses was provided by the B r i t i s h Columbia Ministry of Forests, Research D i v i s i o n . 1 CHAPTER 1 INTRODUCTION 1.1 The L a n d s l i d e Problem L a n d s l i d i n g i n i t s v a r i o u s forms i s a dominant e r o s i o n a l process in removing and t r a n s p o r t i n g s o i l and rock d e b r i s from steep mountainous slopes of the western C o r d i l l e r a . As l a r g e r demands are p l a c e d upon the v a l u a b l e f o r e s t resources of B r i t i s h Columbia, the s t e e p e r , more d i f f i c u l t watersheds are being developed. V a r i o u s economic and environmental impacts r e s u l t i n g from such development are now demanding that slope hazards be e v a l u a t e d and understood p r i o r to development. Past experience i n v a r i o u s p a r t s of the world i n d i c a t e s that both d e f o r e s t a t i o n and road b u i l d i n g may have marked impacts on l a n d s l i d e occurrence (Swanston 1974 , Zeimer 1981 , F r o e h l i c h , 1979 , Dale and James 1977 , and Takeda 1 9 7 6 ) . In the West Kootenay Region of B r i t i s h Columbia, the focus of t h i s study, marked i n c r e a s e s i n stream sediment l o a d have been l i n k e d with l o g g i n g o p e r a t i o n s (Chamberlain and J e f f r e y 1968). Such i n c r e a s e s are a t t r i b u t e d to s o i l d i s t u r b a n c e from skidder l o g g i n g systems which have l e d to both l a n d s l i d i n g and s u r f a c e e r o s i o n . Sedimentation i n streams has a negative e f f e c t on f i s h p o p u l a t i o n s and o c c a s i o n a l l y on l o c a l m u n i c i p a l water s u p p l i e s . In the Slocan V a l l e y of the West Kootenay Region, a l o c a l study has documented numerous examples of both slope and stream degradation r e s u l t i n g from . road b u i l d i n g on steep s l o p e s near stream channels (Slocan V a l l e y Community 1974). Such occurrences 2 are not unique to t h i s r e g i o n . In the Coast Mountains north of Vancouver, B.C., O'Loughlin (1973) estimated that l o g g i n g roads are r e s p o n s i b l e f o r up to 47% of of l a n d s l i d e s which run d i r e c t l y i n t o stream channels. T h i s can be a t t r i b u t e d to the f a c t that i n most areas main access roads are c l o s e to major streams. S t u d i e s to the east of Slocan V a l l e y i n d i c a t e that severe s o i l d i s t u r b a n c e may have an adverse e f f e c t on f o r e s t p r o d u c t i v i t y , p a r t i c u l a r l y at high e l e v a t i o n s ( U t z i g and H e r r i n g 1975). P r o d u c t i v i t y r e d u c t i o n s are g r e a t e s t where s o i l d i s t u r b a n c e i s 'severe' or deep, that i s , where (1) the f o r e s t l i t t e r , A -horizon, and a p o r t i o n of the B-horizon are removed; (2) the s o i l s u r f a c e i s b u r i e d by .25 m or more of d e b r i s ; or (3) the A and B mineral h o r i z o n s are s e v e r e l y compacted. To date, the long-term e f f e c t s of l a n d s l i d i n g on f o r e s t p r o d u c t i v i t y i n B r i t i s h Columbia have not been assessed q u a n t i t a t i v e l y . Road damage caused by l a n d s l i d i n g may not only damage the environment, but may a l s o s i g n i f i c a n t l y i n c r e a s e road maintenance c o s t s and costs- i n c u r r e d by t r a n s p o r t d e l a y s . F o r e s t companies are now r e a l i z i n g that e i t h e r a v o i d i n g road c o n s t r u c t i o n on unstable slopes or e n g i n e e r i n g roads f o r them , though c o s t l y at the o u t s e t , i s o f t e n to t h e i r advantage f i n a n c i a l l y (Gardner 1979). As l a n d s l i d e s are becoming an i n c r e a s i n g l y widespread problem both f i n a n c i a l l y and e n v i r o n m e n t a l l y i n B r i t i s h Columbia, the land manager i s faced with the need for a more comprehensive understanding of the nature and extent of the 3 problem. The three most commonly asked q u e s t i o n s by land managers about p o t e n t i a l l a n d s l i d e s a r e , a c c o r d i n g to Burroughs (1980), "(1) Where are they?, (2)How bad are they?, and (3)What can be done about them? In recent' years, s e v e r a l methods of slope s t a b i l i t y a n a l y s i s have been developed f o r e v a l u a t i n g l a n d s l i d e p o t e n t i a l in f o r e s t e d watersheds (Foggin and Rice 1979, Swanston 1980 and Simons et a l . 1978). There are s e v e r a l l i m i t a t i o n s in these a n a l y s e s : (1) the h e t e r o g e n i e t y of l a n d s l i d e c o n t r o l l i n g f a c t o r s in the n a t u r a l s e t t i n g leads to u n c e r t a i n t i e s i n a s t a b i l i t y a n a l y s i s , (2) data c o l l e c t i o n i s d i f f i c u l t i n l a r g e i n a c c e s s i b l e areas, (3) broad i n t e r p r e t a t i o n s are o f t e n based on inadequate data, and (4) knowledge of the processes i n v o l v e d i s u s u a l l y incomplete. I f these l i m i t a t i o n s are overcome and slope s t a b i l i t y i s determinable, assumptions as to the kind, i n t e n s i t y , and q u a l i t y of e n g i n e e r i n g to be imposed upon the t e r r a i n leads to the assignment of r e l a t i v e hazard r a t i n g s . I t i s these hazard r a t i n g s that are of i n t e r e s t to the land manager. 1.2 Scope Of Study T h i s t h e s i s attempts to e v a l u a t e the s t a b i l i t y of slopes i n four eastward-draining basins i n the V a l h a l l a Mountains west of Slocan Lake, southeastern B r i t i s h Columbia (117° 22'-38'W; 49° 54'-50° 01'N; see F i g u r e 1.1). I t i n c l u d e s the major drainage basins of Wee Sandy Creek and Nemo Creek and the two minor basins of Hoben Creek and Sharp Creek which d r a i n the e a s t - f a c i n g Slocan Lakefront between the two major drai n a g e s . The 4 F i g u r e 1.1 Index map. area encompasses some 160 km2 of which 66 km2 have p o t e n t i a l l y h a r v e s t a b l e timber. T h i s area was chosen because i t i n c l u d e s many steep, p o t e n t i a l l y u nstable s l o p e s , merchantable timber of value to the f o r e s t i n d u s t r y , and some high a e s t h e t i c - r e c r e a t i o n a l v a l u e s . P o t e n t i a l land-use c o n f l i c t s i n t h i s area are of p u b l i c concern and s t u d i e s are needed to determine what 5 impact, i f any, l o g g i n g o p e r a t i o n s w i l l have on the environment. No l o g g i n g or development has o c c u r r e d i n the study area f o r at l e a s t 30 y e a r s . The f i r s t of two o b j e c t i v e s i s to determine, map, and d e s c r i b e the fundamental f a c t o r s c o n t r o l l i n g the s t a b i l i t y of slopes i n the study area. The f a c t o r s i n c l u d e g e o l o g i c s t r u c t u r e , s o i l p r o p e r t i e s , root s t r e n g t h , groundwater c o n d i t i o n s , s l i d e geometry and slope angle. T h i s i s the f i r s t step i n a s s e s s i n g the r e l i a b i l i t y of c o n v e n t i o n a l slope s t a b i l i t y a nalyses in determining hazard r a t i n g s . T h i s o b j e c t i v e a l s o i n c l u d e s the examination of f a c t o r i n t e r a c t i o n s l e a d i n g to n a t u r a l i n s t a b i l i t y . The second o b j e c t i v e i s to determine how l a n d s l i d e c o n t r o l l i n g f a c t o r s i n t e r a c t with p a r t i c u l a r e n g i n e e r i n g p r a c t i c e s to produce l a n d s l i d e s . L a n d s l i d e s caused by e n g i n e e r i n g a c t i v i t i e s on slopes s i m i l a r to those of the study area are a b a s i s f o r the hazard r a t i n g system in the study area. A r a t i o n a l method of l a n d s l i d e p r e d i c t i o n i s t h e r e f o r e based on s t a b i l i t y a n a l y s i s , the e n g i n e e r i n g behaviour of s i m i l a r slopes in other areas, and c e r t a i n assumptions as to the type and q u a l i t y of e n g i n e e r i n g a l t e r a t i o n s to be imposed upon slopes i n the study area. The f i n a l r e s u l t i s a map of slope s t a b i l i t y hazards. T h i s study does not attempt to p r e d i c t s i t e - s p e c i f i c occurrences of l a n d s l i d e s and i s designed only to d e l i n e a t e and r a t e areas which are l i k e l y to produce slope s t a b i l i t y problems. In some cases, f a i l e d s lopes are d e s c r i b e d i n d i v i d u a l l y , p a r t i c u l a r l y where they are c r i t i c a l to the watershed 6 development scheme. P o t e n t i a l problem areas a s s o c i a t e d with low-volume f o r e s t roads, as w e l l as slopes that have or w i l l be logged are emphasized. Primary and secondary highways are a l s o examined but are not c o n s i d e r e d to be as r e l e v a n t to f o r e s t e n gineering problems. A l l areas above t i m b e r l i n e which c o n s t i t u t e the d i v i d e s between watersheds are i n c l u d e d i n the landform mapping but are l e s s important to land-use c o n f l i c t s , and are, t h e r e f o r e , not c o n s i d e r e d i n d e t a i l . Only l a n d s l i d i n g , i . e . the downslope movement of rock, s o i l and d e b r i s under the i n f l u e n c e of g r a v i t y l a r g e l y independent of c o n t r i b u t i n g f o r c e s from agencies such as flowing water or wind (Leopold et a l . 1964) w i l l be c o n s i d e r e d . Other geomorphic hazards, such as snow a v a l a n c h i n g , w i l l not be d i s c u s s e d . The term ' l a n d s l i d i n g ' w i l l be used as an e q u i v a l e n t to 'mass- wasting' or ' s o i l mass-movement' f o r t h w i t h , f o r s i m p l i c i t y . 1.3 Previous Work In The Study Area Reconnaissance s o i l surveys were begun i n 1980 by the Ca s t l e g a r F o r e s t D i s t r i c t of B.C. M i n i s t r y of F o r e s t s ( M i n i s t r y of F o r e s t s 1981a). T h e i r p r e l i m i n a r y report i n c l u d e s b r i e f d e s c r i p t i o n s of the g e o l o g i c environment and s o i l moisture regimes encountered d u r i n g a two-day t r a v e r s e along Nemo Creek. Remarks re g a r d i n g slope s t a b i l i t y and road b u i l d i n g are made but are sketchy and of l i t t l e use to t h i s study. Wee Sandy Creek was not ground checked by the survey. T e n t a t i v e road l o c a t i o n s i n both Nemo and Wee Sandy Creeks were e s t a b l i s h e d i n 1980 by the Nelson Regional Engineer of B.C. 7 M i n i s t r y of F o r e s t s . A report c o n t a i n s b r i e f d e s c r i p t i o n s of p o t e n t i a l slope s t a b i l i t y problems along the proposed road alignments ( M i n i s t r y of F o r e s t s 1981b). 8 CHAPTER 2 STUDY AREA DESCRIPTION > 2.1 Physiography The study area l i e s i n the hig h , steep-walled, s e r r a t e d , east-west t r e n d i n g r i d g e s of the northern V a l h a l l a Range of southeastern B r i t i s h Columbia. L o c a l r e l i e f v a r i e s from 900 to 1350 meters, but the t o t a l r e l i e f i s about 2200 meters and e l e v a t i o n s range from 535 meters at Slocan Lake to 2743 meters at Mount Denver 6 ki l o m e t e r s to the west. The upper end of Nemo Creek i s dominated by Mount Meers to the north, Hela Peak to the south, and a s e r i e s of c i r q u e b a s i n s with f l o o r s at e l e v a t i o n s between 1850 and 2150 meters. Lower Nemo Creek i s f l a n k e d to the north by rugged c l i f f s , which r i s e some 1060 meters from the v a l l e y bottom. Nemo Creek has numerous small t r i b u t a r i e s that enter from c i r q u e b a s i n s on both the north and south s i d e s of the upper v a l l e y and a few minor t r i b u r a r i e s which feed from the s t r a i g h t , steep sl o p e s of the lower v a l l e y . V a l l e y geometry i s t y p i c a l l y U-shaped i n the upper reaches but becomes dominately V-shaped i n the lower v a l l e y . F i g u r e 2.1 i l l u s t r a t e s the U-shaped geometry of the upper Nemo Creek Basi n . The physiography of Wee Sandy Creek Basin i s s i m i l a r to that of Nemo Creek. The upper v a l l e y i s U-shaped i n c r o s s - s e c t i o n but, remarkably, has no t r i b u t a r y c i r q u e b a s i n s to the nor t h . Wee Sandy Creek o r i g i n a t e s at Wee Sandy Lake which occupies a north-south t r e n d i n g hanging t r i b u t a r y v a l l e y at the 9 Figure 2,1. Ae r i a l view of upper Nemo Creek Basin. head of the basin. The lower valley again assumes a V-shaped cross-section, as does Nemo Creek, at the 1370 meter l e v e l . The steepest slopes and c l i f f faces are consistently found on the northern sides of both val l e y s . Both Nemo and Wee Sandy Creek have r e l a t i v e l y gentle stream gradients in the upper valley portions, which then steepen abruptly at mid-valley to descend via rapids and cascades at an average 15% gradient to Slocan to Slocan Lake below. One notable cascade 1 kilometer long occurs approximately 5 km up Wee Sandy Creek and has an average gradient of 27%. Hoben and Sharp Creeks occupy hanging cirque valleys which drain into Slocan Lake between Nemo and Wee Sandy Creek. Both Creeks descend a series of g l a c i a l l y formed steps, some of which 1 0 are occupied by small t a r n s , and then drop a b r u p t l y i n t o the main Slocan V a l l e y . N e i t h e r stream has i n c i s e d s i g n i f i c a n t l y i n t o bedrock and t h e r e f o r e have not developed V-shaped canyons as have Nemo and Wee Sandy Creeks. New Denver G l a c i e r i s perched at the head of Sharp Creek Basin i s the only r e p r e s e n t a t i v e of once e x t e n s i v e v a l l e y g l a c i e r s . 2.2 Bedrock Geology The study area i s s i t u a t e d between two d i s t i n c t g e o l o g i c f e a t u r e s : (1) the Slocan s y n c l i n e to the north and (2) the domal V a l h a l l a Gneiss Complex to "the south. The V a l h a l l a Gneiss Complex i s centered at the core of the V a l h a l l a Dome near Gladsheim Peak, approximately 15 km to the south, and i n c l u d e s the southern h a l f of the study area ( P a r r i s h 1982 and Reesor 1965, see F i g u r e 2.2). F o l i a t i o n s of the gn e i s s dome d i p quaq u a v e r s a l l y and are r e f l e c t e d by a s e r i e s of inward f a c i n g c l i f f s which r i s e s t e e p l y to g e n t l y c u r v i n g r i d g e s e n t i r e l y surrounding the c e n t r a l gneiss c o r e . The hig h c l i f f s of lower Nemo Creek are an expression of northward d i p p i n g f o l i a t i o n s of the V a l h a l l a Dome i n c i s e d by e r o s i o n . The n o r t h - f a c i n g v a l l e y s lopes of Nemo Creek Basin more c l o s e l y approach d i p - s l o p e geometry and are consequently l e s s steep. Gneiss f o l i a t i o n s d i p between 15° and 25° to the NNE throughout the southern h a l f of the study area and can be c l e a r l y observed on the headwall of the upper Hoben Creek c i r q u e b a s i n . At the head of Nemo Creek, the monzonitic Nemo Lake Stock i n t r u d e s the gneiss complex. A mixture of metamorphic and p l u t o n i c rocks c o n s t i t u t e a zone of "mixed g n e i s s " at the ' ; 7 PLUTONICi"* i ' Mk<<<:\ o>y', --v-i)-,N(»>'v-x6 KM -v- LEGEND PLUTONIC granodiorite , leucoquartz monzonite, quartz diorite, quartz monzonite monzonite, granite HIGH GRADE MET AMORPHICS leucogranite gneiss granite gneiss, granodiori te-augen gneiss MEDIUM GRADE METAMORPHICS amphibolite , pelitic schist , ca lc -s i l i ca te metasediments ultramafics LOW GRADE METAMORPHICS argillite , quartzite , slate , pelitic phyllite F i g u r e 2 . 2 G e o l o g y o f t h e s t u d y a r e a . 1 2 i n t r u s i o n boundary (Reesor 1965) Both the g r a n i t i c and gneissic rocks are c o n s i s t e n t l y c o a r s e - g r a i n e d and are of s i m i l a r composition. North of the study area, an assemblage of low to medium grade metamorphic rocks form the s t r u c t u r a l l y complex Slocan S y n c l i n e on the northern boundary of the Nelson B a t h o l i t h and V a l h a l l a Dome. The topography formed on these mechanically weaker rocks i s more subdued than that found f a r t h e r to the south. A b e l t of medium grade intermediate metamorphic rocks, termed the Nemo Lakes B e l t , i s exposed p r i n c i p a l l y i n the drainage of Wee Sandy Creek and f a r t h e r to the west. These rocks p o s s i b l y grade c o n t i n u o u s l y from the p e l i t i c p h y l l i t e s and s l a t e s to the nort h to the l e u c o g r a n i t e gneisses to the south ( P a r r i s h 1982). The p o s i t i o n of the contact between the p e l i t i c s c h i s t s and amp h i b o l i t e s of the Nemo Lakes B e l t and the l e u c o g r a n i t e g n e i s s e s of the V a l h a l l a Complex of the south i s u n c e r t a i n but i s occurs somewhere w i t h i n the Sharp Creek drainage area. The Wragge Creek Stock and East Cariboo stock l i e immediately to the north of Wee Sandy Creek. These m e c h n i c a l l y strong, c o a r s e - g r a i n e d g r a n i t i c rocks have r e s i s t e d g l a c i a l e r o s i o n and thus e x p l a i n the absence of c i r q u e basins on that s i d e of the b a s i n . The Snowslide Creek Stock, s i m i l a r i n composition to the Wragge Creek Stock, occurs to the west and south of upper Wee Sandy Creek Basin (see F i g u r e 2.2). The Slocan Lake F a u l t bounds the study area to the east and precl u d e s the c o r r e l a t i o n of rocks a c r o s s Slocan Lake ( P a r r i s h 1982). The g r a n i t e s (some p o r p h y r i t i c ) of the Nelson B a t h o l i t h l i e immediately to the east of the V a l h a l l a Gneiss Dome as does 13 the Slocan Group ( L i t t l e 1952). 2.3 S u r f i c i a l Geology The West Kootenay region has undergone m u l t i p l e g l a c i a t i o n s (Holland 1976) g i v i n g r i s e to complex d i s t r i b u t i o n s of g e n e t i c m a t e r i a l s i n c l u d i n g morainal (basal and a b l a t i o n ) , g l a c i o f l u v i a l , f l u v i a l and c o l l u v i a l d e p o s i t s i n the study area. A main trunk g l a c i e r once occupied the Slocan V a l l e y to at l e a s t the 1200 meter l e v e l d u r i n g the l a s t g l a c i a t i o n as evidenced by i c e - m a r g i n a l g l a c i o f l u v i a l d e p o s i t s and g l a c i a l f l u t e s between Hoben and Sharp Creek on e a s t - f a c i n g s l o p e s . The t r i b u t a r y g l a c i e r s occupying Nemo, Sharp, Hoben and Wee Sandy Creek Basins probably receeded u p - v a l l e y p r i o r to the disapperarance of the trunk g l a c i e r during f i n a l a b l a t i o n stages, as seen i n a modern example of the d e g l a c i a t i o n stages of a s i m i l a r v a l l e y i n Alaska ( F i g u r e 2.3). F l u t e d knobs to the south of the confluence of Wee Sandy Creek with Slocan Lake suggest that a reentrant of the main trunk g l a c i e r entered i n t o the lower Wee Sandy Creek B a s i n . Lower t r i b u t a r y v a l l e y s are dominated by both t r i b u t a r y and trunk g l a c i a l d e p o s i t s . 1 4 Figure 2.3 Salmon Glacier, near Stewart, B.C. showing the retreat of a tributary glacier prior to the disappearance of the main trunk g l a c i e r . The tributary valley occupies a valley similar in geometry to that of both Nemo and Wee Sandy Creeks. (Photo taken by W.H. Mathews). 2.3.1 Morainal Deposits Ablation morainal blankets and veneers 1 are abundant throughout the study area. Comminution of coarse grained bedrock in Nemo, Hoben and portions of Sharp and Wee Sandy Creek Basins has produced gravelly to sandy morainal deposits with s i l t 'The term 'ablation' refers to the wastage of g l a c i a l ice by melting and evaporation leading to depostion of eng l a c i a l l y and/or supraglacially transported debris, the term 'blanket' means a mantle of unconsolidated materials thick enough to mask minor i r r e g u l a r i t i e s in the underlying unit, but which s t i l l conforms to the general underlying topography (generally greater than 1 m th i c k ) , and the term 'veneer' means a layer of unconsolidated materials too thin to mask the minor i r r e g u l a r i t i e s of the underlying unit surface (between 10 cm and 1 m th i c k ) . 15 f r a c t i o n s g e n e r a l l y l e s s than 20% and only minor c l a y . A b l a t i o n morainal d e p o s i t s i n Wee Sandy Creek ba s i n tend to have f i n e r sand components r e s u l t i n g form comminution of f i n e r grained s c h i s t s . F i g u r e 2.4 i l l u s t r a t e s the obvious d i f f e r e n c e between Gravel Sand Coarse to Fine medium Silt Clay U.S. standard sieve sizes £ 6 d d o Grain diameter, mm F i g u r e 2.4 G r a i n - s i z e d i s t r i b u t i o n s f o r morainal SM s o i l s sampled in Nemo and Wee Sandy Creek B a s i n s . the g r a i n - s i z e d i s t r i b u t i o n s of morainal s i l t y sand d e p o s i t s (SM in the U n i f i e d S o i l C l a s s i f i c a t i o n ) sampled i n Wee Sandy Creek 1 6 Basin versus Nemo Creek B a s i n 1 . These m a t e r i a l s , although widely d i s t r i b u t e d , were seldom observed i n c r o s s - s e c t i o n i n g u l l i e s or on stream banks i n the study area. Consequently, o b s e r v a t i o n s were mostly l i m i t e d to s o i l p i t s where the l a t e r a l c o n t i n u i t y or s t r a t i f i c a t i o n of a depo s i t i s not observable. E n g l a c i a l and s u p r a g l a c i a l m a t e r i a l s w i t h i n a b l a t i o n moraine are complex and f r e q u e n t l y i n c l u d e g l a c i o f l u v i a l lenses and pockets i n other areas (Embleton and King 1968). In one i n s t a n c e , a marked v a r i a t i o n i n te x t u r e was observed w i t h i n 1 meter l a t e r a l l y where a t r e e had overturned exposing u n d e r l y i n g a b l a t i o n moraine a s s o c i a t e d with a g l a c i o f l u v i a l pocket (see samples Nl9-1and N19-2 i n Appendix A). Such c o m p l e x i t i e s make p o s i t i v e i d e n t i f i c a t i o n of a b l a t i o n moraine problematic as most o b s e r v a t i o n s are l i m i t e d to s o i l p i t s . M o r a i n a l d e p o s i t s a s s o c i a t e d with c i r q u e b a s i n s at higher e l e v a t i o n s are t y p i c a l l y blocky or rubbly with fewer f i n e s and in p l a c e s e x h i b i t t e r m i n a l of l a t e r a l moraine morphology. In come cases, t a l u s aprons grade g r a d u a l l y i n t o a b l a t i o n moraine near c i r q u e b a s i n w a l l s . A l o b a t e rock g l a c i e r e x i s t s i n one n o r t h - f a c i n g c i r q u e i n Nemo Creek B a s i n . L a t e r a l or t e r m i n a l moraines are rare in the lower v a l l e y s , as are kame t e r r a c e s . • Compact b a s a l morainal d e p o s i t s are observed only where 1The percentages of coarse fragments g r e a t e r than approximately 2 cm were estimated in the f i e l d and d i s c a r d e d from the sample. Samples were then a i r - d r i e d and s i e v e d with U.S. standard mesh si e v e s a c c o r d i n g to ASTM D1140-54 s p e c i f i c a t i o n s . S i z e f r a c t i o n s l e s s than .425 mm (# 40 sie v e ) were determined by the standard hydrometer method (Bowles 1978). Sample l o c a t i o n s are shown on Map A ( f i l e d s e p a r a t e l y ) . 1 7 morainal b l a n k e t s a s s o c i a t e d with the main trunk g l a c i e r are deeply i n c i s e d by stream e r o s i o n . Sample N-0+80 taken from near lower Nemo Creek y i e l d s percentages of s i l t and c l a y of 43% and 12% r e s p e c t i v e l y . U n l i k e a b l a t i o n moraine i n the area, these compact d e p o s i t s can i n p l a c e s i n c l u d e pockets of pure c l a y . In g e n e r a l , a b l a t i o n moraine and g l a c i o f l u v i a l m a t e r i a l s blanket' u n d e r l y i n g b a s a l moraine. T h i s may e x p l a i n the s u r p r i s i n g absence of observable b a s a l moraine i n the study area. 2.3.2 G l a c i o f l u v i a l D e p o s i t s G l a c i o f l u v i a l d e p o s i t s are c h a r a c t e r i z e d by rounded to subangular, w e l l s o r t e d sands and g r a v e l s with l i t t l e s i l t or c l a y . These d e p o s i t s occur throughout the study area but are p a r t i c u l a r l y common on the e a s t - f a c i n g slopes of the main Slocan V a l l e y . Morphologic f e a t u r e s a s s o c i a t e d with these d e p o s i t s i n c l u d e small t e r r a c e s , r i d g e s , b l a n k e t s , and veneers. G l a c i o f l u v i a l d e p o s i t s i n p l a c e s grade i n t o , or are mixed with, a b l a t i o n morainal m a t e r i a l s of s i m i l a r t e x t u r e and a n g u l a r i t y , and can be d i s t i n g u i s h e d only by the absence of s i l t and c l a y . R e l a t i v e l y short t r a n s p o r t d i s t a n c e s w i t h i n the t r i b u t a r y basins f r e q u e n t l y r e s u l t i n subangular cobbles and g r a v e l s a s s o c i a t e d with i c e - m a r g i n a l or e n g l a c i a l s o r t i n g by g l a c i a l meltwater. 18 2.3.3 F l u v i a l Deposits F l u v i a l d e p o s i t s are c h a r a c t e r i z e d by w e l l to moderately w e l l s o r t e d sands and g r a v e l s a s s o c i a t e d with present-day creeks and streams on f l a t or t e r r a c e d f l o o d p l a i n s and fans. These d e p o s i t s are u s u a l l y c o n f i n e d to narrow f l o o d p l a i n s and small fans w i t h i n 50 m of a c t i v e streams and are a r e a l l y l i m i t e d . 2.3.4 C o l l u v i a l Deposits C o l l u v i a l d e p o s i t s are c h a r a c t e r i z e d by p o o r l y s o r t e d , blocky to rubbly m a t e r i a l s on steep slopes o v e r l y i n g bedrock or accumulated on or at the base of slopes by g r a v i t y - i n d u c e d movement. These d e p o s i t s occur throughout the study area and dominate the steeper t e r r a i n of higher e l e v a t i o n s where they are mostly d e r i v e d from bedrock. The blocky, rubbly t e x t u r e c h a r a c t e r i s t i c of c o l l u v i u m i s s t r o n g l y i n f l u e n c e d by the composition and competence of the g r a n i t e s , g n e i s s e s and s c h i s t s from which i t i s d e r i v e d . C o l l u v i u m occurs most f r e q u e n t l y as veneers and/or blankets mantling steep t e r r a i n i n excess of 30° on upper s l o p e s . Thick fans and aprons are common along the toe s l o p e s of steep rock escarpments at any e l e v a t i o n . Colluvium d e r i v e d from bedrock along the upper p a r t s of v a l l e y s i d e s i n p l a c e s o v e r l i e a b l a t i o n moraine or g l a c i o f l u v i a l d e p o s i t s . 19 2.3.5 Weathering Weathering by b i o l o g i c a l and p h y s i c a l agents has a l t e r e d only s l i g h t l y the n e a r - s u r f a c e p h y s i c a l p r o p e r t i e s of s u r f i c i a l m a t e r i a l s . Humo-ferric P o d z o l i c s o i l s are common throughout the area where i n a c t i v e geomorphic processes allow s o i l development. These s o i l s t y p i c a l l y occur i n coarse to medium t e x t u r e d , a c i d parent m a t e r i a l s , under f o r e s t or heath v e g e t a t i o n i n c o o l to very c o l d humid to perhumid c l i m a t e s (Canada S o i l Survey Committee 1978). Humo-ferric Podzols were never observed deeper than 1 m and P o d z o l i c s o i l development has had l i t t l e e f f e c t on the bulk p h y s i c a l c h a r a c t e r i s t i c s of s u r f i c i a l m a t e r i a l s i n the area. Mechanical weathering of coarse l i t h i c fragments i n g l a c i a l m a t e r i a l s was o c c a s i o n a l l y observed i n Wee Sandy Creek Basin where m i c a - r i c h s c h i s t s have broken down, i n - p l a c e s p r e f e r e n t i a l l y to more g n e i s s i c rock fragments. However, most s u r f i c i a l m a t e r i a l s have not been s i g n i f i c a n t l y weathered s i n c e d e p o s i t i o n . 2.4 Geomorphic Processes Geomorphic processes i n c l u d i n g d e b r i s avalanches, d e b r i s flows, r o c k s l i d e s , rock f a l l s , snow avalanches, water-born e r o s i o n and f l o o d i n g are a c t i v e throughout the steep, r e c e n t l y g l a c i a t e d t e r r a i n of the study area. Each type of process has played a r o l e i n modifying slope morphology s i n c e the l a s t g l a c i a t i o n . The f o l l o w i n g d i s c u s s i o n w i l l be l i m i t e d to those processes which d i r e c t l y a f f e c t or are themselves a f f e c t e d by 20 the a c t i v i t i e s of man. 2.4.1 Debris Avalanche - Debris Flows De b r i s avalanches are r a p i d , shallow f a i l u r e s from steep s l o p e s i n v o l v i n g s l i d i n g , bouncing and r o l l i n g of c o h e s i o n l e s s s u r f i c i a l m a t e r i a l along a r e l a t i v e l y impermeable, mechanically strong bedrock or compact t i l l shear surface (Swanston 1979 and Burroughs 1980). 1 When the s u r f i c i a l m a t e r i a l i s n e a r l y s a t u r a t e d at time of f a i l u r e , d e b r i s a v a l a n c h i n g may r e v e r t to d e b r i s flowage r e s u l t i n g i n the r a p i d downslope t r a n s p o r t of a s l u r r y of s o i l , rocks, and organic d e b r i s d i r e c t l y to stream channels. The combined term 'debris avalanche - d e b r i s flow' i s used where d e b r i s avalanches are i n i t i a t e d by p a r t i a l s o i l s a t u r a t i o n and almost immediately r e v e r t to d e b r i s flows a f t e r i n i t i a l f a i l u r e . In the study area, d e b r i s avalanches occur on long uniform slopes with continuous blan k e t s or veneers of s u r f i c i a l m a t e r i a l , on steep g u l l y s i d e w a l l s undercut by r e c u r r e n t d e b r i s flows or continuous s u r f a c e e r o s i o n , or on steep t e r r a c e faces adjacent to stream channels. The l a r g e s t avalanches occur on long uniform s l o p e s where ample s u p p l i e s of m a t e r i a l are a v a i l a b l e f o r t r a n s p o r t . F i g u r e 2.5 i s an example of a major d e b r i s avalanche on a n o r t h - f a c i n g slope with a smooth 1 T h i s d e f i n i t i o n of d e b r i s avalanche i n c l u d e s the l a n d s l i d e type commonly r e f e r e d to as 'debris s l i d e ' by Varnes (1958,1978). The d i s t i n c t i o n i s problematic as the two processes are c l o s e l y r e l a t e d and v i r t u a l l y i n d i s t i n g u i s h a b l e i n many environments (Blong 1973). 21 unweathered granite gneiss shear surface inclined at 30° in lower Nemo Creek Basin. Figure 2.5. Debris avalanche on a north-facing slope of the lower Nemo Creek Basin. Many debris avalanche - debris flows occur adjacent to g u l l i e s and are subsequently confined to a previously scoured channel. With time, numerous small landslides may accumulate s i g n i f i c a n t amounts of debris in the gully bottoms only to be mobilized later by a major debris flow from above or by 22 e x c e s s i v e storm flow during an extreme storm event. 1 D e b r i s flows i n i t i a t e d by small spoon-shaped d e b r i s avalanches i n l i n e a r depressions are more common than simple planar d e b r i s avalanches. S o u t h - f a c i n g s l o p e s of lower Wee Sandy Creek Basin i n c l i n e d between . 30° and 40° show evidence of numerous V-notch g u l l i e s which have, at some time, given r i s e to d e b r i s avalanche - d e b r i s flows. These chutes can be d i s t i n g u i s h e d from o r d i n a r y e r o s i o n a l g u l l i e s by the presence of levee d e p o s i t s on d e b r i s fans at the g u l l y mouth and/or l a r g e t r a n s p o r t e d boulders on g u l l y s i d e w a l l s or i n the channel. These g u l l i e s may a l s o serve as avalanche paths when they develop i n a l p i n e areas. Many g u l l i e s have not had d e b r i s flows f o r at l e a s t 150 years as i n d i c a t e d by o l d growth f o r e s t stands growing on levee d e p o s i t s and i n g u l l i e s . F i g u r e 2.6 i s an example of an o l d d e b r i s avalanche - d e b r i s flow path subsequently r e f o r e s t e d i n lower Nemo Creek B a s i n . The r e l a t i v e dormancy or a c t i v i t y of a d e b r i s flow i s determined by (1) the area of catchment b a s i n f e e d i n g i n t o the g u l l y , (2) the s t a b i l i t y of s l o p e s i n the d e b r i s source area, (3) the g r a d i e n t of the g u l l y where the flow gains d e s t r u c t i v e momentum and (4) the s i z e and g r a d i e n t of the d e b r i s fan at the g u l l y mouth (Eisbacher 1982). On the n o r t h - f a c i n g s l o p e s of lower Wee Sandy Creek, source m a t e r i a l s f o r d e b r i s flows are d e r i v e d l a r g e l y from g l a c i o f l u v i a l and morainal b l a n k e t s . F i g u r e 1Such an occurrence i s r e f e r e d to by many authors as a 'debris t o r r e n t ' ( W i l f o r d and Schwab 1982 and M i l e s and K e l l e r h a l s 1981). 23 Figure 2.6 Debris avalanche - debris flow path subsequently reforested on the north-facing slope of lower Wee Sandy Creek Basin. 2.7 is the plan view of a particular gully network incised into a g l a c i o f l u v i a l blanket that serves as a debris source area for a recurrent debris flow system. The size of the debris fan at the base suggests that this debris flow system has been only s l i g h t l y active since the last g l a c i a t i o n . There i s l i t t l e evidence of f l u v i a l erosion at the toe of the fan. Debris fans at the mouths of g u l l i e s on north-facing slopes of both Nemo and Wee Sandy Creek Basins are r e l a t i v e l y small in re l a t i o n to the fans on the south-facing slopes. These larger 24 fans develop from more frequent d e b r i s flows o r i g i n a t i n g i n long, l i n e a r , rock-walled g u l l i e s i n c l i n e d in excess of 40° on steep rock c l i f f s and benches where c o l l u v i a l m a t e r i a l s F i g u r e 2.7 Debris flow system on the n o r t h - f a c i n g slope of the lower Wee Sandy Creek Basin showing the d e b r i s source, the main g u l l y , and the d e b r i s f an. accumulate. Heavy r a i n f a l l s , perhaps coupled with r a p i d snowmelt, p e r i o d i c a l l y f l u s h the accumulated d e b r i s from g u l l i e s r e s u l t i n g i n d e p o s i t i o n on the d e b r i s fan. C o l l u v i a l m a t e r i a l s on the s o u t h - f a c i n g slopes are c o n t i n u o u s l y accumulating and serve as e x c e l l e n t d e b r i s source areas. F i g u r e 2.8 shows a recent c o l l u v i a l l y d e r i v e d d e b r i s flow b i f u r c a t e d on a d e b r i s fan. T h i s p a r t i c u l a r flow continued to t r a n s p o r t d e b r i s on slopes as low as 6° i n t o Nemo Creek at the toe of the fan. Fi g u r e 2.9 i s a diagram showing a t y p i c a l p r o f i l e of a d e b r i s flow path measured on the s o u t h - f a c i n g slope of Nemo Creek Basin and the s i z e of the l a r g e s t boulder d e p o s i t e d on each segment of the fan'. Debris fans may have toe sl o p e s as low as 5° or as high 25 Figure 2.8 Debris flow that has bifurcated on a debris fan of upper Nemo Creek Basin. as 20° depending on the width of the valley into which the flow descends. The texture of a debris fan is similar to that of a c o l l u v i a l apron or fan. 26 DEBRIS FLOW BOULDERS CHUTE  1 » 1 m (erosion) BOULDERS DIAMETER >1 m 30° ̂ 3 BOULDERS DIAMETER LARGE 1 m 24° SMALL COBBLES DIAMETER COBBLES 20° 6° 12° 17° .HAZARD CLASSIFICATION HAZARD CLASSIFICATION F l F2 F i g u r e 2.9 P r o f i l e of a d e b r i s flow on a d e b r i s fan i n upper Nemo Creek Basin showing the s i z e s of the l a r g e s t rock fragments d e p o s i t e d as levees along the flow path. Included are the hazard s u b d i v i s i o n s f o r d e b r i s fans d i s c u s s e d i n Chapter 5. 2.4.2 R o c k s l i d e s A r o c k s l i d e i s a r a p i d downslope movement of rock, e i t h e r as an incoherent mass or as a l a r g e unbroken block detached from bedrock, and may have e i t h e r a c u r v i l i n e a r or planar shear s u r f a c e , depending on the nature and o r i e n t a t i o n of c o n t r o l l i n g j o i n t s , bedding, planes, f o l i a t i o n s or other d i s c o n t i n u i t i e s . S e v e r a l l a r g e , deep-seated, r o t a t i o n a l r o c k s l i d e s were i d e n t i f i e d on the n o r t h - f a c i n g s l o p e s of both Nemo and Wee Sandy Creek B a s i n s . These s l i d e s are c h a r a c t e r i z e d by w e l l d e f i n e d arcuate headwall scarp areas a s s o c i a t e d w i t h a l t e r e d v a l l e y - s i d e 27 forms below. Near lower Nemo Creek, the toe of a large rotational rock sli d e i s undergoing toppling f a i l u r e as evidenced by deep cracks shown in Figure 2.10. It i s supposed Figure 2.10 Toppling rock f a i l u r e in lower Nemo Creek Basin. that n o r t h w a r d dipping f o l i a t i o n s , coupled with g l a c i a l l y oversteepened slopes are contributing to these s l i d e s . They are large enough to constitute mappable rock units, but because they either (1) retain their mantle of o r i g i n a l s u r f i c i a l materials or (2) form blocky c o l l u v i a l slopes similar to other non-sliding areas, they have not been mapped i n d i v i d u a l l y . Small rockslides are confined to areas of steep rock and colluvium where f a i l u r e i s controlled by e x f o l i a t i o n j o i n t i n g and perhaps i n i t i a t e d by frost-wedging or seismic a c t i v i t y . 28 2.4.3 R o c k f a l l s C o l l u v i a l aprons and fans at the base of v i r t u a l l y a l l steep rock c l i f f s a t t e s t to the frequency of r o c k f a l l s i n the study a r e a . The.wide j o i n t spacings and high competence of these metamorphic and p l u t o n i c rocks r e s u l t i n the detachment of l a r g e blocks s e v e r a l meters in diameter. In g e n e r a l , r o c k f a l l s are known to be most frequent d u r i n g earthquake events and d u r i n g freeze-thaw p e r i o d s . No f a l l s were observed d u r i n g the course of t h i s study. 2.4.4 E r o s i o n E r o s i o n a l processes are c l o s e l y a s s o c i a t e d with mass- wasting processes as both are mutually interdependent. E r o s i o n may produce l o c a l slope oversteepening which i n c r e a s e s i n s t a b i l i t y which, in t u r n , c o n t r i b u t e s m a t e r i a l to be f u r t h e r eroded. G u l l i e s formed s o l e l y by s u r f a c e e r o s i o n a l processes o c c a s i o n a l l y occur throughout the , study area but are most commonly a s s o c i a t e d with sandy g l a c i o f l u v i a l -terraces and deeper morainal b l a n k e t s . G r a v e l l y - t o - r u b b l y , shallow, w e l l - d r a i n e d s o i l s have r e t a r d e d n a t u r a l e r o s i o n a l processes i n most other areas. 29 2.4.5 S o i l Creep Trees t i p p e d or bowed along t h e i r e n t i r e l e n g t h , i n d i c a t i v e of i n c i p i e n t l a n d s l i d i n g or s o i l creep, were r a r e l y observed i n the study area. The non-viscous p r o p e r t i e s of sandy to g r a v e l l y s o i l s have l i m i t e d s o i l creep processes to those a s s o c i a t e d with the incremental movement of d i s c r e t e p a r t i c l e s a c c e l e r a t e d by t r e e r o o t i n g and t r e e overthrow. Only on slop e s i n c l i n e d in excess of 35° was strong evidence of s o i l creep observed. F i g u r e 2.11 shows the b u t t r e s s i n g e f f e c t s of a t r e e on a 40° slope s u b j e c t to s o i l creep and perhaps some minor s l o u g h i n g . 2.4.6 Snow Avalanching L i n e a r s c a r s and v e g e t a t i o n p a t t e r n s on steep f o r e s t e d s l o p e s that d i s p l a y sharp t r i m l i n e s i n d i c a t e that snow avalanches f r e q u e n t l y occur i n the area. Many avalanche paths reach the v a l l e y bottom, p a r t i c u l a r l y on the s o u t h - f a c i n g slopes of both Nemo and Wee Sandy Creek Basins. Snow avalanches most commonly s t a r t i n steep rocky t e r r a i n , then become concentrated i n chutes or g u l l i e s . C o l l u v i a l aprons or veneers, and/or d e b r i s fans u s u a l l y serve as run-out zones. 30 Figure 2.11 Buttressing effect of tree roots r e s i s t i n g s o i l creep. 2.5 Climate The southern Selkirk mountains are influenced by both maritime and continental a i r masses. The climate i s predominately moist at lower elevations, e.g. 574 mm/yr at Fauquier (elev. 472 m), and increases to wet at upper elevations, e.g. 1055 mm/yr at Sandon (elev. 1067 m). Localized variations in regional weather patterns are s i g n i f i c a n t in the mountainous terrain of the study area and are influenced by loc a l aspect, elevation, r e l a t i v e topographic position, and the 31 e f f e c t s of l o c a l bodies of water or i c e . The seasonal v a r i a t i o n s i n t o t a l monthly p r e c i p i t a t i o n f o r 5 l o c a l weather s t a t i o n s from 1941 to 1970 (see F i g u r e 2.12 f o r s t a t i o n l o c a t i o n s ) i n d i c a t e that p r e c i p i t a t i o n p a t t e r n s i n F i g u r e 2.12 Index map showing l o c a t i o n s of p r i n c i p a l weather s t a t i o n s . Slocan V a l l e y proper are s i m i l a r from s t a t i o n to s t a t i o n at the lower e l e v a t i o n s of the v a l l e y bottom and that p r e c i p i t a t i o n i n c r e a s e s s i g n i f i c a n t l y with e l e v a t i o n i n the New Denver - Sandon area (see F i g u r e 2.13). Extreme 24-hour p r e c i p i t a t i o n i n t e n s i t i e s between 1941 and 1970 are shown i n F i g u r e 2.14 f o r New Denver, Fauquier and Sandon. At a l l s t a t i o n s , the most 32 25 O M H < 200 150 100 i 50 0 -I CRESCENT VALLEY elev. 450 m SOUTH SLOCAN elev. 457 m NEW DENVER elev. 564 m SANDON elev. 1067 m FAUQUIER elev. 472 ra J J MONTH F i g u r e 2.13 Mean t o t a l monthly p r e c i p i t a t i o n f o r s e l e c t e d weather s t a t i o n s near the study area ( A i r S t u d i e s Branch, B.C M i n i s t r y of Environment). intense storms occurred d u r i n g the months of June and September with i n t e n s i t i e s ranging between 41 and 56 mm/day. The unstable a i r p a t t e r n s which dominate the summer season g i v e r i s e to l a r g e thunder c e l l s that a f f e c t l o c a l i z e d areas o n l y . The random nature of these storm events p r e c l u d e s the det e r m i n a t i o n of any a r e a l d i s t r i b u t i o n p a t t e r n . However, i t has been suggested that summer thunderstorms may i n c r e a s e i n frequency and i n t e n s i t y at upper e l e v a t i o n s ( U t z i g 1978). The 48-hour p r e c i p i t a t i o n extremes f o r New Denver are given in F i g u r e 2.15 showing a maximum of 77 mm per 48-hour p e r i o d . R a i n f a l l occurs d u r i n g every month of the year while s n o w f a l l i s l i m i t e d to November through A p r i l . U n f o r t u n a t e l y , no temperature data i s a v a i l a b l e f o r the New Denver - Sandon area. 75 SANDON — NEW DENVER - — FAUQUIER T T 1 1 1 1 1 1 1 , 4 F M A M J J A S O N D MONTH OF L A S T DAY OF PERIOD F i g u r e 2.14 Twenty-four hour p r e c i p i t a t i o n extremes f o r New Denver, Sandon and Fauquier from 1924 to-1979 ( A i r S t u d i e s Branch, B.C. M i n i s t r y of Environment). 25 H 1 1 1 1 1 1 1 1 1 1 f J F M A M J J A S O N D MONTH OF L A S T DAY OF PERIOD F i g u r e 2..15 F o u r t y - e i g h t hour p r e c i p i t a t i o n extremes f o r New Denver, B.C. from 1924 to 1979 ( A i r S t u d i e s Branch, B.C. M i n i s t r y of Environment). 34 2.6 V e g e t a t i o n The study area i s dominated by the Engelmann Spruce - Subalpine F i r (ESSF) and the I n t e r i o r Western Red Cedar - Western Hemlock (ICH) B i o g e o c l i m a t i c Zones a c c o r d i n g to K r a j i n a and Brooke (1969). The ESSF zone occurs above the 1400 meter l e v e l on the upper slopes of a l l basins w i t h i n the study area. The ICH zone dominates lower s l o p e s , p a r t i c u l a r l y those near Slocan Lake. T i m b e r l i n e i s at approximately 1700 meters. Pl a n t and shrub communities are o f t e n i n d i c a t i v e of c e r t a i n c l i m a t i c and h y d r o l o g i c v a r i a b l e s . Some s p e c i e s are r e s t r i c t e d to a p a r t i c u l a r h a b i t a t while others occupy a broad range of h a b i t a t s . The dominant p l a n t and shrub s p e c i e s o c c u r r i n g on d r i e r , w e l l - d r a i n e d s o i l s throughout the study area i n c l u d e Paxistima m y r s i n i t e s , Vaccinium membranaceum, Spiraea b e t u l i f o l i a , Shepherdia canadensis , Mahonia a q u i f o l i u m , Chimaphila umbellata , Linnaea b o r e a l i s , F r a g a r i a vesca , C l i n t o n i a u n i f l o r a , and Goodyera o b l o n g i f o l i a . In somewhat wetter areas where water i s not removed from the s o i l q u i t e as r a p i d l y , G a u l t h e r i a o v a t i f o l i a , Vaccinium membranaceum , P y r o l a c h l o r a n t h a , T i a r e l l a u n i f o l i a t a , Cornus canadensis , Gymnocarpium d r y o p t e r i s , and S m i l a c i n a racemosa become more p r e v e l a n t . In areas where the groundwater t a b l e approaches the s o i l s u r f a c e d u r i n g c e r t a i n times of the year, Oplopanax h o r r i d u s , Ribes l a c u s t r e , Rubus p a r v i f l o r u s } Streptopus a m p l e x i f o l i u s , Athyrium f i l i x - f e m i n a , Equisetum arvense and Carex disperma are more abundant. A complete d e s c r i p t i o n of p l a n t a s s o c i a t i o n s found w i t h i n the ICH and ESSF b i o g e o c l i m a t i c zones i s found i n U t z i g et a l (1978). In areas where the f o r e s t 35 canopy in p a r t i c u l a r l y dense, understory v e g e t a t i o n can be sparse. Geomorphic processes play a major r o l e i n determining v e g e t a t i o n cover. Those slopes i n f l u e n c e d by snow avalanching and c o l l u v i a l a c t i v i t y support shrub communities only, while those s u b j e c t to only p e r i o d i c d i s t u r b a n c e s permit mature f o r e s t development. F o r e s t cover i s t y p i c a l l y dense where e l e v a t i o n l i m i t a t i o n s , t h i n s o i l s , or geomorphic processes do not i n h i b i t t r e e growth. V a r i a t i o n i n f o r e s t cover type i s l a r g e l y governed by c l i m a t e and a v a i l a b l e moisture. In c e r t a i n areas Pinus c o n t o r t a (lodgepole pine) and Pseudotsuga m e n z i e s i i (Douglas f i r ) stands were found to p r e f e r sunnier, d r i e r south aspects at lower e l e v a t i o n s . Otherwise, f o r e s t type v a r i e s l a r g e l y with temperature - moisture changes a s s o c i a t e d with e l e v a t i o n . 36 CHAPTER 3 SLOPE STABILITY IN THE STUDY AREA 3.1 Approaches To Slope S t a b i l i t y Assessment Despite recent advances i n slope s t a b i l i t y a n a l y s i s and knowledge of s o i l and rock p r o p e r t i e s , the complexity and h e t e r o g e n i e t y of most n a t u r a l s l o p e s prevent accurate d e t e r m i n a t i o n of s t a b i l i t y c o n d i t i o n s and a complete understanding of causes of f a i l u r e (O'Loughlin 1981). S o i l mechanics theory, a p p l i e d to s i t e s p e c i f i c a n a l y s i s of s t a b i l i t y , i s accurate i n a s s e s s i n g the s t r e n g t h - s t r e s s r e l a t i o n s h i p s i n a small area. However, past experience has demonstrated that t h i s technique r e q u i r e s c o n s i d e r a b l e g e o t e c h n i c a l e x p e r t i s e , accurate measurement of the e n g i n e e r i n g p r o p e r t i e s of s o i l s i n v o l v e d , and a s p e c i f i c knowledge of the geology and groundwater hydrology at a s i t e . Where the slopes of an e n t i r e watershed are being analysed, such techniques are c o s t l y and i m p r a c t i c a l . Consequently, a number of approaches to a n a l y s i n g f a c t o r s c o n t r o l l i n g slope s t a b i l i t y at the reconnaissance l e v e l have been developed by v a r i o u s r e s e a r c h e r s . Remote sensing coupled with p a t t e r n r e c o g n i t i o n techniques have been used to examine t e r r a i n f o r f e a t u r e s d i s t i n c t i v e of l a n d s l i d e hazards in many areas (Foggin and Rice 1979). T h i s approach i s e f f e c t i v e i f l a n d s l i d e hazards manifested i n s u r f a c e c h a r a c t e r i s t i c s can be photographed or otherwise d e t e c t e d . A second approach, i n v o l v i n g e m p i r i c a l models developed through s t a t i s t i c a l a nalyses of measurable f i e l d and photogrammetric 37 data, attempts to provide a numerical index of slope s t a b i l i t y ( F u r b i s h 1981 and P i l l s b u r y 1976). M u l t i p l e r e g r e s s i o n and d i s c r i m i n a n t f u n c t i o n analyses are common techniques f o r dev e l o p i n g such r e l a t i o n s h i p s . A more common approach i s the s t a b i l i t y f a c t o r technique. F a c t o r s r e l a t e d to l a n d s l i d e occurrence are i n d i v i d u a l l y d e l i n e a t e d on separate maps then superimposed. Factor combinations found to be c o i n c i d e n t with known l a n d s l i d e hazards on the map are i d e n t i f i e d . I d e n t i c a l f a c t o r combinations i n other areas on the map are then assigned an a p p r o p r i a t e hazard r a t i n g . Computer techniques are now a v a i l a b l e that f a c i l i t a t e manipulation and weighting of f a c t o r s so that hazard maps can be q u i c k l y produced from d i g i t i z e d f a c t o r maps (VanDriel 1980). U n f o r t u n a t e l y , each of these approaches n e c e s s a r i l y makes gross assumptions about the o p e r a t i o n of the p h y s i c a l system l e a d i n g to l a n d s l i d i n g . Even in s t a t i s t i c a l l y r i g o r o u s s t u d i e s , assumptions are o f t e n i m p l i c i t i n the a n a l y s i s . T h i s has a number of consequences. E v a l u a t i o n s can be expected to be operator dependent due to the d i f f i c u l t i e s i n weighting the importance of v a r i o u s c l a s s i f i c a t i o n i n d i c e s . If the weighting i s wrong or i f c e r t a i n f a c t o r s are ne g l e c t e d , eg. l o s s of root s t r e n g t h f o l l o w i n g h a r v e s t i n g , the r e s u l t i n g e v a l u a t i o n may be very i n a c c u r a t e . Beven (1981) p o i n t s out t h a t , "Without some u n d e r l y i n g p h y s i c a l l y based s t r u c t u r e on which to base t h i s type of ( s l o p e s t a b i l i t y ) a n a l y s i s , we may be very wrong i n i n t e r p r e t i n g the success or f a i l u r e of our evaluation....whenever p o s s i b l e , we should base our models on •we l l - d e f i n e d p h y s i c a l p r i n c i p l e s , r a t h e r than on e m p i r i c a l 38 r e l a t i o n s h i p s t h a t , to a l a r g e extent, obscure cause and e f f e c t r e l a t i o n s h i p s . " R e cently, g e o t e c h n i c a l models that take i n t o account v a r i a b i l i t y of s o i l p r o p e r t i e s and groundwater c o n d i t i o n s have been developed in order to extend p h y s i c a l p r i n c i p l e s of s t a b i l i t y to l a n d s l i d e hazard mapping systems {Simons et a l . 1978 and Wu and Swanston 1980). Slope e q u i l i b r i u m can be determined i n terms of 'expected f a c t o r of s a f e t y ' and ' p r o b a b i l i t y of f a i l u r e ' , depending on the amount of u n c e r t a i n t y i n v o l v e d i n e i t h e r the measurement or e s t i m a t i o n of slope p r o p e r t i e s . T h i s approach has the advantage of being more e a s i l y t r a n s f e r r e d from one region to another, but l i k e the other approaches, has s e v e r a l assumptions l i m i t i n g i t s u n i v e r s a l a p p l i c a b i l i t y . Moreover, what at the outset may seem l i k e a completely o b j e c t i v e method may a c t u a l l y i n v o l v e many s u b j e c t i v e estimates of model v a r i a b l e s that render i t as s u b j e c t i v e as any other approach. A p h y s i c a l model does, however, pr o v i d e the b a s i s f o r r e c o g n i z i n g where man has l e a s t understanding and opens the way f o r f u r t h e r s t u d i e s which can strengthen model weaknesses. T h i s study i s a f i r s t attempt at a p p l y i n g a p h y s i c a l l y - based model to l a n d s l i d e hazard mapping in B r i t i s h Columbia. I t i s hoped t h a t , i f proven s u c c e s s f u l , the technique w i l l be u s e f u l i n other p a r t s of the p r o v i n c e . 39 3.2 The S t o c h a s t i c G e o t e c h n i c a l Model In s i m p l e s t terms, a l a n d s l i d e occurs i f the shear s t r e s s a c t i n g on the slope i s g r e a t e r than or equal to the shear s t r e n g t h or i n t e r n a l r e s i s t a n c e to shear s t r e s s of the slope m a t e r i a l . The f a c t o r of s a f e t y (FS) of the slope i s the r a t i o of the shear s t r e n g t h (s) to shear s t r e s s ( T ) , SO that FS = S/T (3.1) In t r a d i t i o n a l s o i l mechanics, s o i l shear s t r e n g t h i s represented by the Coulomb equation s = C + a' tan.*' (3.2) . where c' i s e f f e c t i v e 1 s o i l cohesion, e' i s e f f e c t i v e normal s t r e s s , and <t>y i s e f f e c t i v e angle of i n t e r n a l f r i c t i o n . However, in f o r e s t e d watersheds, f a c t o r s i n c l u d i n g root cohesion and t r e e surcharge weight should a s l o be i n c l u d e d i n equation 3.2 (Brown and Sheu 1975 and O'Loughlin 1973). T h i s has l e a d to an expanded v e r s i o n of the shear s t r e n g t h equation represented by s = C + Cr + H c os 2£ [qo/H + (ysat- rwet)M + y(l-M)] tan*' (3.3) f o r f o r e s t e d s l o p e s as formulated by Simons et a l . (1978). 1The term " e f f e c t i v e " r e f e r s to measurements that take i n t o account pore water pressure e f f e c t s . 40 Symbols are d e f i n e d a c c o r d i n g to F i g u r e 3.1. S i m i l a r l y , the simple v e r s i o n of shear s t r e s s due to the t a n g e n t i a l component of g r a v i t a t i o n a l s t r e s s along the b a s a l 6 = slope i n c l i n a t i o n 0 = range of angles of i n t e r n a l f r i c t i o n of s o i l C = s o i l cohesion Cr = root cohesion qo = tree surcharge load ywet = unit weight of s o i l y sat = unit weight of saturated s o i l Y = unit weight of water H = height of s o i l mantle above shear surface Hw = height of water table above shear surface M = Hw/H F i g u r e 3.1. D e f i n i t i o n s of input v a r i a b l e s used in the d e t e r m i n i s t i c g e o t e c h n i c a l model. zone of s l i d i n g i s expressed as T = W.sinp (3.4) 41 where 0 i s slope angle and W i s weight of the s o i l mass above the shear plane; but with the a d d i t i o n of the e f f e c t s of t r e e surcharge l o a d and groundwater, the equation expands to T = H(qo/H + rsat(M) + r(1-M)) s i n * cos* (3.5) It i s assumed in t h i s equation that the s o i l i s homogeneous and i s o t r o p i c , the p i e z o m e t r i c s u r f a c e i s p a r a l l e l to a p l a n a r shear s u r f a c e , and that the e f f e c t s of wind s t r e s s are n e g l i g i b l e . I f equations 3.3 and 3.5 are d i v i d e d d i r e c t l y to determine f a c t o r of s a f e t y , i t i s a l s o assumed that the slope i s p l a n a r and semi- i n f i n i t e . As d i s c u s s e d i n Chapter 2, much of the study area has only t h i n d e p o s i t s of s u r f i c i a l m a t e r i a l s subject to shallow p l a n a r - type f a i l u r e s on a bedrock shear s u r f a c e and d e b r i s flows are g e n e r a l l y c o n f i n e d to narrow g u l l i e s that c r o s s the broader s l o p e s . Many of the slopes g e n e r a l l y s a t i s f y the g e o m e t r i c a l assumptions of the i n f i n i t e slope model. 1 In i t s expanded form, the i n f i n i t e slope model i s expressed as 1 I t may be noted that g l a c i a l d e p o s i t s i n the study area are g e n e r a l l y inhomogeneous. T h i s d i f f i c u l t y i s p a r t i a l l y addressed by Lumb(l970) who found that an inhomogeneous, m u l t i l a y e r s o i l can o f t e n be represented as a homogeneous s o i l i f the s o i l p r o p e r t i e s are averaged p r o p e r l y . 42 c' + Cr + H cos 2*? [qo/H + ( r s a t - ywet)M + (1-M)] tan*' FS = H(qo/H + r s a t (M) + y(l-M)) sinp cos? (3.6) The model i n c l u d e s terms f o r s o i l ( C , r s a t , r, #'), v e g e t a t i o n (Cr, qo), topography ( p ) and groundwater (M) as shown i n F i g u r e 3.1. As with a l l mathematical models, a n a l y s i s of model s e n s i t i v i t y h e l ps d e f i n e which values must be c a r e f u l l y c o l l e c t e d and which can be roughly estimated. R e a l i s t i c ranges of v alues f o r FS v a r i a b l e s are f i r s t s e l e c t e d and the midpoint f o r each value i s used to compute FS. By a l t e r i n g the value of one v a r i a b l e at a time a c r o s s i t s range of va l u e s , the percent change i n FS can be c a l c u l a t e d . The r e s u l t s of the a n a l y s i s are shown i n F i g u r e 3,2. From the f i g u r e i t i s evident that slope e q u i l i b r i u m i s h i g h l y s e n s i t i v e to slope angle, s o i l i n t e r n a l f r i c t i o n , r e l a t i v e p i e z o m e t r i c s u r f a c e , s o i l cohesion and root cohesion. V a r i a t i o n of f a c t o r s such as s o i l d e n s i t y and t r e e surcharge load i s o b v i o u s l y l e s s important. These r e s u l t s c o i n c i d e with general o b s e r v a t i o n s that l a n d s l i d e s are a s s o c i a t e d with steep s l o p e s , seepage, and s o i l s with low shear s t r e n g t h . I t has been argued that the i n f i n i t e slope model f a i l s to account f o r many environmental f a c t o r s known to i n f l u e n c e s l i d e l o c a t i o n (Blong 1981). Many such f a c t o r s , however, d i r e c t l y i n f l u e n c e the valu e s of fundamental v a r i a b l e s i n the model (Table 3.1). The determination of the cause e f f e c t r e l a t i o n s h i p s shown i n Table 3.1 i s the source of much s u b j e c t i v i t y i n the g e o t e c h n i c a l approach. 43 + « A F S +%AX 100% -%AF8 100% Figure 3.2 Diagram showing the s e n s i t i v i t y of the factor of safety FS to variations in model variables. Many of the factors influencing the model variables are time dependent. Figure 3.3 i l l u s t r a t e s the cycle involved in reaching slope equilibrium. Slopes can conceiveably follow any evolutionary path within the cycle, the end result being long- term s t a b i l i t y . The values of model variables are determined by the severity and number of cycles a p a r t i c u l a r slope has undergone since g l a c i a t i o n . For example, one slope may have been continuously subjected to l o c a l steepening by gully action and subsequently burned by a forest f i r e . A p a r t i c u l a r l y intense r a i n f a l l two years later then triggers a landslide. An adjacent slope may have escaped any or a l l of the above events and thus 44 FACTOR INCREASE FUNDAMENTAL VARIABLES IN MODEL FS M Cr B qo REFERENCES RAINFALL PREVIOUS LANDSLIDE DEFORESTATION ROAD BUILDING TREE CANOPY DENSITY PROXIMITY TO DRAINAGE DEPRESSION WET CLIMATE SLOPE FLOODING EARTHQUAKE SHADED ASPECT 0 Cleveland 1973 0 Terzaghi and Peck 1967| - Brown and Sheu 1975 Prellwitz 1975 + L i 1974 0 O'Loughlin 1973 ± Schumm 1968 0 Megahan 1972 0 Youd 1973 ± Lee 1963 Table 3.1 E f f e c t s of v a r i o u s f a c t o r s on the v a r i a b l e s fundamental to the i n f i n i t e slope model. remained s t a b l e . From F i g u r e 3.3 i t i s apparent that road b u i l d i n g i s the most s i g n i f i c a n t f a c t o r as i t can a l t e r the model v a r i a b l e s <t>, C, Cr, M, and p s i m u l t a n e o u s l y . A s t o c h a s t i c v e r s i o n of the d e t e r m i n i s t i c i n f i n i t e slope model has been developed to allow f o r the input of a r e a l i s t i c range of values f o r 0 , C, and Cr. Monte C a r l o t e s t i n g of v a r i o u s s t a t i s t i c a l d i s t r i b u t i o n s of these v a l u e s by Ward et a l . (1978) has determined that assumed uniform ( r e c t a n g u l a r ) d i s t r i b u t i o n s of input v a r i a b l e s r e s u l t i n the most c o n s e r v a t i v e estimates of FS, and that the FS d i s t r i b u t i o n i s best d e s c r i b e d as Gaussian (normal). The assumed uniform d i s t r i b u t i o n i s p a r t i c u l a r l y convenient because an upper and lower l i m i t (or d i s c r e t e range) can be assign e d without the need f o r s t a t i s t i c a l l y d e r i v e d standard d e v i a t i o n s . The d e r i v a t i o n of the s t o c h a s t i c g e o t e c h n i c a l model i s d e s c r i b e d i n d e t a i l i n Appendix B. The methodology f i r s t i n v o l v e s the de t e r m i n a t i o n of a 4 5 F i g u r e 3 . 3 A l t e r a t i o n s to slope e q u i l i b r i u m subsequent to g l a c i a t i o n l e a d i n g to long-term s t a b i l i t y . r e a l i s t i c range of values f o r <p, Cr and C over the e n t i r e slope u n i t . S i n g l e values of p, M, H, r, and qo are then estimated as e i t h e r average or 'worst case' values f o r the s l o p e . Whereas the v a r i a b l e s 0 and M are probably the most c r i t i c a l to the r e s u l t i n g value of FS, the assignment of a s i n g l e value, assumed r e p r e s e n t a t i v e of an e n t i r e s l o p e , i s sometimes p r o b l e m a t i c a l . T h i s d i f f i c u l t y w i l l be d i s c u s s e d i n s e c t i o n s 3 . 5 and 3 . 6 . I t i s important at t h i s p o i n t to decide whether or not a l l model assumptions are s a t i s f i e d f o r the slope being analysed. I f not, an a l t e r n a t i v e method of s t a b i l i t y a n a l y s i s i s r e q u i r e d . The value s a s s i g n e d to the model v a r i a b l e s are then run through a s e r i e s of equations which r e s u l t i n the output of an expected 4 6 d i s t r i b u t i o n of FS f o r the s l o p e . The value of FS at the peak of the frequency d i s t r i b u t i o n curve i s the 'expected value of f a c t o r of s a f e t y ' E [ F S ] . The area under that p o r t i o n of the curve with values of FS < 1.0 y i e l d s the ' p r o b a b i l i t y of f a i l u r e ' P, so that f a i l u r e P so that P = p[FS<1.0]. (3.7) Values of E[FS] and P can then be used to d e s c r i b e the s t a t e s of e q u i l i b r i u m of va r i o u s slope u n i t s throughout the area. The model, though extremely s i m p l i f i e d , demonstrates i n a semi- q u a n t i a t i v e s e n s e 1 , the behaviour of a slope having c e r t a i n f a c t o r combinations. I t e x p l a i n s i n r e a l p h y s i c a l terms some of the i n t u i t i v e judgements made q u a l i t a t i v e l y by v a r i o u s r e s e a r c h e r s and provi d e s a framework on which to base r e l a t i v e hazard c l a s s i f i c a t i o n . The f o l l o w i n g s e c t i o n s d e a l " with the measurement and d e l i n e a t i o n of the fundamental v a r i a b l e s found i n the study a r e a . Slopes with c h a r a c t e r i s t i c s which do not allow the a p p l i c a t i o n of the model are t r e a t e d s e p a r a t e l y . 1The term ' s e m i - q u a n t i t a t i v e ' suggests that many of the input v a r i a b l e s are somewhat s u b j e c t i v e l y determined. 47 3.3 S o i l Shear Strength S o i l shear s t r e n g t h i s a f u n c t i o n of e f f e c t i v e angle of i n t e r n a l f r i c t i o n (*'), e f f e c t i v e cohesion ( C ) and the e f f e c t i v e s t r e s s (*') as d e s c r i b e d by equation 3.2. The value of <f> depends on g r a i n s i z e d i s t r i b u t i o n , p a r t i c l e shape, p a r t i c l e i n t e r l o c k i n g , and s o i l d e n s i t y . Larger v a l u e s of <t> are a s s o c i a t e d with dense, w e l l graded, angular m a t e r i a l s (Rahn 1969). The value of c' i s a r e f l e c t i o n of the a t t r a c t i v e f o r c e s between f i n e s o i l p a r t i c l e s . The value of i s a f u n c t i o n of the s o i l d e n s i t y y and pore water pressure u, so that tf' = e - u (3.8) where c = yHcos 2£. 3.3.1 E s t i m a t i o n Of S o i l Shear S t r e n g t h T r a d i t i o n a l l y , values of <t> and C are determined by d i r e c t l y t e s t i n g s o i l s i n a mechanical shear apparatus. Large i n - s i t u shear t e s t i n g equipment, designed f o r s o i l - r o o t networks, has been developed f o r use by O'Loughlin (1973) and Gray (1970) but i s somewhat u n r e l i a b l e , c o s t l y , and time consuming. These and other types of i n - s i t u apparatuses are a l s o very d i f f i c u l t to use i n reconnaissance surveys because roads are r e q u i r e d f o r t h e i r t r a n s p o r t a t i o n and i n s t a l l a t i o n . Shear boxes or t r i a x i a l shear t e s t i n g equipment can be used in the l a b o r a t o r y as a second a l t e r n a t i v e . However, depending on the s i z e of the apparatus, <t> and C values obtained can be 48 i n a c c u r a t e when g r a v e l l y s o i l s with l a r g e coarse c l a s t s are t e s t e d . Moreover, l a r g e samples are r e q u i r e d i f coarse g r a i n e d s o i l s are to be a c c u r a t e l y represented. Where l o g i s t i c s are d i f f i c u l t and s o i l s are g r a v e l l y , the t e s t i n g of a l a r g e number of r e p r e s e n t a t i v e samples can be pr o b l e m a t i c . A l t e r n a t i v e methods f o r shear s t r e n g t h determination i n c l u d e the i n f e r e n c e of reasonable ranges of 0 and C from e i t h e r slope morphology or from past experience with s o i l s of an i d e n t i c a l e n g i n e e r i n g c l a s s i f i c a t i o n . Although such i n f e r e n c e s are h i g h l y s u b j e c t i v e and much l e s s a c curate than a c t u a l shear t e s t i n g , the methods o f f e r the advantage of being b e t t e r able to account f o r l o c a l v a r i a t i o n s i n <t> and C r e s u l t i n g from the v a r i a b i l i t y of g l a c i a l and f l u v i o g l a c i a l d e p o s i t i o n a l c o n d i t i o n s . E r o s i o n a l slopes on c o h e s i o n l e s s s o i l s sometimes r e f l e c t the <t> parameter i f f a c t o r s such as root cohesion and pore water pr e s s u r e s have no i n f l u e n c e on the s o i l mechanics (Krynine and Judd 1957). Slopes s u b j e c t to dry r a v e l g e n e r a l l y behave a c c o r d i n g to the equation tan* = tan*, a l l o w i n g the i n f e r e n c e of <t> v a l u e s - f o r s o i l s with 'loose' r e l a t i v e d e n s i t i e s and no cohesion. These <t> values f o r loose m a t e r i a l s are e s p e c i a l l y u s e f u l when p r e d i c t i n g the s t a b i l i t y of road c u t s or f i l l s (Wilson 1976). U n f o r t u n a t e l y , e r o s i o n a l s l o p e s c l e a r l y u n i n f l u e n c e d by the e f f e c t s of v e g e t a t i o n , groundwater, or snow ava l a n c h i n g are extremely r a r e i n the study area. Moreover, the m a j o r i t y of slop e s to be c r i t i c a l l y e v a l u a t e d f o r impacts of f o r e s t h a r v e s t i n g are n e c e s s a r i l y f o r e s t e d ! T h i s method i s , t h e r e f o r e , of l i m i t e d u t i l i t y . 49 If a s u r f i c i a l d e p o s i t i s c l a s s i f i e d a c c o r d i n g to i t s g e n e s i s , a l t e r a t i o n , and t e x t u r e , i t s range of p r o p e r t i e s should be s i m i l a r to those found i n another s u r f i c i a l d e p o s i t having the same gen e s i s , a l t e r a t i o n and t e x t u r e (Wilson 1976 and Wilson et a l . 1982). 1 Whereas s o i l p h y s i c a l p r o p e r t i e s , i n c l u d i n g shear s t r e n g t h , are l a r g e l y dependent on g r a i n s i z e d i s t r i b u t i o n and the p l a s t i c i t y of the f i n e f r a c t i o n , the U n i f i e d C l a s s i f i c a t i o n System (USC) has been developed on t h i s b a s i s f o r e n g i n e e r i n g purposes (see Appendix C f o r d e f i n i t i o n s ) . E n g i n e e r i n g experience with the USC throughout the world has allowed engineers to compile expected ranges of <t> and C as shown i n Table 3.2. U n f o r t u n a t e l y , d e l i n e a t i n g the a r e a l d i s t r i b u t i o n of USC s o i l c l a s s e s i s d i f f i c u l t because d e p o s i t s are g e n e r a l l y d i s t r i b u t e d a c c o r d i n g to g e n e s i s , not g r a i n s i z e d i s t r i b u t i o n . I t i s t h e r e f o r e necessary to c h a r a c t e r i z e mappable genetic m a t e r i a l s ( i . e . a b l a t i o n moraine, etc.) a c c o r d i n g to t h e i r t y p i c a l USC c l a s s i f i c a t i o n . Such a c h a r a c t e r i z a t i o n provides the l i n k between past e n g i n e e r i n g experience i n determining <t> and C with mappable s o i l u n i t s i n the study area. Sources of u n c e r t a i n t y with t h i s method i n c l u d e the u n r e l i a b i l i t y or l a c k of u n i v e r s a l a p p l i c a b i l i t y of ranges of <t> and C r e p o r t e d i n the l i t e r a t u r e , the problems i n v o l v e d with a c c u r a t e l y c l a s s i f y i n g and mapping m a t e r i a l s a c c o r d i n g to the USC and g e n e s i s , and the wide range of values r e s u l t i n g from the i d e n t i f i c a t i o n of more 1 W i l s o n and h i s co-authors develop t h i s concept i n t o an approach to the a r e a l assessment of s o i l e n g i n e e r i n g p r o p e r t i e s termed 'pe d o t e c h n i c a l e n g i n e e r i n g 1 . 50 CLASSIFICATION AVERAGE * VALUE RANGES REFERENCES 2 use SUBGROUP LOOSE DENSE GW 36-40 40-45 BA,BO,M GW-GC 31--38 BA,M SANDY 33-36 36-42 BA,H,M GB 34-36 36-38 BO,M GM 34 e s t . M GC 31 e s t . M sw 33-36 36-41 BA,H,M SP DRY COARSE 32-35 35-38 BA,BO,M WET COARSE 31-34 34-37 BA,M MEDIUM 31-34 34-39 BA,BO,M FINE 28-32 32-37 BA,BO,M,H SM MOIST 29-33 30-34 BA,BO,M SATURATED 25-29 29-32 BA,BO,M SC 28-34 M 2BA M = = BAZANT(1979) BO = BOWLES(1979) H = HOUGH(1957) = MOORE(1969) Table 3.2 Estimated angles of i n t e r n a l f r i c t i o n (*) f o r c o h e s i o n l e s s s o i l s than one USC c l a s s w i t h i n a s i n g l e complex g e n e t i c m a t e r i a l . 3 Values of 0 and C a l s o vary a c c o r d i n g to the r e l a t i v e d e n s i t y encountered i n the f i e l d . T h i s can be c r u d e l y estimated with a r e i n f o r c i n g rod and hammer, a method d e v i s e d by the USDA (1975), and should be c o n s i d e r e d when determining a p p r o p r i a t e <t> and C v a l u e s (see Appendix D f o r complete d e s c r i p t i o n of methodology). 3 T h i s i s n e c e s s a r i l y the case with complex g l a c i a l m a t e r i a l s that have <t> and C values which vary over a broad range. 51 3.3.2 Range Of S o i l Shear Strength Values In order to determine the d i s t r i b u t i o n of g e n e t i c m a t e r i a l s and hence <t> and C i n the study area, the T e r r a i n C l a s s i f i c a t i o n System (TCS) was employed i n mapping u n i t s on the b a s i s of t e x t u r e , g e n e t i c m a t e r i a l , surface e x p r e s s i o n and modifying p r o c e s s . Developed by the Province of B r i t i s h Columbia, the system i s designed to be e s p e c i a l l y u s e f u l i n r e c e n t l y g l a c i a t e d t e r r a i n (E.L.U.C. 1976). The r e s u l t s of the mapping p r o j e c t are found as Map B ( t e r r a i n map f i l e d s e p a r a t e l y ) . An e x p l a n a t i o n of the symbols and terminology are found on the legend of the map, and i n Appendix E. The t e r r a i n map demonstrates that the s u r f i c i a l m a t e r i a l s i n the study area are extremely complex with many map u n i t s r e q u i r i n g composite symbols. During the course of the mapping, samples were taken of the c h a r a c t e r i s t i c g e n e t i c m a t e r i a l s and l a t e r c l a s s i f i e d a c c o r d i n g to USC. Where sampling was not p o s s i b l e , USC c l a s s e s were estimated i n the f i e l d a c c o r d i n g to standard c r i t e r i a . I t was found that the USC c l a s s e s are governed to a l a r g e extent by mode of d e p o s i t i o n and can be i n f e r r e d from knowledge of s o i l t e x t u r e and g e n e s i s . 1 Indeed, the p a r t i c l e s i z e and g r a d a t i o n requirements of coarse g r a i n e d USC c l a s s e s are very s i m i l a r to the c r i t e r i a a p p l i e d i n i n f e r r i n g g e n e s i s . T e r r a i n u n i t s r e c o g n i s e d i n the f i e l d were found to i n c l u d e 1 U n f o r t u n a t e l y , the t e x t u r e d e s i g n a t i o n of TCS does not r e l a t e d i r e c t l y to the USC. 52 a p r e d i c t a b l e number of U n i f i e d S o i l c l a s s e s and are a c c o r d i n g l y grouped in F i g u r e 3.4. The range of <t> and C valu e s f o r v a r i o u s g e n e t i c m a t e r i a l s were c a l c u l a t e d and ranked a c c o r d i n g to r e l a t i v e s t r e n g t h . As one would expect, c o l l u v i a l and morainal s o i l s have higher <t> values with a g r e a t e r range than s i l t y sandy f l u v i a l and f l u v i o g l a c i a l d e p o s i t s . S i l t y sandy (SM) s o i l s , with s i l t percentages of l e s s than 20% by weight, occur i n a v a r i e t y of g e n e t i c m a t e r i a l types. S i l t y (ML) s o i l s of low p l a s t i c i t y r a r e l y occur as pockets w i t h i n complex a b l a t i o n morainal and f l u v i o g l a c i a l d e p o s i t s , but are not continuous enough to add a s i g n i f i c a n t weak component. Mor a i n a l s o i l s have ranges of <t> of up to 16° f o r 'loose' to ' f i r m ' d e p o s i t s . 1 R e l a t i v e l y homogeneous sandy f l u v i a l d e p o s i t s , on the other hand, have <t> values which may vary w i t h i n a 5° range. Thus the complexity or u n p r e d i c t i b i l i t y of the s o i l i s r e f l e c t e d i n the range of probable <t> v a l u e s i n F i g u r e 3.4. S u r f i c i a l d e p o s i t s observed i n s o i l p i t s w i t h i n 1 meter of the s u r f a c e almost always e x h i b i t f i r m r e l a t i v e d e n s i t i e s as determined by m u l t i p l e probings with a r e i n f o r c i n g rod. I t i s p o s s i b l e that m a t e r i a l s are denser deeper w i t h i n the d e p o s i t . However, the predominance of t h i n b l a n k e t s or veneers, g i v i n g r i s e to shallow planar f a i l u r e s i n the study area, suggest that the f i r m $ values are most a p p l i c a b l e to s t a b i l i t y c a l c u l a t i o n s . D isturbance by road b u i l d i n g may loosen the m a t e r i a l and cause a d e c l i n e i n The loose <t> v a l u e s can be used i n c a l c u l a t i o n s of 1 I n areas where compact t i l l s are common, the range of p o s s i b l e <t> v a l u e s can be much higher than 16°. I n - s i t u compact t i l l s are much stronger than remolded or d i s t u r b e d m a t e r i a l s . 53 I STRENGTH] CLASS TCS u s e LOOSE d> VALUES 25 c FIRM <(> VALUES 4 5 ° / 2 5 ° A5° s*sFu .sM 4gT SM 27° 32° 2 9 ° 34 c f g F G frM fgM ^rM SM GM 27 c 34 c 29° 36° g F G gkF^ kgF^ sF sF sF SP GP SM GM SP SM SP 27° 36 c * * * * * * * * * 3 0 ° 34° 2 9 ° 38° 3 2 ° 3 6 ° ksF kF eF GP SP 3 0 ° 35° 3 1 ° 37° rM gM bM srM SW SM GW GM 27 e 40° * * * * 29 c 43 aC rC arC GW 3 5 ° 4 0 ° * * ** 3 8 ° 4 3 ° F i g u r e 3 . 4 . R a n g e s o f <t> v a l u e s f o r b o t h l o o s e a n d f i r m s u r f i c i a l m a t e r i a l s i n t h e s t u d y a r e a . The a s t e r i s k s r e p r e s e n t p l o t s o f 0 v a l u e s i n f e r r e d f r o m a n g l e s o f r e p o s e on r o a d c u t s a n d f i l l s n e a r t h e s t u d y a r e a . p o s s i b l e r o a d c u t o r r o a d f i l l s t a b i l i t y . The e s t i m a t e d r a n g e s o f <t> g i v e n i n F i g u r e 3 . 4 a p p e a r r e a s o n a b l e when c o m p a r e d w i t h t h e r e p o s e a n g l e s o f c u t a n d f i l l 54 s l o p e s on roads b u i l t i n s i m i l a r m a t e r i a l s to the south of the study area. In a l l but one case, the assumed range encompasses the <t> values i n f e r r e d from the measured s l o p e s . The p l o t s of these measurements are shown as a s t e r i s k s on F i g u r e 3.4. F u r t h e r work i s needed to cross-check the assumed ranges with a c t u a l shear t e s t data and repose angles measured i n s i m i l a r m a t e r i a l s . Estimates of s o i l cohesion are more d i f f i c u l t to determine i n the f i e l d . A t t e r b e r g l i m i t t e s t s were performed on a l l samples c o l l e c t e d i n order to determine the p l a s t i c i t y and cohesion c h a r a c t e r i s t i c s of the f i n e f r a c t i o n . In almost a l l cases, s i l t y sands (SM) and s i l t y g r a v e l s (GM) d i d not have measurable p l a s t i c i t y and e x h i b i t e d l i t t l e c o hesion. S i m i l a r r e s u l t s from A t t e r b e r g t e s t s have been r e p o r t e d by Swanston (1970) who determined by shear t e s t i n g cohesion values approaching 0 f o r s i l t y - l o a m s o i l s d e r i v e d from a b l a t i o n moraine i n southeast A l a s k a . With the e x c e p t i o n of compact t i l l and the o c c a s i o n a l s i l t or c l a y lens i n a morainal or g l a c i o f l u v i a l d e p o s i t , a l l s o i l s i n the study area are assumed to have n e g l i g i b l e cohesion. 3.4 Root Strength Cohesion imparted to s o i l s by t r e e r o o t s has an important e f f e c t on s o i l s t r e n g t h w i t h i n the root zone. In the study area, t h i s zone i s c o n f i n e d to the upper meter of s o i l . Roots can c o n t r i b u t e s t r e n g t h to the s o i l by b i n d i n g and r e i n f o r c i n g s o i l p a r t i c l e s , anchoring to the u n d e r l y i n g bedrock s u r f a c e or l a t e r a l l y to adjacent root networks, t r a n s f e r i n g surcharge loads to the substratum, and inducing negative pore pr e s s u r e s by root 55 c a p i l l a r y t e n s i o n (O'Loughlin 1973). The apparent root cohesion Cr i s a term which d e s c r i b e s the combined e f f e c t s of r o o t s on s o i l s t r e n g t h and i s one of the most d i f f i c u l t parameters i n the g e o t e c h n i c a l model to a c c u r a t e l y determine. 3.4.1 E s t i m a t i o n Of Root Strength Brown and Sheu (1975) and Wu et a l . (1979) d e s c r i b e f i v e p o s s i b l e ways of e s t i m a t i n g root cohesive s t r e n g t h i n c l u d i n g (1) b a c k - a n a l y s i s of a slope f a i l u r e ; (2) a n a l y s i s of s o i l creep; (3) a n a l y s i s of i n d i v i d u a l t r e e overthrows; (4) i n - s i t u measurements such as those by O'Loughlin (1973); and (5) s o i l - root . s t r e s s - f i e l d m odelling with known root t e n s i l e s t r e n g t h s . Method 2 i s i m p r a c t i c a l and can only be a p p l i e d to cohesive s o i l s having v i s c o u s behaviour. Method 3 i s a l s o i m p r a c t i c a l as such a computation r e q u i r e s a known wind v e l o c i t y imparting the o v e r t u r n i n g moment and ignores the p o s s i b l e presence of r o o t - r o t l e a d i n g to the overthrow. Method 4 r e q u i r e s i n - s i t u shear t e s t i n g and was deemed i m p r a c t i c a l because of the i n a c c e s s i b i l i t y of many p a r t s of the study area. Method 1, l i k e the other methods, has many p o s s i b l e sources of e r r o r because many input v a l u e s assumed c o r r e c t i n a back a n a l y s i s are as d i f f i c u l t to a c c u r a t e l y determine as root s t r e n g t h , such as seepage c o n d i t i o n s at time of f a i l u r e or s o i l s t r e n g t h . Consequently, s t u d i e s of slope f a i l u r e s by d i f f e r e n t r e s e a r c h e r s have produced c o n f l i c t i n g root s t r e n g t h values f o r i d e n t i c a l f a i l u r e s (see O'Loughlin 1973 versus Morton 1975). Method 5 can be employed when data on root t e n s i l e s t r e n g t h are a v a i l a b l e . T h i s may prove to be an extremely u s e f u l method as 56 r e s e a r c h i n t o the s t r e n g t h and r o o t i n g h a b i t s of v a r i o u s t r e e s p e c i e s develops. 3.4.2 Range Of Root Cohesion Values F o r e s t cover maps compiled by B.C. M i n i s t r y of F o r e s t s , when supplemented with data from a e r i a l photos, provide the b a s i s f o r determining the d i s t r i b u t i o n of slopes l i k e l y to be i n f l u e n c e d by root cohesion. Values of Cr vary a c c o r d i n g to f o r e s t type, r o o t i n g depth, s o i l depth, t r e e d e n s i t y and t r e e s i z e . No methodology c u r r e n t l y e x i s t s f o r a c c u r a t e l y d e l i n e a t i n g the d i s t r i b u t i o n of Cr v a l u e s . A c e r t a i n r e a l i s t i c range of Cr val u e s can only be c a l c u l a t e d from s i t e - s p e c i f i c l a n d s l i d e back a n a l y s e s , or estimated from p r e v i o u s work i n s i m i l a r f o r e s t s elsewhere. A d e b r i s avalanche found w i t h i n 10 km of the study area f i t s the c r i t e r i a f o r root cohesion d e t e r m i n a t i o n by back- a n a l y s i s . L a n d s l i d e D-14 occu r r e d i n a shallow c o l l u v i a l s o i l on a planar shear s u r f a c e at the colluvium-bedrock i n t e r f a c e where water d i v e r t e d by a l o g g i n g road completely s a t u r a t e d the slope (see F i g u r e 3.5). The f a c t that the f a i l u r e occurs on a uniform p l a n a r slope and has a l e n g t h to depth r a t i o g r e a t e r than 10 al l o w s the use of the i n f i n i t e slope model. Input values of C = 0 and <t> = 38°-43° are taken from Table 3.3, qo = 250 kg/m2 i s estimated from the l o c a l f o r e s t , r s a t = 1960 kg/m3 can be 1The values qo and r s a t , though somewhat i n a c c u r a t e l y estimated, should not int r o d u c e too much e r r o r , as FS i s much l e s s s e n s i t i v e to these than the other parameters (see F i g u r e 3.2). 57 estimated for firm GW s o i l 1 , H = 1.0 meters and * = 35° were d i r e c t l y measured and rw = 1000 kg/m3 is a physical constant. Firsthand observations of water issuing from the head scarp Figure 3.5. Landslide D-14 showing the f a i l u r e of a thin veneer of s o i l on a planar bedrock shear surface. strongly suggest that the s o i l was saturated at the time of f a i l u r e , i . e . M = 1.0 (refer to Figure 3 . 1 ) . Assuming that the factor of safety was 1.0 at the time of f a i l u r e , back-analysis yie l d s root cohesion values ranging between 301 kg/m2 and 409 kg/m2 for <f> values of between 38° and 43°. In-situ shear testing by Endo and Tsuruta (1968) in birch forests produced values of root cohesion ranging between 200 and 1200 kg/m2. Using a similar technique, O'Loughlin (1973) produced lower values ranging between 8 and 186 kg/m2 for 58 s e l e c t e d s i t e s i n southwestern B r i t i s h Columbia. O'Loughlin's t e s t s were conducted i n c o a s t a l cedar-hemlock f o r e s t s o i l s s i m i l a r to those encountered i n the study area. U n f o r t u n a t e l y , O'Loughlin's shear t e s t s only i n c l u d e d r o o t s with diameters l e s s than 3 to 4 cm, which only p a r t i a l l y c o n t r i b u t e to the o v e r a l l root c o h e s i o n . Doing f u r t h e r work with back a n a l y s i s techniques, O'Loughlin (1973) found that root cohesion v a l u e s more r e a l i s t i c a l l y f a l l w i t h i n the 160 to 300 kg/m2 range. Swanston (1970) d e r i v e d values ranging from 350 to 450 kg/m2 i n mountain t i l l s o i l s of southeastern Alaska using the same technique. These v a l u e s , though fraught with assumptions i n t h e i r c a l c u l a t i o n , do provide some estimate of the magnitude of reinforcement t r e e s c o u l d impart to s o i l s . I t appears that i n view of the f o r e g o i n g , Cr c o u l d c o n c e i v a b l y have values any where between 0 and 450 kg/m2, depending on the o r i e n t a t i o n of the shear plane r e l a t i v e to the r o o t i n g zone. Endo and Tsuruta's 1200 kg/m2 value f o r b i r c h f o r e s t s i s d i s r e g a r d e d as being unique to hardwood f o r e s t s . With only 1 data p o i n t f o r Cr v a l u e s i n the region of study a v a i l a b l e , i t i s v i r t u a l l y i m p ossible to d e l i n e a t e any a r e a l d i s t r i b u t i o n of Cr. I t i s t h e r e f o r e necessary to s u b j e c t i v e l y assume that i n a l l f o r e s t e d areas of the study area values of Cr w i l l range between 0 and 450 kg/m 2. In non-forested areas, the value of Cr i s assumed to be 0. 59 3.5 Groundwater Piez o m e t r i c pressures induced by groundwater lower the e f f e c t i v e normal s t r e s s on a slope and thereby reduce s o i l s t r e n g t h . In a d d i t i o n , groundwater can c o n t r i b u t e to s u r f a c e e r o s i o n , subsurface p i p i n g , r e d u c t i o n of cohesive s t r e n g t h , seepage p r e s s u r e , or l i q u i f a c t i o n d u r i n g earthquakes. Chamberlain (1972) introduced an ' i n t e r f l o w ' model of groundwater flow a p p l i c a b l e to f o r e s t s o i l s t y p i c a l of mountainous t e r r a i n i n B r i t i s h Columbia. The model d e s c r i b e s the process of water movement p a r a l l e l to the s o i l s u r f a c e caused by boundary c o n d i t i o n s , such as impermeable bedrock, s u f f i c i e n t l y r e s t r i c t i v e to prevent normal v e r t i c a l i n f i l t r a t i o n and p e c o l a t i o n to the water t a b l e . During an intense r a i n f a l l , water i s t r a n s m i t t e d v i a the extremely permeable s u r f a c e organic h o r i z o n (duff l a y e r ) to i n d i v i d u a l t r e e root channels. The water i s then conveyed through these h i g h l y conductive c o n d u i t s to a b a s a l zone composed of matted ro o t s at the i n t e r f a c e with bedrock or compact t i l l . The b a s a l zone d r a i n s r a p i d l y with high h y d r a u l i c c o n d u c t i v i t y and does not allow a true watertable or p i e z o m e t r i c s u r f a c e to form. The s o i l i s termed 'open' as i t does not allow the complete s a t u r a t i o n of s o i l matrix between root channels. The s o i l has a n a t u r a l drainage network. Chamberlain (1972) f u r t h e r suggests that root development r e l a t i v e to s o i l depth i s the most important of the s o i l forming f a c t o r s with respect to the 'openness' of a s o i l . I f r o o t s p e n e t r a t e to the f i r s t impermeable l a y e r , i t i s l i k e l y that an open s o i l w i l l develop; i f not, i n f i l t r a t i n g water w i l l be t r a n s m i t t e d to the l e s s permeable s o i l matrix between root 60 c o n d u i t s and r e s u l t i n a c l o s e d s o i l of lower h y d r a u l i c c o n d u c t i v i t y (deVries and Chow 1978). These c o n s i d e r a t i o n s are important i n p r e d i c t i n g the probable groundwater p o s i t i o n s on a s l o p e . Some of O'Loughlin's (1973) work i l l u s t r a t e s t h i s p o i n t . A steep convex slope with a 1 meter mantle of bouldery sandy loam over compact t i l l was monitored with piezometers. Measurements of p i e z o m e t r i c head du r i n g r a i n f a l l s of up to 80 mm/day i n d i c a t e d a maximum r i s e of only 10 cm. I t i s i n f e r r e d that r a p i d conduction of water through the root mat at the compact t i l l s u r f a c e d i d not allow p i e z o m e t r i c p r e s s u r e s to r i s e s i g n i f i c a n t l y . Deeper s o i l s , other than tough impermeable compact t i l l s , a llow f u l l root p e n e t r a t i o n and minimal mat formation. Water i s r a p i d l y t r a n s m i t t e d to the base of the root zone but i s subsequently i n c o r p o r a t e d i n t o a deeper flow path. In t h i s case, the groundwater t a b l e p o s i t i o n i s governed l a r g e l y by the h y d r a u l i c c o n d u c t i v i t y of the s o i l below the root zone. Depending on the r e l a t i v e slope p o s i t i o n of the s i t e , the groundwater t a b l e i s more l i k e l y to reach s t e a d y - s t a t e e q u i l i b r i u m . At the toe of a slo p e , groundwater input from upslope coupled with r a i n f a l l i n f i l t r a t i o n can cause the pi e z o m e t r i c s u r f a c e to r i s e to ..the s u r f a c e and c r e a t e temporary or permanent seepage (Freeze 1980). P i e z o m e t r i c pressures are more l i k e l y to i n f l u e n c e slope s t a b i l i t y i n these areas. Areas of permanent seepage or e l e v a t e d p i e z o m e t r i c s u r f a c e s can a l s o occur i n open s o i l s where drainage depressions c o n c e n t r a t e i n t e r f l o w . P i e z o m e t r i c s t u d i e s by O'Loughlin (1973) in steep l i n e a r drainage d e p r e s s i o n s i n shallow s o i l s 61 approximately 1 meter deep i n d i c a t e d 70 to 90 cm r i s e s i n p i e z o m e t r i c head given an 80 mm/day maximum r a i n f a l l i n put. Likewise, l i n e a r depressions i n shallow permeable s o i l s of the study area o c c a s i o n a l l y show s i g n s of ephemeral runoff i n d i c a t i n g complete s o i l s a t u r a t i o n . These l o c a l i z e d seepage areas, though not a r e a l l y e x t e n s i v e , have been s i g n i f i c a n t to l a n d s l i d e occurrence i n many areas (Bishop and Stevens 1967 and O'Loughlin 1973). 3.5.1 E s t i m a t i o n Of P i e z o m e t r i c Pressures The t r a d i t i o n a l method of monitoring slopes with piezometers was not f e a s i b l e i n the study area. Where p o s s i b l e , s u r f a c e i n d i c a t o r s are used to decipher subsurface groundwater c o n d i t i o n s . Swamps, s p r i n g s and seeps were mapped as discharge areas, i . e . where the water t a b l e i s at or near the ground s u r f a c e . M o t t l e d s o i l s or f r e e water observed i n s o i l p i t s a l s o i n d i c a t e a p e r i o d i c attainment of a ne a r - s u r f a c e water t a b l e . Where more d i r e c t groundwater i n d i c a t o r s are not a v a i l a b l e , v e g e t a t i o n types have been used to i n f e r groundwater c o n d i t i o n s with some success (Pole and S a t t e r l u n d 1978). C e r t a i n p l a n t s p e c i e s may occupy a broad range of h a b i t a t s while others may be r e s t r i c t e d t o , and thus i n d i c a t i v e of, s p e c i f i c moisture c o n d i t i o n s . When p l a n t s with h a b i t a t s r e s t r i c t e d to seepage areas are observed on steep s l o p e s , slope i n s t a b i l i t y can be suspected. The use of p l a n t i n d i c a t o r s f o r a s s e s s i n g e c o l o g i c a l moisture regimes r e q u i r e s an e x i s t i n g v e g e t a t i o n c l a s s i f i c a t i o n scheme f o r the b i o g e o c l i m a t i c subzone under c o n s i d e r a t i o n or a 62 reconnaissance of the area tp e s t a b l i s h r e l a t i o n s h i p s between v e g e t a t i o n i n d i c a t o r s and the range of moisture and n u t r i e n t c o n d i t i o n s . Such a scheme e x i s t s f o r southeastern B r i t i s h Columbia (Comeau et a l . 1982). When using p l a n t i n d i c a t o r s , the f i r s t s tep i s to determine the subzone by r e f e r i n g to b i o g e o c l i m a t i c maps p u b l i s h e d by B.C. M i n i s t r y of F o r e s t s . Southeastern B.C. has been d i v i d e d i n t o three c l i m a t i c r e g i o n s , each encompassing areas of s i m i l a r c l i m a t i c p a t t e r n s and each i n c l u d i n g a number of subzones. In the study area, the subzones i n c l u d e the Kootenay-Columbia Moist Southern I n t e r i o r Cedar-Hemlock Subzone (iCHal) and the Moist Southern Engelmann Spruce-Subalpine F i r Subzone (ESSFc). P l a n t s p e c i e s f o r a given moisture regime i n the subzone can then be compared with p l a n t s observed at a p a r t i c u l a r s i t e i n the study a r e a . Moisture regime d e f i n i t i o n s are given i n Table 3.3. The absence.of w e t - s i t e p l a n t i n d i c a t o r s does not guarantee the absence of o c c a s i o n a l n e a r - s u r f a c e s o i l s a t u r a t i o n . On the other hand, the presence of w a t e r - l o v i n g p l a n t a s s o c i a t i o n s can u s u a l l y guarantee that the water t a b l e i s n e a r - s u r f a c e at l e a s t sometime duri n g the year. By d e f i n i t i o n , mesic s i t e s are i n a s t a t e of e q u i l i b r i u m between p r e c i t a t i o n input and subsurface outflow. A mesic s i t e i n a wet b i o g e o c l i m a t i c subzone r e c i e v e s a much higher p r e c i p i a t i o n input than a mesic s i t e i n a dry subzone. Moisture regimes are t h e r e f o r e not q u a n t i t a t i v e l y e q u i v a l e n t from subzone to subzone and must be s c r u t i n i z e d with r e s p e c t to groundwater on a subzonal b a s i s . The ICHal and ESSFc subzones average p r e c i p i t a t i o n inputs ranging between 70 and 150 cm/yr 63 VERY XERIC Water removed extremely ra p i d l y i n r e l a t i o n to supply; s o i l i s moist for a n e g l i g i b l e time af t e r ppt. XERIC Water removed very r a p i d l y i n r e l a t i o n to supply; s o i l i s moist for b r i e f periods following ppt. SUBXERIC Water removed rap i d l y i n r e l a t i o n to supply; s o i l i s moist for short periods following ppt. SUBMESIC Water removed r e a d i l y i n r e l a t i o n to supply; water available for moderately short periods following ppt. MESIC Water removed somewhat slowly in r e l a t i o n to supply; s o i l may remain moist for a small, but s i g n i f i c a n t period of the year. SUBHYGRIC Water removed slowly enough to keep the s o i l wet for a s i g n i f i c a n t part of the growing season; some temper- ary seepage and possibly mottling below 20 cm. HYGRIC Water removed slowly enough to keep the s o i l wet for most of the growing season; permanent seepage and mottling present; possibly weak gleying. SUBHYDRIC Water removed slowly enough to keep the water table at or near the surface for most of the year; gleyed mineral or organic s o i l s ; permanent seepage less than 30 cm below the surface. Table 3.3. D e f i n i t i o n s of moisture regimes (from Walmsley et a l . 1980) . r e s p e c t i v e l y . Whereas p l a n t s do not r e f l e c t r a p i d changes i n pi e z o m e t r i c head caused by intense r a i n f a l l , drainage d e p r e s s i o n s with subxeric to mesic p l a n t i n d i c a t o r s c o u l d have p i e z o m e t r i c s u r f a c e s at or near the ground s u r f a c e at some b r i e f time d u r i n g the the year. Dynamic f l u c t u a t i o n s are c r i t i c a l to slope s t a b i l i t y and i n 1 t h e s e areas, p l a n t s f a i l to a c c u r a t e l y 64 i n d i c a t e c r i t i c a l subsurface groundwater c o n d i t i o n s . Examination of s o i l p i t s or exposed s o i l p r o f i l e s o f f e r s an op p o r t u n i t y to v e r i f y the determination of moisture regime made usi n g p l a n t i n d i c a t o r s . A coarse t e x t u r e d s o i l may have a water t a b l e j u s t below the r o o t i n g zone that i s , t h e r e f o r e , not i n d i c a t e d by the v e g e t a t i o n , which can be de t e c t e d only by exc a v a t i n g a s o i l p i t . On d i s t u r b e d s i t e s , more emphasis should be p l a c e d on r e s u l t s gained by examining the s o i l , as many of these i n d i c a t o r s may invade d r i e r , d i s t u r b e d s i t e s or may be d i s p l a c e d by other invader s p e c i e s (Walmsley et a l . 1980). Even though the use of p l a n t i n d i c a t o r s can be a v a l u a b l e t o o l f o r r e c o g n i z i n g moisture regimes, i t cannot completely r e p l a c e the examination of s o i l m a t e r i a l s . Of primary i n t e r e s t to slope s t a b i l i t y a n a l y s i s i s the r a t i o of the v e r t i c a l d i s t a n c e between the shear s u r f a c e and the water t a b l e to the v e r t i c a l s o i l t h i c k n e s s above the p o t e n t i a l shear plane. T h i s r a t i o M can be g e n e r a l i z e d f o r d i f f e r e n t moisture regimes i f the depth to f a i l u r e plane i s estimated. I t i s important to do t h i s , as reconnaissance mapping of groundwater i n the study area i s best accomplished by d e l i n e a t i n g moisture regime d i s t r i b u t i o n s and r e l a t i n g them to values of M. 65 3.5.2 Estimated E f f e c t s Of Groundwater In Study Area Surface drainage r e s u l t i n g from heavy r a i n f a l l i s only e v i d e n t in g u l l i e s scoured to bedrock in the study a r e a . H i g h l y permeable sandy to g r a v e l l y s o i l s , i n t e r l a c e d with h i g h l y conductive root networks are e v i d e n t l y capable of h a n d l i n g high r a i n f a l l inputs without reaching f u l l s a t u r a t i o n i n most areas. Such 'open' s o i l h y d r o l o g i c behaviour only allows s o i l s at r e c e i v i n g s i t e s , i . e . drainage d e p r e s s i o n s , toe s l o p e s , g u l l i e s , e t c . , to reach s a t u r a t i o n as has been observed i n s i m i l a r s o i l s elsewhere (Chamberlain 1972, d e V r i e s and Chow 1978, P a t r i c and Swanston 1968 and Swanston 1967). The v a r i a t i o n of the parameter M with respect to r a i n f a l l input on s i m i l a r steep s o i l s approximately 1 meter deep in southwestern B.C. i s shown i n F i g u r e 3.6 f o r comparison. For these data, the shear s u r f a c e i s assumed to be at the s o i l - b e d r o c k or soil-compact t i l l i n t e r f a c e . F i g u r e 3.6 shows that convex shedding s l o p e s do not have a p p r e c i a b l e r i s e s i n M with l e s s than 50 mm of r a i n f a l l . The r e c o r d 56 mm/day p r e c i p i t a t i o n extreme recorded at Fauquier would have only r a i s e d M to 0.04, assuming study area s o i l s behave s i m i l a r l y . On the other hand, given the same p r e c i p i t a t i o n extreme on concave r e c e i v i n g s l o p e s , the v a r i a b l e M c o u l d c o n c e i v a b l y r i s e to between 0.4 and 0.9 with the given p r e c i p i t a t i o n i n p u t . These trends can be c o r r e l a t e d with moisture regimes f r e q u e n t l y observed i n the study area. Planar to convex slo p e s in shallow s o i l s are g e n e r a l l y s u b x e r i c to submesic and grade to mesic where s o i l s deepen or on shaded a s p e c t s . The subxeric to mesic moisture regimes dominate the steeper slopes where only a 66 U J 1.0 0.8 2 5 ii O t 0.6 U J Q O L OL U J U J 0 4 < 0 C 0.2 S w a n s t o n ( 1 9 6 7 ) O ' L o u g h l i n ( 1 9 7 3 ) c o n c a v e rece i iving s l o p e s ; c o n v e x s h e d d i n g s l o p e s 10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 P R E C I P I T A T I O N m m / d a y 9 0 F i g u r e 3.6. V a r i a t i o n of the r e l a t i v e p i e z o m e t r i c head M with r e s p e c t to 24 hr r a i n f a l l inputs as determined by Swanston (1967) and O'Loughlin (1973). s l i g h t r i s e i n p i e z o m e t r i c head would be expected during a heavy ra i n s t o r m . On the other hand, drainage depressions and'subdued g u l l i e s observed on the same slop e s sometimes support subhygric to h y g r i c v e g e t a t i o n , p a r t i c u l a r l y on the lower p o r t i o n s of the s l o p e . In these areas, r e l a t i v e p i e z o m e t r i c head i s much more l i k e l y to reach values above 0.4. A r a p i d r i s e i n p i e z o m e t r i c head can a l s o be expected i n g u l l i e s not supporting subhygric to h y g r i c p l a n t s as moisture f l u c t u a t i o n s may be too s h o r t - l i v e d to allow phreatophytes to develop. On lower slopes where s u r f i c i a l d e p o s i t s are g e n e r a l l y t h i c k e r and f l a t t e r , subhygric and h y g r i c moisture regimes are more common. F i g u r e 3.7 i s a schematic c r o s s ^ s e c t i o n of a t y p i c a l slope i n the study area showing the r e l a t i o n s h i p between 67 slo p e , s o i l -depth, t e r r a i n c l a s s , and moisture regime. Hygric s i t e s are almost e n t i r e l y c o n f i n e d to areas where seepage i s evident at the base of l o c a l l y steepened t e r r a c e faces and F i g u r e 3.7 T y p i c a l p r o f i l e of an i d e a l i z e d h i l l s l o p e i n the study area showing moisture regimes, probable groundwater t a b l e p o s i t i o n s and s u r f i c i a l m a t e r i a l types. f l o o d p l a i n s near streams. Using both s u r f a c e and p l a n t i n d i c a t o r s , the a r e a l d i s t r i b u t i o n of groundwater was roughly d e l i n e a t e d . Areas not t r a v e r s e d d u r i n g the course of the survey were s u b j e c t i v e l y d e l i n e a t e d on the b a s i s of slope p o s i t i o n and morphology. Much of t h i s work was done c o n c u r r e n t l y with Greg U t z i g , a s o i l s and p l a n t e c o l o g i s t a l s o working i n the study a r e a . 68 The r e s u l t s of the moisture regime mapping p r o j e c t are to be i n c l u d e d i n the r e p o r t e n t i t l e d " T e r r a i n and E c o l o g i c a l C l a s s i f i c a t i o n of the V a l h a l l a Mountains Study Area" by G.F. U t z i g , of f i l e with B.C. M i n i s t r y of F o r e s t , Arrow D i s t r i c t O f f i c e . Moisture regime d e l i n e a t i o n s o f t e n correspond to t e r r a i n u n i t s with the exception of l i n e a r g u l l i e s , drainage depressions and t e r r a c e f r o n t s . Observations of groundwater t a b l e p o s i t i o n s i n r e l a t i o n to moisture regimes adjacent to the study area were made at road cuts where p o s s i b l e . T h i s , coupled with o b s e r v a t i o n s in the study area and p r e v i o u s l y p u b l i s h e d data lead to the f o l l o w i n g g e n e r a l i z a t i o n s f o r both ICHal and ESSFc b i o g e o c l i m a t i c subzones: (1) X e r i c to mesic s i t e s on veneers and t h i n b l a n k e t s of s u r f i c i a l m a t e r i a l g e n e r a l l y e x h i b i t i n t e r f l o w at the bedrock i n t e r f a c e . T h i s flow i s sometimes r a p i d i n drainage depressions a f t e r storms. (2) X e r i c to mesic s i t e s on t h i c k e r b l a n k e t s of s u r f i c i a l m a t e r i a l may have wate^r t a b l e s approaching the s u r f a c e in drainage depressions but are u s u a l l y deeper than 1 meter. (3) Subhygric to h y g r i c s i t e s are most commonly found i n s o i l s deeper than 1 meter and u s u a l l y have water t a b l e s w i t h i n 1 meter of the s u r f a c e (4) O c c a s i o n a l l y subhygric to h y g r i c p l a n t i n d i c a t o r s occur on shallow w e l l - d r a i n e d slopes where p l a n t roots are a b l e to tap permanent i n t e r f l o w i n minor d e p r e s s i o n s . (5) Subhydric to h y d r i c s i t e s always have watertables at the s u r f a c e . On s t r a i g h t slopes where groundwater i s not c o n c e n t r a t e d and 69 where p o t e n t i a l planar d e b r i s s l i d e s are l e s s than 2 m t h i c k , the maximum value of M f o r v a r i o u s moisture regimes i s estimated to be those shown i n Table 3.4. These values do not apply to MOISTURE REGIME M MOISTURE REGIME M Very X e r i c 0 Subhygric .5 Xer i c .1 Hygric 1.0 Subxeric .1 Subhydric 1.0 Submesic .1 Hydric 1.0 Mesic .2 Table 3.4. Maximum values of M f o r v a r i o u s moisture regimes, i n ESSFal and ICHal subzones where shear planes are l e s s than 2 m deep. deep r o t a t i o n a l s l i d e s nor to drainage d e p r e s s i o n s . The hydrology of l a r g e slopes must not be g e n e r a l i z e d without t r e a t i n g l o c a l l i z e d c o n c e n t r a t i o n s of groundwater s e p e r a t e l y . 3.6 Slope Angle The g e o t e c h n i c a l model i s h i g h l y s e n s i t i v e to the i n c l i n a t i o n of the shear plane p as shown i n F i g u r e 3.2. Whereas p o t e n t i a l s l i d e s to be analysed are of the shallow planar v a r i e t y , the slope angle i s assumed to be i d e n t i c a l to the slope of the shear plane. 70 3.6.1 Measurement Of Slope Angle Maps d e l i n e a t i n g u n i t s with slopes o c c u r r i n g w i t h i n a c e r t a i n i n t e r v a l can be produced by three methods i n c l u d i n g : (1) photogrammetric measurement of a e r i a l photos; (2) a n a l y s i s of topographic maps produced e i t h e r by ground surveys or photogrammetry; and (3) a c t u a l f i e l d measurements. 3.'6.2 D i s t r i b u t i o n Of Slope Angles In Study Area Slope c l a s s e s are best s u b d i v i d e d i n t o i n t e r v a l s which, when d e l i n e a t e d , have boundaries c o i n c i d e n t with n a t u r a l breaks i n slope found i n the study area. One n a t u r a l break occurs at between 33° and 35° above which s l o p e s are dominated by g r a v i t y p r o c e s s e s , that i s , c o l l u v i a l fans and c l i f f s with pockets of c o l l u v i u m . However c o l l u v i a l fans are sometimes a f f e c t e d by avalanches o r i g i n a t i n g from steeper slopes above and are a l t e r e d to more concave, f l a t t e r slope p r o f i l e s (Caine 1969). Young (1972) mentions that f o r a slope with continuous v e g e t a t i o n and s o i l cover, without rock outcrops or s c a r s i n the root mat, the upper l i m i t of the slope angle i s t y p i c a l l y 30° to 36°. Most uniform f o r e s t e d slopes of primary i n t e r e s t to t h i s study are i n c l i n e d at l e s s than t h i s l i m i t . Concave t i l l mantled slopes t y p i c a l of g l a c i a t e d v a l l e y s g e n e r a l l y have slope i n c l i n a t i o n s f a l l i n g w i t h i n the 15° to 35° range. Slopes of l e s s than 15° are found e i t h e r on e i t h e r the e a s t - f a c i n g slopes of the main Slocan V a l l e y or on c i r q u e b a s i n f l o o r s and U-shaped v a l l e y bottoms. Though u s u a l l y not dominated by g r a v i t y processes, these slopes sometimes serve as run-out zones fo r snow avalanches and 71 l a n d s l i d e s i n the steeper v a l l e y s , p a r t i c u l a r l y on the lower d e b r i s fans. On the b a s i s of these general o b s e r v a t i o n s , the slopes of the study area are somewhat a r b i t r a r i l y grouped i n t o the 4 c l a s s e s shown i n F i g u r e 3.8. HORIZONTAL DISTANCE - - - --- - ---- - e - - - - • - F i g u r e 3.8. Slope c l a s s i n t e r v a l s used f o r the study area showing the types of slop e s o c c u r r i n g w i t h i n each c l a s s . Wherever p o s s i b l e , slopes were measured d u r i n g ground t r a v e r s e s and p l o t t e d d i r e c t l y on a e r i a l photos. Study area s i z e d i d not allow a complete survey of a l l slope angles i n the study area; t h e r e f o r e a e r i a l photos and topographic maps were employed to i n t e r p o l a t e from known to unknown s l o p e s . Enlargement of 1:50,000 topographic maps (100 f t contour i n t e r v a l ) to 1:20,000 per m i t t e d slope u n i t s to be d e f i n e d by the bar-template method of Chapman (1952). I t was found that 10° to 72 15° slope c l a s s i n t e r v a l s were r e q u i r e d to d e l i n e a t e c a r t o g r a p h i c a l l y readable u n i t s i n view of the extremely complex topography encountered. The end r e s u l t of t h i s slope survey i s Map C (slope map f i l e d s e p a r a t e l y ) . The u n i t boundaries are l a r g e l y a r t i f i c i a l and subject to i n a c c u r a c i e s i n t r o d u c e d by: machine e r r o r i n producing the 1:50,000 map and then e n l a r g i n g i t to 1:20,000; operator e r r o r i n u s i n g the bar-template method; and the averaging e f f e c t of slope c l a s s e s which do not account for v a r i a t i o n s i n topography w i t h i n the map u n i t . Ground t r u t h measurements i n d i c a t e that slope u n i t boundaries are a r t i f i c i a l and somewhat erroneous i n many are a s . A e r i a l photos can be used to d e l i n e a t e more n a t u r a l and a c c u r a t e slope c l a s s boundaries when coupled with slope the t e r r a i n u n i t maps p r e v i o u s l y d i s c u s s e d . T e r r a i n mapping i n d i c a t e d that breaks i n slope are o f t e n a s s o c i a t e d with changes in m a t e r i a l type, eg. a t a l u s slope below a rock face or a steep c o l l u v i a l s l ope below a f l a t t e r bench mantled with a b l a t i o n moraine. By comparing the t e r r a i n map with the slope map, many boundaries are seen to roughly correspond. Where major d i s c r e p a n c i e s appear, comparison with a e r i a l photos enables a d e c i s i o n to be made as to which boundary i s most a c c u r a t e . Where p o s s i b l e , the r e s u l t s of t h i s process are compared with ground measurements. An example from lower Nemo Creek Basin i s shown in F i g u r e 3.9. The f i n a l slope map resembles both the t e r r a i n map and the t o p o g r a p h i c a l l y d e r i v e d map and i s a more accurate r e p r e s e n t a t i o n of slope i n c l i n a t i o n than the t o p o g r a p h i c a l l y d e r i v e d slope map when compared with ground measurements. In u n i t s with multimodal slope c l a s s d i s t r i b u t i o n s , e.g. 73 (a) slope map derived from 1:50,000 topographic contours (b) terrain map (c) slope map Interpreted from maps (a) and (b) F i g u r e 3 . 9 T h r e e maps o f a p o r t i o n o f l o w e r Nemo C r e e k B a s i n s h o w i n g (a) t h e o r i g i n a l s l o p e map d e r i v e d f r o m . a t o p o g r a p h i c map; (b) t h e o r i g i n a l t e r r a i n map; a n d (c ) t h e more a c c u r a t e s l o p e map r e s e m b l i n g t h e f i r s t two m a p s . 74 terraced or hummocky t e r r a i n , the steepest mode i s assumed to be the most c r i t i c a l and i s , consequently, used as the b a s i s f o r slope c l a s s d e s i g n a t i o n . Map (c) of F i g u r e 3.9 has a somewhat l a r g e r area covered by c l a s s 3 and c l a s s 4 s l o p e s than map ( a ) . T h i s i s because slope u n i t s are a s s i g n e d to the c r i t i c a l (or s t eepest) slope c l a s s ; unmappable f l a t t e r s e c t i o n s such as benches, e t c . w i t h i n the u n i t are n e c e s s a r i l y ignored. 3.7 M i s c e l l a n e o u s F a c t o r s Tree surcharge weight and s o i l bulk d e n s i t y 1 are the l a s t two f a c t o r v a r i a b l e s which r e q u i r e c o n s i d e r a t i o n i n the g e o t e c h n i c a l model. The model i s l e s s s e n s i t i v e to these v a r i a b l e s and they w i l l , t h e r e f o r e , not be c o n s i d e r e d i n as much d e t a i l as the other v a r i a b l e s . Expected bulk d e n s i t i e s of c o h e s i o n l e s s s o i l s i n the U n i f i e d C l a s s i f i c a t i o n System are given i n Table 3.5. Bulk d e n s i t y and p o r o s i t y f o r f i r m s o i l s are assumed to l i e midway between the loose and dense v a l u e s . Time and access l i m i t e d i n - s i t u bulk d e n s i t y determinations in the study area. Those values determined, u s i n g a m o d i f i e d volume measure technique ( U t z i g and H e r r i n g 1975), f a l l w i t h i n the upper l i m i t s of expected ranges f o r g r a v e l l y s o i l s with f i r m to dense r e l a t i v e d e n s i t i e s , and are given i n Table 3.6. These valu e s can be combined to d e r i v e values of rwet and r s a t f o r the d i f f e r e n t g e n e t i c m a t e r i a l s as was done f o r <f> v a l u e s i n s e c t i o n 1 T h i s term i s not e q u i v a l e n t to 'unit weight' used i n some t e x t s . Bulk d e n s i t y i s mass per u n i t volume (kg/m 3) while u n i t weight i s f o r c e per u n i t volume (kN/m 3). 75 3.3.2. Brown and Sheu (1975) found that surcharge loads due to f o r e s t s are t y p i c a l l y 250 kg/m2 with extremes as high as 500 USC dry* LOOSE sat n FIRM dry sat n dry DENSE sat n GP 1400 1420 1860 1940 ;— 2320 2460 GW 1425 1440 .46 1880 1960 .29 2340 2480 .12 GM 1600 2000 1985 2250 .26 2370 2500 .11 GC 1600 2000 .41 1985 2250 .26 2370 2500 .11 SP 1330 1345 .50 1610 1760 .40 1890 2180 .29 SW 1360 1380 .75 1740 1875 .55 2115 2370 .35 SM 1390 1410 .47 1710 1840 .35 2035 2275 .23 SC 1390 1410 .47 1710 1840 .35 2035 2275 .23 kg/m' Table 3.5. Average bulk d e n s i t i e s f o r d i f f e r e n t U n i f i e d S o i l C l a s s e s (from Bowles 1979, Sowers 1979 and Hough 1957). kg/m2. These values vary a c c o r d i n g to the spacing and s i z e of t r e e s and can be d e r i v e d from f o r e s t mensuration data, i f a v a i l a b l e . F o r e s t cover maps do e x i s t f o r the study area, but f a i l to pro v i d e enough data f o r surcharge load d e t e r m i n a t i o n . A value of 250 kg/m2 i s assumed as a estimate of the average surcharge l o a d f o r f o r e s t e d s l o p e s i n the study area. 76 3.8 Slope E q u i l i b r i u m In The Study Area It i s now important to examine how the v a r i o u s model NO. REL . DENS. use n% w% dry (kg A O i n - s i t u (kg/m3) sat (kg/m3) D-l FIRM SW 35.1 11.6 1720 1921 2324 Nl+100 FIRM SM 38.5 9.0 1630 1781 2257 R-1 FIRM SM 42.6 17.2 1520 1790 2158 N-0 DENSE SM 20.0 9.3 2120 2327 2544 Nl+100-2 DENSE SM 20.4 8.8 2110 2304 2540 Nl+55 DENSE SM 14.3 7.7 2270 2455 2594 Table 3.6. I n - s i t u bulk d e n s i t i e s determined i n the study area. v a r i a b l e s d e l i n e a t e d i n the previous s e c t i o n s combine to a f f e c t slope s t a b i l i t y . The p r o f i l e of a t y p i c a l slope i n the study area i s shown i n F i g u r e 3.10. Model parameters, when assigned to v a r i o u s segments of the slope and combined a c c o r d i n g to F i g u r e 3.3, y i e l d the expected f a c t o r s of s a f e t y E[FS] and p r o b a b i l i t i e s of f a i l u r e P shown, f o r both f o r e s t e d and non- f o r e s t e d slope segments. Some i n t e r e s t i n g i n t e r r e l a t i o n s h i p s become apparent. F i r s t , the steeper slopes are u s u a l l y d r i e r and the f l a t t e r slopes are u s u a l l y wetter. These two v a r i a b l e s compensate f o r each other and r e s u l t i n l e s s extreme FS and P v a l u e s . The exception to t h i s r u l e i s , of course, the t e r r a c e f r o n t where wet c o n d i t i o n s and steep s l o p e s combine to produce p r o b a b i l i t i e s of 100%. Second, the e f f e c t s of root cohesion are more c r i t i c a l to s t a b i l i t y on the steeper s l o p e s . T h i s p o i n t 77 agrees with observed i n c r e a s e s i n l a n d s l i d e occurrence f o l l o w i n g l o s s of root s t r e n g t h a f t e r l o g g i n g on s i m i l a r s l o p e s i n Alaska F i g u r e 3.10. P r o f i l e of a t y p i c a l slope i n the study area showing the r e l a t i v e s t a b i l i t y r e s u l t i n g from v a r i o u s combinations of q u a n t i f i e d v a r i a b l e s of the s t o c h a s t i c g e o t e c h n i c a l model. and S.W. B r i t i s h Columbia (Swanston 1974 and O'Loughlin 1973). T h i r d , slopes i n c l i n e d at l e s s than 20° have high FS values and low p r o b a b i l i t i e s of f a i l u r e . From the model i t i s p o s s i b l e to e x p l a i n i n semi- q u a n t i t a t i v e terms the observed d i s t r i b u t i o n of many l a n d s l i d e s i s the study area and surrounding r e g i o n . However, i t i s somewhat unclear as to what the s t a b i l i t y values r e a l l y mean i n 78 terms of f o r e s t management and proposed e n g i n e e r i n g . The p r o b a b i l i t i e s cannot be used to a c t u a l l y p r e d i c t the number of l a n d s l i d e s l i k e l y to occur on a p a r t i c u l a r s lope, nor the l i k e l i h o o d of a l a n d s l i d e o c c u r r i n g w i t h i n a c e r t a i n time p e r i o d . Such q u a n t i t a t i v e p r e d i c t i o n s can only be made where model v a r i a b l e s are l e s s s u b j e c t i v e l y determined and when p r o b a b i l i t i e s are c a l i b r a t e d and compared with observed events. The values of E[FS] and P are best used as r e l a t i v e i n d i c e s of s t a b i l i t y . These i n d i c e s can be used to compare the e q u i l i b r i u m of s l o p e s i n the study area with s l o p e s that have responded unfavorably to f o r e s t e n g i n e e r i n g i n other areas. From such comparisons, the i n d i c e s can be grouped to form hazard c l a s s e s of use to f o r e s t managers and engineers. Many slop e s do not s a t i s f y the assumptions of the s t o c h a s t i c g e o t e c h n i c a l model. They i n c l u d e : rocky slopes and c l i f f s with l i t t l e s u r f i c i a l m a t e r i a l ; l i n e a r or s i t e - s p e c i f i c g u l l i e s and t e r r a c e f r o n t s which cannot be d e l i n e a t e d as map polygons; d e b r i s fans dominated by flow processes; and t a l u s fans formed by d i s c r e t e r o c k f a l l and minor r o c k s l i d e s . The probable behaviour of these s l o p e s must a l s o be p r e d i c t e d by comparison with experience on s i m i l a r slopes elsewhere. However, r a t h e r than develop i n d i v i d u a l schemes f o r the determination of slope e q u i l i b r i u m f o r these slope c l a s s e s , i t i s assumed that the behaviour i s homogeneous throughout the u n i t . In other words, i n the case of steep rocky s l o p e s , i t i s assumed that s i m i l a r e n g i n e e r i n g problems are l i k e l y to be encountered anywhere w i t h i n the u n i t ; l i k e w i s e on t e r r a c e f r o n t s and d e b r i s fans. 79 CHAPTER 4 LANDSLIDE HAZARD C L A S S I F I C A T I O N 4.1 H a z a r d s On S l o p e s M a n t l e d W i t h S u r f i c i a l M a t e r i a l A r e c o n n a i s s a n c e s u r v e y o f l a n d s l i d e s a s s o c i a t e d w i t h f o r e s t e n g i n e e r i n g on s l o p e s m a n t l e d w i t h s u r f i c i a l m a t e r i a l s was c o n d u c t e d n e a r t h e s t u d y a r e a i n o r d e r t o : (1) r e l a t e p r e - e x i s t i n g s l o p e c o n d i t i o n s a n d r e l a t i v e s t a b i l i t y i n d i c e s t o r e s u l t i n g i m p a c t s , (2) i d e n t i f y t h o s e e n g i n e e r i n g p r a c t i c e s most l i k e l y t o p r o m o t e f a i l u r e s a n d (3) d e v e l o p a h a z a r d c l a s s i f i c a t i o n s y s t e m b a s e d on r e l a t i v e s t a b i l i t y w h i c h c a n s t r a t i f y s l o p e s i n t h e s t u d y a r e a a c c o r d i n g t o l i k e l y i m p a c t s , g i v e n c e r t a i n a s s u m p t i o n s a s t o t h e k i n d , i n t e n s i t y , a n d q u a l i t y o f e n g i n e e r i n g t o be i m p o s e d on t h e s l o p e . 4 . 1 . 1 E n g i n e e r i n g P r o b l e m s N e a r S t u d y A r e a L a n d s l i d e s a s s o c i a t e d w i t h f o r e s t r o a d s n e a r t h e s t u d y a r e a i n i t i a t e e i t h e r a t t h e r o a d c u t o r r o a d f i l l a n d may c o n t i n u e p r o g r e s s i v e l y d o w n s l o p e , o r r e t r o g r e s s i v e l y u p s l o p e . T h e s e s l i d e s a r e g e n e r a t e d by c h a n g e s i n v a l u e s o f fi, M, <t>, C r , a n d H c a u s e d by s t e e p e n i n g s l o p e s w i t h c u t s a n d f i l l s , i n t e r c e p t i n g s u b s u r f a c e g r o u n d w a t e r f l o w , l o o s e n i n g s u r f i c i a l m a t e r i a l s , l o a d i n g t h e s l o p e w i t h s i d e c a s t m a t e r i a l , o r d e s t r o y i n g c o h e s i o n by t r e e r o o t r e m o v a l . C u t s l o p e f a i l u r e s a r e d o c u m e n t e d i n a r e a s where t h e r o a d p r i s m i n t e r c e p t s s u b s u r f a c e f l o w a l l o w i n g s e e p a g e p r e s s u r e s t o r e d u c e t h e e f f e c t i v e n o r m a l s t r e s s a n d i n d u c e f a i l u r e . 80 Landslides D19, D20 and S1 are examples of this type of f a i l u r e (see Appendix F for landslide data and l o c a t i o n s ) . Two of the three s l i d e s were r e l a t i v e l y minor, involving less than 72 m3, and of l i t t l e consequence to the road. Landslide D19, on the other hand, blocked approximately 25 meters of road with 225 m3 of saturated sandy morainal debris (see Figure 4.1). Evidence of Figure 4.1 Cut-slope f a i l u r e s caused by seepage on the cut face of a logging road in the Cariboo Creek area (Landslide D19 - see Appendix F). piping along root conduits can be seen as holes on the cut face associated with free water within 0.7 meters of the ground surface. Vegetation at this p a r t i c u l a r s i t e indicates subhygric to hygric moisture conditions i n f e r r i n g a near-surface groundwater table. Failures of this type occur on wet s i t e s 81 where s o i l i s s u f f i c i e n t l y deep to allow the development of an e l e v a t e d p i e z o m e t r i c s u r f a c e i n a c l o s e d s o i l , but were never observed i n shallow s o i l s where i n t e r f l o w processes predomiate. Minor r a v e l l i n g of m a t e r i a l from cut slope faces i s a much more common phenomenon near the study area. The usual road c o n s t r u c t i o n p r a c t i c e i s to b u l l d o z e cut slopes to near v e r t i c a l angles with the allowance that the s u r f i c i a l m a t e r i a l w i l l e v e n t u a l l y r e a d j u s t a c c o r d i n g to i t s angle of i n t e r n a l f r i c t i o n . Minor cut slope f a i l u r e s have been known to cause water drainage problems in other regions where slou g h i n g m a t e r i a l chokes i n s i d e drainage d i t c h e s and d i v e r t s water over the road bed onto unprotected f i l l s lopes (Burroughs et a l . 1976 and O'Loughlin 1973). T h i s , however, was not found to be a problem near the study area. Shallow c o l l u v i a l s o i l s may i n p l a c e s r a v e l r e t r o g r e s s i v e l y long d i s t a n c e s upslope from a rock cut as observed near l a n d s l i d e D4 where 0.5 to 1.0 meters of c o l l u v i u m continues to r a v e l from more than 50 meters upslope. The volumes of m a t e r i a l i n v o l v e d i n r a v e l l i n g are l a r g e l y a f u n c t i o n of the c o n t i n u i t y of the c o l l u v i a l veneers between rock b u t t r e s s e s and the a b i l i t y of t r e e roots to anchor s o i l m a t e r i a l to bedrock. The r o l e of r a p i d subsurface water i n t e r f l o w i n i n i t i a t i n g these f a i l u r e s i s u n c e r t a i n . Where slope angles approach the loose angles of i n t e r n a l f r i c t i o n of s i d e c a s t m a t e r i a l , f i l l s l opes may f a i l p r o g r e s s i v e l y , e i t h e r to the base of the slope ( o f t e n a creek bed), or u n t i l some o b s t r u c t i o n such as a rock outcrop or t r e e stump b u t t r e s s e s the f i l l . S l i d e s of t h i s type were i n a l l cases 82 r e s t r i c t e d to slopes i n c l i n e d i n excess of 34° near the study area, s i m i l a r to trends observed i n other regions of the P a c i f i c Northwest (O'Loughlin 1973, U t z i g and H e r r i n g 1975 and Swanston 1969). F a i l u r e s are induced by: f i l l s a t u r a t i o n due to the o b s t r u c t i o n of subsurface or s u r f a c e water flow; e r o s i o n due to water running u n c o n t r o l l e d down the f i l l s l o p e ; or d e t e r i o r a t i o n of organic d e b r i s b u t t r e s s i n g f i l l s l o p e s . L a n d s l i d e D1 i s an example of a l a r g e d e b r i s avalanche - d e b r i s flow t r i g g e r e d by both s u r f a c e and subsurface water flow i n t e r c e p t i o n by an o l d skidder road near Wragge Creek. A spoon- shaped shear s u r f a c e at the zone of i n i t i a t i o n occured w i t h i n the f i l l m a t e r i a l but, upon i n c o r p o r a t i o n of water, transformed i n t o a V-shaped s u r f a c e i n v o l v i n g approximately 9800 m3 of morainal blanket m a t e r i a l (see F i g u r e 4.2). Recurrent f a i l u r e s along t h i s g u l l y now r e q u i r e annual maintenance at three main haul road c r o s s i n g s along the s l i d e path and c o n t r i b u t e sediment to Wragge Creek. Where water i s allowed to run u n c o n t r o l l e d down f i l l s l o p e s , the r e s u l t i n g e r o s i o n w i l l eat away at the road s u r f a c e and undercut the embankment s u f f i c i e n t l y to cause l a n d s l i d i n g . L a n d s l i d e D3 on Shannon Creek Road i s an example of where the improper placement of a c u l v e r t has l e d to e r o s i o n which in turn promoted a c c e l e r a t e d r a v e l l i n g ( F i g u r e 4.3). Water d i v e r t e d onto n a t u r a l slopes can a l s o i n i t i a t e l a n d s l i d e s such as D14 d e s c r i b e d i n s e c t i o n 3.4.2 (see F i g u r e 3.5). L a n d s l i d e s i n i t i a t e d s o l e l y due to l o s s of root cohesion f o l l o w i n g l o g g i n g were never observed on uniform slope mantled with s u r f i c i a l m a t e r i a l near the study area. 8 3 Figure 4.2. V-shaped p r o f i l e of the path of a debris avalanche- debris flow caused by the saturation of road f i l l material near Wragge Creek Road. 4.1.2 Hazard Classes S t a b i l i t y indices, when calculated for slopes adjacent to landslides, can be used for dir e c t comparison with slopes to be developed in the study area. Values of E[FS] and P calculated for natural slopes adjacent to 14 landslides i n i t i a t e d by engineering are given in Table 4.1. The values of model variables were inferred in the same fashion as slopes in the study area. Results indicate that 11 of 14 s i t e s had 84 Figure 4.3. F i l l slope erosion and r a v e l l i n g resulting from improper water control at the culvert e x i t . p r o b a b i l i t i e s of f a i l u r e greater than 4% prior to engineering and that at the 3 more stable s i t e s , obvious engineering errors, such as use of organic debris in road f i l l s , flooding by water diversion, or improper switchback layout were responsible for landsliding. If the small cut slope f a i l u r e occurring on the P = 4% slope l i s t e d in Table 4.1 is ignored, landslides of major consequence a l l occur on slopes with natural p r o b a b i l i t i e s of f a i l u r e greater than 10%. The s t a b i l i t y indices for natural slopes are used for 85 NO 8 M.R.* TCS BEFORE ENGINEERING USC 4>° H Cr Cs M E[FS] P Cr M AFTER B ENGINEERING H o>° E[FS) P DI 32° 2-3 fgMb GW-GM 29-36 2 0 0-450 0 . 1 1 . 10 17% 0 .2 34° 5.0 29-36 .85 99% D2 30° 3 rCv GM 38-43 1 0 0-450 0 . 1 1.66 0% 0 . 1 40° 3.0 35-40 .87 100% D3 44° 4 rCa GW 38-43 2 0 0-450 0 .2 .91 87% 0 .2 44° 2.0 35-40 .72 100% D5 26° 3 rMb GW 29-43 2. 0 0-450 0 . 1 1.59 0% 0 . 1 35° .5 27-40 .92 73% Dllb 36° 2 gF Gb GP 29-38 2. 0 0-450 0 . 1 .99 52% 0 . 1 38° .5 27-36 .76 100% D12 39° 3 sF Gb SP 29-38 2 0 0-450 0 . 1 .90 83% 0 . 1 42° 2.0 27-36 .65 100% . D14 35° 3 rCv GW 38-43 1 0 0-450 0 . 1 1.40 0% 0 -450 1.0 35° 1.0 38-43 .88 82% D15 36° 3 G sF b ,SC 29-38 2 0 0-450 0 . 1 .99 52% 0 .2 65° 1.0 29-38 .28 100% D17 34° 3-4 sMb SW 34-38 2 0 0-450 0 .2 1.09 13% D18 38° 3 gMt SW 34-38 2. 0 0-450 0 . 1 1.00 48% 0 -450 .2 45° 4.0 34-38 .71 100% D19 28° 5 sMb SP-SW 32-38 2. 0 0-450 0 .5 1.12 11% 0 .5 62° 1.5 32-38 .28 100% D20 30° 3 £sMb SW-SM 29-38 2. 0 0-450 0 . 1 1.23 4% 0 . 1 52° 2.0 29-38 .49 100% SI 36° 2-3 gF GC GP 29-38 2 0 0-450 0 . 1 .99 52% 0 . 1 75° .5 29-38 . 17 100% Al 40° 3-4 gF Gt GW-GP 29-38 2. 0 0-450 0 .2 .87 89% 0 .2 40° .5 27-36 .67 100% *Moisture Regime Table 4.1 Values of E[FS] and P c a l c u l a t e d f o r n a t u r a l slopes adjacent to 14 l a n d s l i d e s . comparison between areas because, depending on the type of f a i l u r e i n v o l v e d , the values used i n c a l c u l a t i n g the s t a b i l i t y of a p a r t i c u l a r l a n d s l i d e may be q u i t e d i f f e r e n t from those used i n n a t u r a l slope s t a b i l i t y c a l c u l a t i o n s . For example, l a n d s l i d e D2 oc c u r r e d on a n a t u r a l slope with E[FS] = 1.66 and P = 0%. However, t a k i n g i n t o account the lo o s e n i n g of s u r f i c i a l m a t e r i a l by d i s t u r b a n c e , the steepening of the slope angle by road prism c o n s t r u c t i o n , and the d e s t r u c t i o n of s t a b i l i z i n g t r e e r o o t s caused by b u l l d o z i n g r e s u l t s i n E[FS] = .87 and P = 100% f o r the a c t u a l l a n d s l i d e (see Table 4.1). 1 The p r e d i c t i o n of s t a b i l i t y c o n d i t i o n s at the time of f a i l u r e can only be made with s i t e 'The i n f i n i t e slope model, though not u s u a l l y a p p l i e d to short f i l l s l o p e s , does give an approximate estimate of s t a b i l i t y . 86 s p e c i f i c knowledge of the type of e n g i n e e r i n g a l t e r a t i o n to be imposed upon the n a t u r a l s l o p e . I t i s assumed that e n g i n e e r i n g p r a c t i c e s common i n the re g i o n of study w i l l be a p p l i e d to slope s of the study area i n determining the hazards on slopes with c e r t a i n n a t u r a l s t a b i l i t y i n d i c e s . High P valu e s and low E[FS] values sometimes c a l c u l a t e d f o r slope s showing no evidence of n a t u r a l i n s t a b i l i t y p r i o r to road c o n s t r u c t i o n r e f l e c t the c o n s e r v a t i v e estimates of input value ranges and s i m p l i f y i n g assumptions used in the model. U n f o r t u n a t e l y , time d i d not permit the in v e r s e approach of a n a l y s i n g s l o p e s for s t a b i l i t y i n d i c e s i n areas where l a n d s l i d e s have not o c c u r r e d . I t i s p o s s i b l e t h a t , given b e t t e r f o r e s t e n g i n e e r i n g p r a c t i c e s , a l a r g e percentage of the s l i d e s o c c u r r i n g on slopes with P valu e s g r e a t e r than 10% would not have o c c u r r e d . Indeed, i n many areas, s l i d e s do not occur on slopes that are extremely steep and appear to be p o t e n t i a l l y u n s t a b l e . However,' i t seems reasonable that slopes with p r o b a b i l i t i e s of f a i l u r e g r e a t e r than 10% i n the study area be ass i g n e d to the 'high hazard' c l a s s , as a s i n g l e l a n d s l i d e event, though o c c u r r i n g only once on an a r e a l l y e x t e n s i v e slope, may have c r i t i c a l environmental and f i n a n c i a l i m p l i c a t i o n s . Moreover, there i s no guarantee that l a n d s l i d e p r e v e n t a t i v e techniques w i l l be f i n a n c i a l l y f e a s i b l e or t e c h n i c a l l y p o s s i b l e on these s l o p e s . The four s l i d e s which occurred on more s t a b l e s l o p e s had a l a r g e r component of human e r r o r r e s p o n s i b l e f o r t h e i r i n i t i a t i o n , and l a n d s l i d e s o c c u r r i n g on slop e s with E[FS] gr e a t e r than 1.6 were not observed. The 'moderate hazard'. c l a s s i s t h e r e f o r e assigned to the s t a b i l i t y index i n t e r v a l P < 10% 87 and E[FS] < 1.6, and the 'low hazard' c l a s s i s assigned to a l l s l o p e s with E[FS] values g r e a t e r than 1.6. Roads developed on slopes which show sign s of i n c i p i e n t f a i l u r e , as i n d i c a t e d by s l i d e morphology, exposed t r e e r o o t s , r e c e n t l y b u r i e d s o i l h o r i z o n s , e t c . p r i o r to road c o n s t r u c t i o n , were not observed near the study a r e a . I t i s assumed that slopes f i t t i n g t h i s c r i t e r i a i n the study area are of the 'very high hazard' c l a s s , as the process i s a l r e a d y a c t i v e . For mapping purposes, the l a n d s l i d e hazard c l a s s e s d i s c u s s e d above are given the symbols S4, S3, S2, and S1 corresponding to the very high, h i g h , moderate, and low hazard c l a s s e s r e s p e c t i v e l y , and d e l i n e a t e d on the slope s t a b i l i t y map (Map D f i l e d s e p a r a t e l y ) . The l e t t e r 'S' designates the t e r r a i n s u b d i v i s i o n i n c l u d i n g s l o p e s mantled with s u r f i c i a l m a t e r i a l , and the numbers designate the r e l a t i v e hazard w i t h i n the t e r r a i n u n i t . The 'S' t e r r a i n s u b d i v i s i o n only i n c l u d e s those slopes which can be analysed with the s t o c h a s t i c , g e o t e c h n i c a l model, i . e . slopes with more than 70% cover of b l a n k e t s (b) or veneers (v) of u n c o n s o l i d a t e d s u r f i c i a l m a t e r i a l (see Map B). Slopes with fan ( f ) or apron (a) s u r f a c e e x p r e s s i o n s are not i n c l u d e d . 4.1.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques A number of e n g i n e e r i n g techniques would have helped prevent some of the l a n d s l i d e s that o c c u r r e d on S3 and S2 hazard s l o p e s near the study a r e a . The f a i l u r e to c o n t r o l both s u r f a c e and subsurface water on the road prism i s a r e c u r r i n g problem i n the r e g i o n . Some s o l u t i o n s have been presented by v a r i o u s authors i n c l u d i n g Enberg (1963) who demonstrates that s o l u t i o n s 88 to drainage problems have two p a r t s : (1) l o n g i t u d n a l drainage (drainage p a r a l l e l to the road) and (2) l a t e r a l drainage (drainage at r i g h t angles to the road). Haupt et a l . (1963) found that road s u r f a c e s cambered toward the cut slope with drainage d i t c h e s adjacent to the cut slope are the most s u c c e s s f u l i n c o n t r o l l i n g e r o s i o n on e r o d i b l e g r a n i t i c s o i l s . However, t h i s technique has c o n t r i b u t e d to a number of l a n d s l i d e s i n the study area as i t tends to c o n c e n t r a t e water i n areas where ponding and road f i l l s a t u r a t i o n i s p o s s i b l e . The c o n c e n t r a t e d water, when d r a i n e d l a t e r a l l y and disposed of downslope, can f u r t h e r aggravate slope s t a b i l i t y . In areas where s o i l s are h i g h l y r e s i s t a n t to r i l l e r o s i o n , cambering toward the s i d e c a s t i s o f t e n the best s o l u t i o n as i t avoids the problems a s s o c i a t e d with water c o n c e n t r a t i o n and the danger of u n c o n t r o l l e d f l o o d i n g of unstable s l o p e s below. Where s o i l s are too e r o d i b l e to allow cambering toward the s i d e c a s t , c e r t a i n p r o v i s i o n s can be made f o r p r o t e c t i n g f i l l s l o p e s from concentrated l a t e r a l drainage, and road beds from ponding and s a t u r a t i o n . F i r s t , s l o p e s can be p r o t e c t e d with r i p r a p or c u l v e r t s . Where p o s s i b l e , c u l v e r t s should be p l a c e d i n areas where n a t u r a l drainage channels e x i s t or where slopes are as f l a t as p o s s i b l e . Second, c u l v e r t s should be spaced as f r e q u e n t l y as p o s s i b l e i n order to minimize the amount of c o n c e n t r a t e d water handled by any one c u l v e r t . T h i r d , i n s i d e d i t c h e s should be p r o p e r l y p i t c h e d and r e g u l a r l y maintained. Areas where su r f a c e and p l a n t i n d i c a t o r s of subhygric to h y g r i c moisture c o n d i t i o n s e x i s t should be avoided wherever p o s s i b l e . Where e x c e s s i v e seepage on the cut face i s 89 encountered, p e r f o r a t e d pipe can be i n s t a l l e d to h e l p lower the p h r e a t i c s u r f a c e and c a r r y water to the i n s i d e d i t c h . If slumping c o n t i n u e s , gabion or l o g c r i b s t r u c t u r e s can be c o n s t r u c t e d to h e l p b u t t r e s s the s l o p e . Cutslopes can a l s o be s t a b i l i z e d by a v a r i e t y of bio-mechanical techniques ( S c h i e c h t l 1980) . Where p o s s i b l e , road widths and cut and f i l l s lope lengths should be minimized (Gardner 1979). U s e f u l methods f o r road prism design on p o t e n t i a l l y u n s t a b l e f o r e s t e d s l o p e s have been developed by P r e l l w i t z (1975) and Hendrickson and Lund (1974) based on s t a b i l i t y a n a l y s e s . M i n i m i z i n g s o i l d i s t u r b a n c e by l i m i t i n g s k i d d e r road d e n s i t y and haul road mileage w i l l s i g n i f i c a n t l y decrease the r i s k of l a n d s l i d e occurrence on both S3 and S2 s l o p e s i n the study a r e a . 4.2 Hazards On Steep Rocky Slopes T h i s t e r r a i n s u b d i v i s i o n c o n s t i t u t e s the major p o r t i o n of slopes i n the study area where r o c k s l i d e s or r o c k f a l l s r e s u l t i n g from f r o s t wedging and other mechanical weathering processes dominate. S t a b i l i t y i s l a r g e l y c o n t r o l l e d by the mechanical p r o p e r t i e s of the coherent i n - s i t u rock mass. 90 4.2.1 E n g i n e e r i n g Problems Near The Study Area Roads on steep rocky sl o p e s of the- h i g h l y competent g r a n i t i c t e r r a i n i n the study area i n v a r i a b l y r e q u i r e b l a s t i n g d u r i n g t h e i r c o n s t r u c t i o n . Unfavourable d i s c o n t i n u i t y o r i e n t a t i o n s l e a d i n g to wedge f a i l u r e s from rock cuts were only noted i n areas where 'overbreak' due to e x c e s s i v e dynamite charges has e x t e n s i v e l y f r a c t u r e d the cut face, l e a d i n g to p e r i o d i c r o c k f a l l s t r i g g e r e d by j o i n t water p r e s s u r e s or f r o s t wedging (see F i g u r e 4.4). Damage to p r o d u c t i v e s o i l s , timber d e s t r u c t i o n and d e b r i s avalanche i n i t i a t i o n where rock i s thrown downslope can a l s o r e s u l t from o v e r b l a s t i n g on steep s l o p e s . Because slope angles are g e n e r a l l y g r e a t e r than the angle of i n t e r n a l f r i c t i o n of s i d e c a s t m a t e r i a l , l a r g e s c a r s descending long p o r t i o n s of the slope f r e q u e n t l y r e s u l t . L a n d s l i d e s D9, D10, D13 and A5 i n Appendix F are a l l examples of rock cuts where s i d e c a s t m a t e r i a l has descended i n t o the adjacent creek bed. Environmental damage can be s i g n i f i c a n t i n these cases. Near s i t e D4, a l a r g e scar v i s i b l e from at l e a s t 25 km away has r e s u l t e d from the f a i l u r e of rock s i d e c a s t . 91 Figure 4.4. Rock f a i l u r e on lower Shannon Creek Road. 4.2.2 Hazard Classes High impacts resulting from engineering on steep rocky slopes near the study area w i l l undoubtedly occur on similar slopes in the study area. A l l steep rocky slopes delineated as Rs on the terrain map are therfore designated as being of the 'very high hazard' class and delineated as R3 on the slope s t a b i l i t y map (Map D). Composite units on the ter r a i n map, according to this designation, must include at least 30% Rs in order to be assigned to the R3 hazard c l a s s . 92 Rock slopes i n c l i n e d at l e s s than 35° do not f i t the c r i t e r i a f o r t h i s hazard c l a s s . The high mechanical s t r e n g t h of bedrock coupled with slope angles below the angle of repose of m a t e r i a l being mechanically weathered make these slopes f a i r l y s t a b l e , depending on the o r i e n t a t i o n and i n c l i n a t i o n of d i s c o n t i n u i t i e s w i t h i n the rock mass. B l a s t i n g i s u s u a l l y r e q u i r e d i n road c o n s t r u c t i o n except i n f l a t t e r areas where subgrade m a t e r i a l can be imported and placed over the bedrock s u r f a c e . Because b l a s t e d m a t e r i a l s w i l l u s u a l l y r e s t i n place on s i d e c a s t s l o p e s and w i l l not thr e a t e n the downslope areas, these slopes are designated as being i n the 'low hazard' c l a s s and are d e l i n e a t e d as R1 on the slope s t a b i l i t y map (Map D). 4.2.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques Burroughs et a l . (1976) emphasizes the need f o r proper b l a s t i n g techniques when c o n s t r u c t i n g roads through hard bedrock in steep t e r r a i n . The o b j e c t i v e i s to minimize cut slope overbreak, and prevent the throwing of m a t e r i a l downslope. ' P r e s p l i t t i n g ' the cut slope , f o l l o w e d by c o n t r o l l e d p r o duction b l a s t i n g w i l l u s u a l l y accomplish both o b j e c t i v e s thus a l l o w i n g f r a c t u r e d rock to be trucked to s a f e r areas without damaging slopes below. 93 4.3 Hazards On C o l l u v i a l Aprons And Fans The c o l l u v i a l apron and fan t e r r a i n s u b d i v i s i o n s i n c l u d e those sl o p e s which serve zones of accumulation of rock d e b r i s f a l l i n g from c l i f f s above. Morphology i s governed l a r g e l y by the mechanical p r o p e r t i e s of the c o l l u v i a l m a t e r i a l , r o c k f a l l frequency, and r o c k f a l l height (Carson 1977). 4.3.1 E n g i n e e r i n g Problems Near The Study Area A r e c e n t l y c o n s t r u c t e d road on a 40° c o l l u v i a l apron i n the Wragge Creek drainage provides the only example of the eng i n e e r i n g behaviour of c o l l u v i a l aprons near the study area. At s i t e s W1, W2, and W3, minor r a v e l l i n g of rubble and blocks from the road cut onto the road bed suggests that m a t e r i a l has some i n c i p i e n t i n s t a b i l i t y . However, the road i s l a r g e l y s t a b l e due to the p o s i t i v e e f f e c t s of p a r t i c l e a n g u l a r i t y , t r e e root cohesion, and the f a c t that c o l l u v i a l aprons d e r i v e d from hi g h c l i f f s are n a t u r a l l y more s t a b l e than those d e r i v e d from low c l i f f s . Because the road was c o n s t r u c t e d only one year p r i o r to the survey, i t i s p o s s i b l e t h a t , with time, problems a s s o c i a t e d with c o n t i n u a l r a v e l l i n g from cut slopes or f i l l s l o p e s f o l l o w i n g the d e t e r i o r a t i o n of b u t t r e s s i n g t r e e s w i l l r e q u i r e annual a t t e n t i o n . In other r e g i o n s , roads c o n s t r u c t e d on c o l l u v i a l aprons f r e q u e n t l y i n v o l v e the r a v e l l i n g of l a r g e volumes of m a t e r i a l from long d i s t a n c e s upslope ( B a i l y 1971 and Burroughs et a l . 1976). \ 94 4.3.2 Hazard C l a s s L a n d s l i d e problems on c o l l u v i a l aprons and fans i n the study area are l i k e l y to occur i n view of past experience with s i m i l a r s l o p e s elsewhere. Where i n f r e q u e n t r o c k f a l l s have allowed f o r e s t development and where aprons l i e at the base of high c l i f f s , s lopes can be expected to be more s t a b l e , a l b e i t s u b j e c t to r o c k f a l l hazards. In g e n e r a l , c o l l u v i a l aprons and fans, i d e n t i f i e d as Ca and Cf on Map B, should be given a 'high hazard' r a t i n g d e l i n e a t e d by the symbol R2 on the slope s t a b i l i t y map. 4.3.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques C o l l u v i a l aprons and fans should be avoided wherever p o s s i b l e u n l e s s other a l t e r n a t i v e s are not a v a i l a b l e . The most common and s u c c e s s f u l c o n t r o l measure f o r p r e v e n t i n g cut slope r a v e l i s b u t t r e s s i n g by gabion s t r u c t u r e s . 'End-hauling' excavated m a t e r i a l to safe areas w i l l a l s o h e l p a l l e v i a t e the downslope r a v e l l i n g problem. 4.4 Hazards On Debris Fans Slopes formed by d e b r i s flow d e p o s i t i o n have morphologies s i m i l a r to both f l u v i a l and c o l l u v i a l fans and are i n c l i n e d anywhere between 5° and 35°. Where d e p o s i t i o n a l processes are s t i l l a c t i v e , some p a r t i c u l a r e n g i n e e r i n g problems e x i s t . 95 4.4.1 E n g i n e e r i n g Problems In Other Regions Roads c o n s t r u c t e d on a c t i v e or r e c e n t l y a c t i v e d e b r i s fans near the study area were not encountered d u r i n g t h i s study. However, experience with these t e r r a i n f e a t u r e s elsewhere can be used to p r e d i c t the types of hazards l i k e l y to be a s s o c i a t e d with them. Debris fans have m a t e r i a l p r o p e r t i e s s i m i l a r to those of c o l l u v i a l f a n s . However, because they are formed by s l u r r y d e p o s i t i o n , they are g e n e r a l l y more s t a b l e than g r a v i t y dominated c o l l u v i a l s lopes of the R2 hazard c l a s s i f i c a t i o n . Roads which c r o s s a c t i v e d e b r i s flow channels are threatened by p e r i o d i c flows which can cause severe damage. S l u r r i e s with boulders i n excess of one meter i n diameter have been known to descend these channels with l i t t l e forewarning and great d e s t r u c t i v e n e s s (Nasmith and Mercer 1979, M i l e s and K e l l e r h a l s 1981 and E i s b a c h e r 1982). The f l a t t e r p o r t i o n s of the lower fans pose l i t t l e hazard to e n g i n e e r i n g except at the i s o l a t e d channel to which a d e b r i s flow may be c o n f i n e d . However, areas where channels are not w e l l i n c i s e d , flows may a f f e c t a much l a r g e r a r ea. On the upper fans, e n g i n e e r i n g p r o p e r t i e s of the substratum are s i m i l a r to those of c o l l u v i a l fans, p a r t i c u l a r l y on d e b r i s fans at the base of high c l i f f s . 96 4.4.2 Hazard C l a s s Because slope angles on d e b r i s fans vary over a broad range, i t i s u s e f u l to somewhat a r b i t r a r i l y d i v i d e the t e r r a i n s u b d i v i s i o n i n t o two hazard c l a s s e s at the 20° slope angle. I t i s f e l t that slopes i n c l i n e d i n excess of 20° (upper fans) are much more l i k e l y to i n v o l v e hazards a s s o c i a t e d with cut and f i l l s lope r a v e l l i n g . Hazards a s s o c i a t e d with lower fans are c o n f i n e d to areas where p e r i o d i c d e b r i s flows are l i k e l y to run. S t e e p l y i n c l i n e d upper d e b r i s fan areas are a s s i g n e d to the 'high hazard' c l a s s and are d e l i n e a t e d on the slope s t a b i l i t y map (Map D) with the F2 symbol. Lower d e b r i s fans have a 'moderate hazard' r a t i n g and are d e l i n e a t e d with the FI symbol. I t i s noted that the FI c l a s s i s s i m i l a r to the S1 c l a s s as they are f r e q u e n t l y found adjacent to one other i n the v a l l e y bottom. However, the d e b r i s flow hazard causes the F1 c l a s s to be given a 'moderate' rather than 'low' hazard r a t i n g . 4.4.3 P r e v e n t a t i v e And Remedial En g i n e e r i n g Techniques C e r t a i n p r o t e c t i v e measures have been s u c c e s s f u l l y employed i n other areas where roads c r o s s d e b r i s t o r r e n t paths. They i n c l u d e masonry or rock f i l l checkdams and s i l l s to r e t a i n d e b r i s m a t e r i a l above the road bed, l a r g e c o r r u g a t e d s t e e l sheet c u l v e r t s used with c o n c r e t e b r i d g e s , and stone and l o g c r i b - t y p e r e t a i n i n g w a l l s on channel embankments ( H e i n r i c h 1978, H a t t i n g e r 1978 and Gagoshidze 1969). These measures reduce water v e l o c i t y by o b s t r u c t i n g d e b r i s channels, i n c r e a s e bridge s t r e n g t h e n a b l i n g d e b r i s pressure to be withstood, and p r o t e c t channel 97 s l o p e s from e r o s i o n near road c r o s s i n g s . C u l v e r t s should be l a r g e enough to c a r r y p e r i o d i c i n f l u x e s of f l o w i n g d e b r i s with l i t t l e damage to road embankments. A rule-of-thumb i s to design the approximate c r o s s - s e c t i o n of the c u l v e r t a c c o r d i n g to the approximate c r o s s - s e c t i o n of the d e b r i s channel ( H a t t i n g e r 1978). Upper d e b r i s fans should be t r e a t e d l i k e c o l l u v i a l fans i n areas other than d e b r i s flow paths. 4.5 Hazards On T e r r a c e s And G u l l i e s Steep l i n e a r t e r r a c e f r o n t s and g u l l y s l o p e s are c r i t i c a l f e a t u r e s found w i t h i n the hazard c l a s s e s d e s c r i b e d i n the p r e v i o u s s e c t i o n s . Many l a n d s l i d e s o c c u r r i n g n a t u r a l l y i n the study area and a s s o c i a t e d with roads i n the region are c o n c e n t r a t e d i n the areas where slo p e s are l o c a l l y steepened and in groundwater discharge areas. T e r r a c e s and g u l l i e s are u s u a l l y too small to be mapped as u n i t s at the 1:20,000 s c a l e and can only be represented as l i n e a r symbols w i t h i n the other hazard u n i t s . 4.5.1 E n g i n e e r i n g Problems Near The Study Area E n g i n e e r i n g problems a s s o c i a t e d with t e r r a c e f r o n t s and g u l l y s l o p e s are i d e n t i c a l to those d e s c r i b e d f o r steep slopes mantled with s u r f i c i a l m a t e r i a l . V a r i o u s f a c t o r s combine to cause l a n d s l i d e s which may e f f e c t not only the road prism i t s e l f , but a l s o the stream channels at the base of the s l o p e . These slopes are a f f e c t e d by l o s s of root cohesion f o l l o w i n g d e f o r e s t a t i o n . Since l o g g i n g o c c u r r e d about 15 years 98 ago, numerous slides have developed on terrace fronts near Shannon Creek, one of which is shown in Figure 4.5. These sl i d e s Figure 4.5. Landslide on a terrace slope near Shannon Creek i n i t i a t e d by loss of root cohesion (Landslide D3 - see Appendix F). are not related to skidder roads nor to flooding from water diversion. The occurrence of these landslides 10 years or more after logging suggests that the slow deterioration of tree roots was responsible. It is also possible that decreased evapotranspiration may have allowed piezometric pressures to 99 r i s e . 1 Where s i d e c a s t m a t e r i a l s choke steep g u l l i e s at a road c r o s s i n g , p e r i o d i c i n f l u x e s of conc e n t r a t e d s u r f a c e runoff f o l l o w i n g a heavy r a i n f a l l may m o b i l i z e the d e b r i s and cause a l a r g e - s c a l e d e b r i s flow (Ziemer 1981). However, problems of t h i s nature were not found near the study a r e a . 4.5.2 Hazard C l a s s Because l a n d s l i d e s are f r e q u e n t l y a s s o c i a t e d with g u l l i e s and t e r r a c e s , the 'very high hazard' c l a s s i s assign e d to these f e a t u r e s . No d i s t i n c t i o n i s made between g u l l i e s and t e r r a c e f r o n t s on the slope s t a b i l i t y map (Map D), as problems a s s o c i a t e d with each are s i m i l a r . 4.5.3 P r e v e n t a t i v e And Remedial E n g i n e e r i n g Techniques E n g i n e e r i n g techniques given f o r S4 and S3 slopes can be a p p l i e d to these l i n e a r f e a t u r e s . Where roads c r o s s g u l l i e s , s p e c i a l a t t e n t i o n should be given to the proper containment of d e b r i s by c o r r e c t l y p l a c i n g c u l v e r t s , t r u c k i n g excavated m a t e r i a l to safe areas, and b u t t r e s s i n g road cut s l o p e s . P r o v i s i o n s f o r drainage of both subsurface and sur f a c e water should be c o n s e r v a t i v e l y designed to allow f o r runoff .surges d u r i n g storm events due to subsurface flow c o n c e n t r a t i o n . Roads l o c a t e d on t e r r a c e benches should not d i v e r t water 'The e f f e c t of d e f o r e s t a t i o n on p i e z o m e t r i c pressures i s u n c e r t a i n , as re s e a r c h e r s have presented c o n f l i c t i n g t h e o r i e s t h a t r e q u i r e f u r t h e r t e s t i n g (deVries and Chow 1978, Chamberlain 1972, O'Loughlin 1973 and L i n s l e y 1975). 100 over the break i n slope and seepage areas on t e r r a c e faces should be avoided, p a r t i c u l a r l y near streams. Trees should not be harvested on these slopes as root cohesion may be c r i t i c a l to slope s t a b i l i t y . 4 . 6 Summary Of The Hazard C l a s s i f i c a t i o n System In summary, hazard c l a s s e s i n the study area are d i v i d e d p r i m a r i l y a c c o r d i n g to n a t u r a l t e r r a i n s u b d i v i s i o n s , then f u r t h e r s u b d i v i d e d a c c o r d i n g to the observed e n g i n e e r i n g behaviour of s i m i l a r slopes i n other areas. F i g u r e 4 . 6 i l l u s t r a t e s the use of the hazard c l a s s i f i c a t i o n system on a h y p o t h e t i c a l s l o p e , and can be used as a quick r e f e r e n c e to the d e f i n i n g c r i t e r i a of each c l a s s . I t i s noted that many land-use planners p r e f e r to use a 3 to 4 c l a s s hazard r a t i n g system f o r d e c i s i o n making purposes, as more complex systems tend to confuse and complicate the i s s u e , p a r t i c u l a r l y when m u l t i p l e - u s e d e c i s i o n s are being made. For t h i s reason, the four r a t i n g s : low, moderate, high, and very h i g h were a s c r i b e d to the nine more s p e c i f i c hazard c l a s s e s d i s c u s s e d i n t h i s c h a p t e r . Thus, both the engineer and the planner can more e a s i l y u t i l i z e the i n f o r m a t i o n . 101 TERRAIN SUBDIVISION DEBRIS FAN MANTLE OF UNCONSOLIDATED MATERIAL ROCK AND COLLUVIUM HAZARD CLASS F2 J F1 S1 I S2 S3 84 J S1 R2 R3 R1 RELATIVE HAZARD high j mod low I mod high very | high | low high very high low J | 1 i c O 1 I 1 a <̂  o u 1 I 1 w O 0 k_ 0 HAZARD j I a c ! C D pron rtt CO 1 • c a CRITERIA 1 # • • i CO _ o j "35 ! | o 1 O 11 U -U i CD > « 1 v 0. | c o ! o 1 " S I o CM | O 1 ® 1 * 1 o A j o 1 j o CM C D >(S 1 a. 1 ! V A 1 u - /!•• v . ! m 1 U J u. 1 % ! U J ! TCS CFf Mb Ca Rs Rm F i g u r e 4.6. A h y p o t h e t i c a l slope i l l u s t r a t i n g the use of the l a n d s l i d e hazard c l a s s i f i c a t i o n system. 4.7 D i s t r i b u t i o n Of Hazard C l a s s e s High hazard R3 and R2 slopes are by f a r the most a r e a l l y e x t e n s i v e of any hazard c l a s s i n the study area as i t i n c l u d e s the rugged, r e c e n t l y g l a c i a t e d topography of the a l p i n e areas between a l l four drainage b a s i n s . In p l a c e s , rock r i b s are exposed on the lower slopes of the v a l l e y s and pose formidable b a r r i e r s to road c o n s t r u c t i o n . One such r i b occurs to the southeast of the mouth of Wee Sandy Creek where R3 s l o p e s encroach on both s i d e s of the v a l l e y . F a r t h e r u p v a l l e y , R3 1 02 slope s again occur adjacent to one another on o p o s i t e s i d e s of the v a l l e y i n the v i c i n i t y of the l a r g e cascade d e s c r i b e d i n Chapter 2. In the lower p o r t i o n s of the v a l l e y s , R2 slopes occur most f r e q u e n t l y at the base of the high s o u t h - f a c i n g c l i f f s of both Nemo and Wee Sandy Creek B a s i n s . I t i s sometimes d i f f i c u l t to d i s t i n g u i s h between c o l l u v i a l b l a n k e t s and c o l l u v i a l aprons, i n which case the composite symbol R2-S3 i s used. ' D i f f i c u l t i e s a l s o a r i s e where steep rocky t e r r a i n u n i t s i n c l u d e numerous but di s c o n t i n u o u s c o l l u v i a l aprons, such as slopes to the southeast of Wee Sandy Lake. In these areas the composite symbol R3-R2 i s employed. In the upper Wee Sandy Creek Basin, S4 slop e s occur where streams have i n c i s e d i n t o morainal blankets or d e b r i s fans. F a r t h e r downvalley, f a i l u r e s are o c c u r r i n g on steep uniform s l o p e s adjacent to R3 and S3 u n i t s , and on the upper slopes of g u l l y networks fee d i n g d e b r i s flow channels. In p l a c e s , t e r r a c e f r o n t s are e x t e n s i v e enough along lower Wee Sandy Creek to be mappable as S4 hazard u n i t s . Some of the most ex t e n s i v e and ob v i o u s l y unstable S4 slopes are found i n lower Nemo Creek B a s i n . The lower n o r t h - f a c i n g s l o p e s along approximately 5 km of the lower v a l l e y show evidence of recent l a n d s l i d e a c t i v i t y , i n c l u d i n g the l a r g e d e b r i s avalanche shown i n F i g u r e 2.5. High hazard S3 slopes i n c l u d e a major p o r t i o n of f o r e s t s l o p e s of both Nemo and Wee Sandy Creek B a s i n s . They f r e q u e n t l y e x i s t as the t r a n s i t i o n u n i t between R3 or R2 hazard c l a s s e s upslope and f l a t t e r S2 or S1 hazard u n i t s downslope, along the concave p r o f i l e t y p i c a l of g l a c i a t e d v a l l e y s (see F i g u r e 4.6). 1 03 Moderate to low hazard S2 and S1 slop e s are l a r g e l y c o n f i n e d to the v a l l e y bottoms or c i r q u e b a s i n f l o o r s of the study area, except f o r the long uniform S2 and S1 slop e s on the n o r t h - f a c i n g s i d e s of both lower Nemo and Wee Sandy Creek Basins, and the e a s t - f a c i n g slopes of the main Slocan V a l l e y . In a l p i n e areas where g l a c i a l m a t e r i a l s have been d e p o s i t e d on r e l a t i v e l y f l a t rock benches, a r e a l l y e xtensive S2 and S1 slopes occur c l o s e l y a s s o c i a t e d with g l a c i a l l y scoured R1 rock benches. High to moderate hazard F2 and F1 d e b r i s fan u n i t s are widely d i s t r i b u t e d along the base of the s o u t h - f a c i n g c l i f f s of the major basins as d i s c u s s e d i n Chapter 2, and a l s o i n the upper Sharpe Creek Drainage. F1 slop e s are c o n f i n e d to the V a l l e y bottoms and are commonly adjacent to S2 and S1 slopes i n these a r e a s . G u l l i e s i n c i s e d i n t o s u r f i c i a l m a t e r i a l s or bedrock commonly occur with R3 and S3 slop e s on the south s i d e s of both Nemo and Wee Sandy Creek Basins. F l u v i a l t e r r a c e s are only found along the major or t r i b u t a r y creek channels of Wee Sandy and Nemo Creek. 1 04 CHAPTER 5 ROAD CORRIDOR ASSESSMENTS 5.1 General The u t i l i t y of the hazard c l a s s i f i c a t i o n system presented i n Chapter 4 can be demonstrated by l o o k i n g i n d e t a i l at main lo g g i n g road alignments i n Nemo and Wee Sandy Creek Basins. The primary o b j e c t i v e i s to d e l i n e a t e the most envi r o n m e n t a l l y sound yet e c o n omically f e a s i b l e road alignments. S e v e r a l road o p t i o n s were t e n t a t i v e l y l o c a t e d and flagged by B.C. M i n i s t r y of F o r e s t s d u r i n g the summer of 1981. These road l o c a t i o n s , shown on the slope s t a b i l i t y map, were engineered to meet grade and l o c a t i o n requirements f o r summer lo g g i n g o p e r a t i o n s . What impact, i f any, these roads w i l l have on the s t a b i l i t y of slopes can be determined by i d e n t i f y i n g the hazard c l a s s e s t r a v e r s e d by each. The f i v e separate road options w i l l be d i s c u s s e d i n d e t a i l . 5.2 Nemo Creek Road Options Three road o p t i o n s were t e n t a t i v e l y l o c a t e d f o r access i n t o Nemo Creek B a s i n . The longest of the three i s opt i o n A and begins at Slocan Lake approximately 2 km south of the mouth of Wee Sandy Creek. The nature of the hazards a s s o c i a t e d with t h i s road l o c a t i o n can be i n t e r p r e t e d from the slope s t a b i l i t y map. For the f i r s t 4 km, the road t r a v e r s e s low hazard S1 slopes of the lower Sharp Creek Basin, c r o s s e s a rock r i b to the north of Hoben Creek v i a a 'notch' e a s i l y d i s c e r n a b l e on the map, then 105 unavoidably t r a v e r s e s some shallow s o i l s r e q u i r i n g i n t e r m i t t e n t b l a s t i n g . At km 6.7, the road begins to t r a v e r s e p o t e n t i a l l y hazardous R3 and S3 slopes which i n some areas may r e q u i r e b l a s t i n g . These slopes are unavoidable because of the road grades needed to a t t a i n a bench adjacent to steep R3 slopes near Nemo Creek at km 9.0. The road then descends, v i a the bench, i n t o the Nemo Creek Basin proper. At km 8.9, the road c r o s s e s a g u l l y which w i l l r e q u i r e s p e c i a l treatement i n order to prevent d e b r i s a v a l a n c h i n g and subsequent stream s i l t a t i o n . C o n t i n u i n g up the bas i n on the north s i d e of Nemo Creek, the road t r a v e r s e s a s e r i e s of moderately hazardous S2 and F1 slopes with s c a t t e r e d boulders s e v e r a l meters i n diameter d e r i v e d from the high c l i f f s to the north. The substratum i s mostly s t a b l e i n these a r e a s . However, a t o t a l of 5 a c t i v e d e b r i s flow paths are c r o s s e d which would r e q u i r e s p e c i a l p r e c a u t i o n a r y e n g i n e e r i n g . The road then c r o s s e s Nemo Creek at km 12.3 i n order to a v o i d a p a r t i c u l a r l y t h r e a t e n i n g snow avalanche path on the north s i d e of the creek. Terrace f r o n t s o c c u r r i n g on the south s i d e of Nemo Creek demand p a r t i c u l a r care i n l o c a t i n g a bridge c r o s s i n g where eng i n e e r i n g techniques can maintain s t a b i l i t y . C o n t i n u i n g to the west on the south s i d e of Nemo Creek, the road t r a v e r s e s both S3 and F2 slopes that have been i n c i s e d to form t e r r a c e f r o n t s near the creek. The road should not be l o c a t e d near these t e r r a c e f r o n t s . At km 14.0, the road e n t e r s the broader upper Nemo Creek Basin where the main haul road i s terminated. A summary of t o t a l l e ngths of road c o r r i d o r A t r a v e r s i n g 106 each hazard c l a s s i s given i n Table 5.1. Included i n the Table i s a t a l l y of the t o t a l number of g u l l i e s , d e b r i s flow channels, and t e r r a c e f r o n t s t r a v e r s e d , and the , number of k i l o m e t r e s RO AD  O PT IO N TO TA L LE NG TH  KILOMETRES OF EACH HAZARD CLASS TRAVERSED GU LL IE S CR OS SE D DE BR IS  F LO WS  C RO SS ED  TE RR AC ES  C RO SS ED  RO AD  O PT IO N TO TA L LE NG TH  KILOMETRES OF EACH RELATIVE HAZARD IN TE RM IT TE NT  B LA ST IN G CO NT IN UO US  B LA ST IN G GU LL IE S CR OS SE D DE BR IS  F LO WS  C RO SS ED  TE RR AC ES  C RO SS ED  RO AD  O PT IO N TO TA L LE NG TH  cn CN C/1 m to to PS CO Bi PH CN ft. S3 -R 3 Sl -R l S2 -R 1 R3 -R 1 S2 -R 3 VE RY  H IG H HI GH  MO DE RA TE  LO W IN TE RM IT TE NT  B LA ST IN G CO NT IN UO US  B LA ST IN G GU LL IE S CR OS SE D DE BR IS  F LO WS  C RO SS ED  TE RR AC ES  C RO SS ED  A 14.0 5 . 6 2.6 1.5 0 0 .3 0 1.8 .2 .9 .9 .2 0 0 0 2.9 4.6 5.5 2.0 0 1 5 1 BI 9.4 .3 3.6 1 . 6 .3 0 .3 0 1.8 .2 0 0 1.1 .2 0 .3 2.3 5.5 .3 1. 1 .2 3 5 1 B2 9.4 .8 3.2 1.1 .3 0 .3 .2 1.8 .2 0 0 1. 1 .4 0 .3 2.2 6.1 .8 1.1 .6 3 5 2 C 9.0 .9 2.5 .4 .6 0 . 1 .6 1.3 1.5 0 0 0 . 1 1.0 .6 4.2 3.8 .9 1.0 .7 2 3 1 D 11.0 2.6 2 . 6 1.0 . 6 .3 . 1 .8 1.3 1.5 0 0 0 0 0 .8 3.4 3.9 2.9 0 1.1 6 3 1 S4, R3 - VERY HIGH S3, R2, F2 - HIGH S2, F l - MODERATE SI, RI - LOW F i g u r e 5.1. Hazards t r a v e r s e d by v a r i o u s proposed road c o r r i d o r s i n Nemo and Wee Sandy Creek Basins. l i k e l y to r e q u i r e e i t h e r i n t e r m i t t e n t or continuous b l a s t i n g d u r i n g road c o n s t r u c t i o n . Nemo Creek road o p t i o n B begins on Slocan Lake approximately 1.8 km south of the mouth of Nemo Creek. I t i s approximately 4.6 km s h o r t e r than o p t i o n A and pr o v i d e s a more d i r e c t access to the Nemo Creek B a s i n . T r a v e r s i n g southwest from Slocan Lake, the road encounters w i t h i n 0.5 km a major g u l l y apt to cause s t a b i l i t y problems i f the road i s not p r o p e r l y engineered. The road then switches back, again c r o s s e s the g u l l y , and t r a v e r s e s S2 slopes l i k e l y to r e q u i r e some minor 1 07 b l a s t i n g because of shallow s u r f i c i a l m a t e r i a l s . At km 1.9, the road begins to descend towards Nemo Creek v i a a l o c a l l y steepened S4 slope showing evidence of recent l a n d s l i d i n g . In t h i s area, slopes are steeper than 35° and i n c l u d e g u l l i e s with spoon-shaped scarp faces at t h e i r heads. At km 2.4, the road emerges from the high r i s k area onto a low hazard S1 slope but soon encounters . shallow s o i l s l i k e l y to r e q u i r e i n t e r m i t t e n t b l a s t i n g d u r i n g road c o n s t r u c t i o n . At km 3.3, road o p t i o n B s p l i t s i n t o (1) an upper, route l a b e l e d B1 which l i n k s up with road o p t i o n A at km 4.3, and (2) a lower route with a more fa v o u r a b l e grade that w i l l , u n f o r t u n a t e l y , r e q u i r e b l a s t i n g d u r i n g road c o n s t r u c t i o n f o r 0.1 to 0.2 km on a R3 slope immediately adjacent to Nemo Creek. The lower route, l a b e l e d B2, i n t e r s e c t s an a c t i v e s l i d e at km 4.6, then j o i n s road o p t i o n A at km 5.0. From t h i s p o i n t , o p t i o n s B1 and B2 f o l l o w the same route as o p t i o n A. Table 5.1 summarizes the t o t a l l e n g t h of road c o r r i d o r s B1 and B2 t r a v e r s i n g each hazard c l a s s and the t o t a l number of l i n e a r hazard f e a t u r e s c r o s s e d . Road op t i o n s A, B1 and B2 each have economic and environmental advantages and disadvantages. When and where environmental c o n s i d e r a t i o n s are c r i t i c a l to the development scheme, the road o p t i o n l i k e l y to minimize l a n d s l i d e occurrence should be chosen, which i n t h i s case would be o p t i o n A. However, i f f i n a n c i a l c o n s i d e r a t i o n s dominate the p i c t u r e , and i f the f i n a n c i a l and environmental consequences of l a n d s l i d i n g are ac c e p t a b l e , perhaps o p t i o n B1 should be chosen because of i t s lower c o n s t r u c t i o n c o s t s . 108 5.3 Wee Sandy Creek Road Options Two c o r r i d o r o ptions were t e n t a t i v e l y l o c a t e d f o r access i n t o Wee Sandy Creek Bas i n . The longest of the two (option D, see slope s t a b i l i t y map and Table 5.1) begins at Slocan Lake where road o p t i o n A begins 2 km south of Wee Sandy Creek. For the f i r s t three k i l o m e t r e s , the road t r a v e r s e s low hazard S1 and S2 slopes of the lower Sharp Creek Bas i n . At km 3.0, the road encounters a steep R3 slope veneered with c o l l u v i u m which w i l l r e q u i r e continuous b l a s t i n g f o r approximately 0.7 km. At k i l o m e t r e 3.7, the road e n t e r s a notch on a bedrock dominated r i d g e , then descends i n t o the Wee Sandy Creek Basin proper i n t e r s e c t i n g three major g u l l i e s , a l l of which have at some time i n v o l v e d d e b r i s flows. Once near Wee Sandy Creek, the road descends a t e r r a c e f r o n t and then c r o s s e s the creek at km 6.6 i n order to a v o i d steeper s l o p e s and snow avalanche paths on the south s i d e . C o n t i n u i n g to the west, the road t r a v e r s e s a s e r i e s of F1 and F2 s l o p e s , c r o s s e s three r e c e n t l y a c t i v e d e b r i s flow paths, and takes a double switch-back on a d e b r i s fan slope i n order to gain e l e v a t i o n and maintain proper grade. At t h i s p o i n t , the road c r o s s e s an unavoidable s e r i e s of high hazard S3 and R3 slope s l i k e l y to have some impact on Wee Sandy Creek immediately adjacent to the south. A c t i v e l y f a i l i n g S4 slopes are v i r t u a l l y unavoidable between km 9.0 and 10.2 because of the extreme steepness of the canyon i n t h i s a r ea. At km 11.0, the road e n t e r s the broader upper Wee Sandy Creek Basin where the main road i s terminated. A summary of the t o t a l l e n g t h s of road 109 c o r r i d o r D t r a v e r s i n g each hazard c l a s s and the number of c r i t i c a l l i n e a r f e a t u r e s c r o s s e d i s given i n Table 5.1. Wee Sandy Creek road o p t i o n C begins at the same p o i n t as o p t i o n D then t r a v e r s e s d i r e c t l y to the north towards the mouth of Wee Sandy Creek. T h i s more d i r e c t route c r o s s e s moderate to steep S2 and R3-S2 s l o p e s , some of which have shallow veneers of s u r f i c i a l m a t e r i a l over competent bedrock which would r e q u i r e i n t e r m i t t e n t b l a s t i n g d u r i n g road c o n s t r u c t i o n . Approaching Wee Sandy Creek at km 2.0, the road unavoidably encounters extremely steep R3 s l o p e s l i k e l y to produce unstable d e b r i s which w i l l descend to the creek below. If a maximum a l l o w a b l e f a v o u r a b l e grade i s maintained, the road can emerge onto a t e r r a c e bench at km 2.5 and a v o i d a h i g h l y unstable t e r r a c e f r o n t showing evidence of a c t i v e l a n d s l i d i n g d i r e c t l y i n t o the creek. Any attempt to l o c a t e a road a c r o s s t h i s t e r r a c e f r o n t would almost c e r t a i n l y r e s u l t i n l a n d s l i d e s c a using environmental damage. Fa r t h e r to the west, road o p t i o n C c r o s s e s two d e b r i s flow g u l l i e s before descending a t e r r a c e f r o n t and c r o s s i n g Nemo Creek at km 4.3. At km 5.0, the road i n t e r s e c t s an f o l l o w s road o p t i o n D to upper Wee Sandy Creek B a s i n . A summary of the t o t a l l e ngths of road o p t i o n C t r a v e r s i n g each hazard c l a s s and number of c r i t i c a l l i n e a r f e a t u r e s c r o s s e d i s given i n Table 5.1. Road o p t i o n s C and D attempt to minimize c o s t and environmental impacts. From Table 5.1 and the above d e s c r i p t i o n , i t i s evident that p o r t i o n s of road r e q u i r i n g b l a s t i n g and i n v o l v i n g environmental damage due to l a n d s l i d i n g are unavoidable with e i t h e r o p t i o n . F i n a n c i a l and environmental 1 10 c o n s i d e r a t i o n s suggest that o p t i o n D i s a b e t t e r c h o i c e . However, because a host of c o n s i d e r a t i o n s other than l a n d s l i d e p o t e n t i a l a f f e c t the c h o i c e of f i n a l road l o c a t i o n , the u l t i m a t e d e c i s i o n i s l e f t to the land planner. 111 CHAPTER 6 SUMMARY AND CONCLUSIONS The f i r s t of two o b j e c t i v e s of t h i s t h e s i s was to map, d e s c r i b e and determine the p r e d i c t i b i l i t y of fundamental f a c t o r s c o n t r o l l i n g the s t a b i l i t y of slopes i n the study area. F a c t o r s deemed important i n l a n d s l i d e occurrence i n other regions i n c l u d e g e o l o g i c , geomorphologic, h y d r o l o g i c , pedologic and v e g e t a t i v e v a r i a b l e s i n a d d i t i o n to e n g i n e e r i n g c o n s i d e r a t i o n s . However, from o b s e r v a t i o n s of the nature and type of dominant l a n d s l i d e processes c u r r e n t l y a c t i v e i n the study area, i t was p o s s i b l e to i n f e r • which f a c t o r s are l i k e l y to a f f e c t l a n d s l i d i n g . The general o b s e r v a t i o n that i n i t i a l f a i l u r e s occur on p l a n a r shear s u r f a c e s w i t h i n 2 meters of the ground s u r f a c e p a r t i a l l y s a t i s f i e s the assumptions of a p h y s i c a l l y based g e o t e c h n i c a l model. The model g r e a t l y s i m p l i f i e s the number of f a c t o r s to be c o n s i d e r e d but can only be a p p l i e d to c e r t a i n t e r r a i n u n i t s . Advantages of the p h y s i c a l model are that i t p r o v i d e s a framework on which to b u i l d a hazard c l a s s i f i c a t i o n system f o r uniform slopes mantled with s u r f i c i a l m a t e r i a l , the s l o p e s most l i k e l y to be a f f e c t e d by f o r e s t e n g i n e e r i n g . Moreover, because i t i s l e s s operator-dependent than other e m p i r i c a l models which are based on the s t a t i s t i c a l a n a l y s i s of s u b j e c t i v e l y chosen and e v a l u a t e d f a c t o r s , i t has the advantage of being more e a s i l y t r a n s f e r r e d from region to r e g i o n . However, some d i f f i c u l t i e s were encountered i n a c c u r a t e l y q u a n t i f y i n g the fundamental v a r i a b l e s of the model i n the study a r e a . I t was f i r s t necessary to d e l i n e a t e those slopes i n the study area which c o u l d be e v a l u a t e d a c c o r d i n g to the 1 1 2 g e o t e c h n i c a l model. T h i s was done using the T e r r a i n C l a s s i f i c a t i o n System (TCS) to map slope u n i t s p r i m a r i l y on the b a s i s of g e n e t i c m a t e r i a l and s u r f a c e e x p r e s s i o n . I t was found that l e s s than one h a l f of the s l o p e s i n the study area c o u l d be e v a l u a t e d a c c o r d i n g to the model. Of the slopes that c o u l d be evaluated, many were complex assemblages of g e n e t i c m a t e r i a l , v a r i a b l e topography, and groundwater c o n d i t i o n s . In a d d i t i o n , parameters such as s o i l shear s t r e n g t h , root s t r e n g t h , p i e z o m e t r i c p r e s s u r e , and depth to shear plane were d i f f i c u l t to measure i n the f i e l d due to l o g i s t i c a l and time c o n s t r a i n t s . T h e r e f o r e , i t was necessary to develop the means to roughly estimate the l i k e l y values of the fundamental l a n d s l i d e c o n t r o l l i n g f a c t o r s over these s l o p e s . The u n c e r t a i n t i e s i n v o l v e d i n the estimates can be p a r t i a l l y accounted f o r i n a s t o c h a s t i c v e r s i o n of the g e o t e c h n i c a l model. Genetic m a t e r i a l s were found to be extremely complex l e a d i n g to d i f f i c u l t i e s i n c h a r a c t e r i z i n g shear s t r e n g t h p r o p e r t i e s . The method of e s t i m a t i n g <t> values developed in t h i s t h e s i s f a i l s to a c c u r a t e l y determine a b s o l u t e values at a p a r t i c u l a r s i t e . However, i t does appear s u c c e s s f u l i n determining the r e l a t i v e values a s s o c i a t e d with d i f f e r e n t g e n e t i c m a t e r i a l s . For example, even though the a b s o l u t e ranges of <f> values f o r c o l l u v i a l and morainal s o i l s c o u l d not be determined very a c c u r a t e l y , the f a c t that c o l l u v i a l s o i l s are g e n e r a l l y stronger than l e s s angular f l u v i o g l a c i a l m a t e r i a l s , and that morainal s o i l s are more u n p r e d i c t i b l e than e i t h e r c o l l u v i a l or f l u v i o g l a c i a l m a t e r i a l s i s born out i n the a n a l y s i s . 1 1 3 A p a r t i c u l a r d i f f i c u l t y encountered was that of a c c u r a t e l y d e l i n e a t i n g on the map the d i s t r i b u t i o n of slope angles, the f a c t o r most fundamental to slope s t a b i l i t y . In many areas, s l o p e s are obscured by the t r e e canopy and cannot be a c c u r a t e l y e v a l u a t e d with a e r i a l photographs. Moreover, topographic complexity demanded that slope c l a s s e s be given 10° i n t e r v a l s , an i n t e r v a l over which a f a i r l y wide range of slope e q u i l i b r i u m v a l u e s can be c a l c u l a t e d . T h i s , however, i s a d i f f i c u l t y that i s not unique to t h i s methodology and i s one of the primary reasons why a r e a l assessments of s t a b i l i t y cannot r e p l a c e s i t e - s p e c i f i c e v a l u a t i o n s . S e m i - q u a n t i t a t i v e v a r i a b l e s i n t h i s t h e s i s a f f e c t or are themselves a f f e c t e d by many of the f a c t o r s d e s c r i b e d by authors i n other r e g i o n s . For example, e m p i r i c a l e v a l u a t i o n s i n many regions have drawn a d i r e c t r e l a t i o n s h i p between d e f o r e s t a t i o n and l a n d s l i d e occurrence, sometimes with l i t t l e e x p l a n a t i o n as to why t h i s i s the case. The p h y s i c a l model, on the other hand, c l a r i f i e s the f a c t that l o s s of root cohesion f o l l o w i n g d e f o r e s t a t i o n i s , i n f a c t , the f a c t o r d i r e c t l y i n f l u e n c i n g l a n d s l i d i n g . S i m i l a r l y , i n the study area, the s t o c h a s t i c model demonstrates not only the f a c t t h a t l a n d s l i d e s are commonly a s s o c i a t e d with t e r r a c e f r o n t s , but a l s o that e l e v a t e d p i e z o m e t r i c p r e s s u r e s and l o c a l l y steepened slopes are c o n t r i b u t i n g to them. F a c t o r s c o n t r o l l i n g s t a b i l i t y i n areas not evaluated a c c o r d i n g to the g e o t e c h n i c a l model were i n f e r r e d from knowledge of slope morphology and g e n e s i s . For example, hazards due to d e b r i s flows c o u l d be i n f e r r e d from evidence of past d e b r i s flow 1 14 a c t i v i t y such as l a r g e boulders d e p o s i t e d on levees adjacent to channels. The second o b j e c t i v e of t h i s t h e s i s was to examine l a n d s l i d e s i n i t i a t e d by e n g i n e e r i n g a c t i v i t i e s i n environments s i m i l a r to those of the study area and develop a hazard r a t i n g system based on past experience. S t a b i l i t y i n d i c e s , when c a l c u l a t e d f o r slopes adjacent to l a n d s l i d e s induced by e n g i n e e r i n g a c t i v i t i e s , were used f o r comparison with slopes to be developed i n the study area. The r e s u l t s i n d i c a t e d that due to n a t u r a l f a c t o r s and e n g i n e e r i n g misjudgement, l a n d s l i d e s tend to occur p r i m a r i l y on slopes with p r o b a b i l i t i e s of f a i l u r e g r e a t e r than 10% as determined by the s t o c h a s t i c g e o t e c h n i c a l model. E n g i n e e r i n g problems a s s o c i a t e d with these f a i l u r e s i n c l u d e inadequate p r o v i s i o n s f o r water drainage, improper f i l l s lope c o n s t r u c t i o n and i n c o r p o r a t i o n of organic d e b r i s i n f i l l m a t e r i a l s . High p r o b a b i l i t i e s of f a i l u r e were a l s o c a l c u l a t e d for s l opes showing no evidence of i n s t a b i l i t y e i t h e r before or a f t e r road c o n s t r u c t i o n , i n d i c a t i n g that c o n s e r v a t i v e estimates of model input values l e a d to higher than expected p r o b a b i l i t i e s of f a i l u r e . F o r t u n a t e l y , t h i s i s of l i t t l e consequence i f the s t a b i l i t y i n d i c e s are grouped to form hazard c l a s s e s a c c o r d i n g to e n g i n e e r i n g experience. From the f a c t that s l o p e s with p r o b a b i l i t i e s of f a i l u r e g r e a t e r than 10% near the study area host a wide v a r i e t y of l a n d s l i d e problems, i t can be i n f e r r e d t h a t slopes with greater than 10% v a l u e s i n the study area w i l l l i k e w i s e i n v o l v e s i m i l a r types of problems, r e g a r d l e s s of what the the a c t u a l 10% value may mean i n r e a l p h y s i c a l terms. In the absence of more a c c u r a t e l y determined model input v a l u e s and 1 1 5 s t a t i s t i c a l a n a l y s es of p r o b a b i l i t y d i s t r i b u t i o n s f o r slopes of the study area, the r e l a t i v e hazard r a t i n g system i s the only recourse a v a i l a b l e . In areas where the g e o t e c h n i c a l model does not apply because of l i m i t i n g assumptions, hazards are a s s i g n e d a c c o r d i n g to past engineering experience i n s i m i l a r n a t u r a l t e r r a i n u n i t s . The e n g i n e e r i n g behaviour i s then assumed to be homogeneous throughout the u n i t . U n i t s c l a s s e d as 'very h i g h hazard' i n c l u d e those slopes which show s i g n s of a c t i v e f a i l u r e as i n d i c a t e d by morphology and v e g e t a t i o n , as w e l l as steep rocky s l o p e s . 'High hazard' slopes i n c l u d e c o l l u v i a l fans, upper p a r t s of d e b r i s fans, and slopes mantled with s u r f i c i a l m a t e r i a l s having p r o b a b i l i t i e s of f a i l u r e g r e a t e r than 10%. Slopes with 'moderate' l a n d s l i d e hazards i n c l u d e lower d e b r i s fans and and slopes mantled with s u r f i c i a l m a t e r i a l s having p r o b a b i l i t i e s of f a i l u r e l e s s than 10% and expected f a c t o r s of s a f e t y l e s s than 1.6. 'Low hazard' slopes i n c l u d e s l o p e s mantled with s u r f i c i a l m a t e r i a l s having expected f a c t o r s of s a f e t y g r e a t e r than 1.6 and g e n t l y s l o p i n g bedrock dominated t e r r a i n . The u n c e r t a i n t i e s i n v o l v e d i n c h a r a c t e r i z i n g n a t u r a l slope c o n d i t i o n s over l a r g e areas where a v a i l a b l e data are l i m i t e d , have no doubt l e d to some broad g e n e r a l i z a t i o n s , which, at the l o c a l l e v e l , are i n e r r o r . 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E c k e l , ed. NAS-NRC P u b l i c a t i o n 54, p.20-47. Varnes, D.J. 1978. Slope movement types and p r o c e s s e s . In: L a n d s l i d e s : A n a l y s i s and C o n t r o l . S p e c i a l Report 176, T r a n s p o r t a t i o n Research Board, Washington, D.C, p.11-33. Walmsley, M., U t z i g , G., V o i d , T., Moon, D. and VanBarneveld, J . 1980. D e s c r i b i n g Ecosystems in the F i e l d . R.A.B. T e c h n i c a l Paper 2, B.C. M i n i s t r y of Environment and B.C. M i n i s t r y of F o r e s t s . Ward, T.J., L i , R.M. and Simons, D.B. 1978. L a n d s l i d e p o t e n t i a l and p r o b a b i l i t y c o n s i d e r i n g randomness of c o n t r o l l i n g f a c t o r s . Report on f i l e at E n g i n e e r i n g Research Center, Colorado State U n i v e r s i t y , F o r t C o l l i n s , Colorado. W i l f o r d , D.J. and Schwab, J.W. 1982. A summary r e p o r t on mass wasting in Rennel Sound. Report in progress, B.C. M i n i s t r y of F o r e s t s , Research Branch. Wilson, G. 1976. E n g i n e e r i n g pedology and i t s a p p l i c a t i o n to slope s t a b i l i t y problems i n B r i t i s h Columbia. 29th Canadian G e o t e c h n i c a l Conference on Slope S t a b i l i t y , 13-16 October, 1976, p.v-12 - v-37. Wilson, G., McCormack, D.E. and Moon, D.E. 1982. P e d o t e c h n i c a l aspects of t e r r a i n a n a l y s i s . Jour, of Geotech. Eng. Div., American S o c i e t y of C i v i l Engineers, i n p r e s s . Wu, T.H., McKinnel, W.P.III, and Swanston, D.N. 1979. Strength of t r e e r o o t s and l a n d s l i d e s on P r i n c e of Wales I s l a n d , A l a s k a . Can. Geotech. Jour., v.16, n.1, p.19-33. Wu, T.H. and Swanston, D.N. 1980. Risk of l a n d s l i d e s i n shallow s o i l s and i t s r e l a t i o n to c l e a r c u t t i n g i n southeastern A l a s k a . F o r e s t Science, v.26, n.3, p. 495-510. 123 Youd, T.L. 1973. L i q u i f a c t ion, flow and a s s o c i a t e d ground f a i l u r e . USGS C i r c u l a r 688. Young, A. 1972. Slopes. O l i v e r and Boyd, Edinburgh, 288p. Ziemer, R.R. 1981. Management of s t e e p l a n d e r o s i o n : an overview. Jour, of Hydrology (NZ), i n p r e s s . APPENDIX A SOIL TEST DATA 125 NO P.I. STONES COBBLES GRAVEL SAND SILT CLAY USC COARSE MEDIUM FINE WS1 — 0 10 20 14.1 25.1 24.9 4.8 1.1 SP-SM WS2 — 0 5 50 4.4 . 7.2 13.7 18.3 1.4 GM WS3 15.3 0 0 0 7.7 33.5 14.1 34.4 8.5 SM WS4 — 5 20 45 7.4 10.0 7.4 4.7 0.4 GW WS5 — 3 40 20 5.6 7.5 11.2 9.6 3.0 GM WS6 — 1 10 20 12.7 16.0 24.2 16.0 0.1 SM WS7 — 0 0 8 4.9 19.9 30.2 34.9 2.0 SM WS20 — 0 5 25 13.9 21.2 23.7 9.7 1.5 SP-SM WS21 — 1 10 5 10.0 12.6 28.2 30.0 3.1 SM WS23 — 5 5 5 6.4 10.7 42.1 23.4 2.4 SM WS25 — 5 20 30 12.6 12.0 12.6 7.2 0.6 GP-GM WS26 — 2 25 25 7.3 10.6 12.9 15.9 1.2 GM WS27 — 0 0 0 2.2 8.9 40.0 44.2 4.6 SM N10 — 0 0 20 10.8 20.2 22.0 24.0 2.9 SM Ni l — 30 15 20 5.5 11.6 7.1 1.0 1.1 GP N12 — 10 10 15 9.9 20.9 15.7 15.9 2.6 SM N13 — 10 15 20 8.8 15.8 11.6 16.0 2.8 GM N14 — 5 5 20 10.3 25.4 15.4 15.4 3.4 SM N15 — 0 0 0 0.6 1.2 32.4 53.9 11.9 ML N17 — 0 10 40 12.3 20.7 14.8 1.9 0.3 GP N18 — 5 10 30 10.1 14.0 13.6 15.4 1.9 GM N19-1 — 5 15 20 4.8 16.6 20.8 15.9 1.8 GM N19-2 — 5 15 20 15.6 29.9 12.3 1.4 0.7 SP N20 — 5 20 15 15.0 22.8 10.2 10.5 1.4 SW-SM N21 — 5 10 20 17.1 25.9 15.1 6.3 0.5 SW-SM N22 — 0 2 7 3.4 11.4 32.0 36.3 7.8 SM NO+80 2.7 0 1 5 4.4 13.2 21.4 42.8 12.2 ML Nl+lOO ~ 1 5 35 10.3 15.8 13.2 18.1 1.7 SM Nl+100-2 5.6 1 5 35 8.0 19.1 12.7 15.7 3.4 SM N2+40 2.8 5 ;•• 5 15 9.3 22.8 16.3 19.3 7.2 SM N3+25 — 5 25 35 8.0 11.2 8.0 6.6 1.1 GW-GM N4+07 — — — 25.5 15.0 26.2 14.9 16.9 1.3 SM Rl — 1 10 30 10.8 15.9 13.9 16.6 1.5 SM R2 3.7 — — 7.6 7.4 15.5 22.4 34.4 13.2 SM R3 — — — 5.9 4.3 23.4 45.8 17.4 3.1 SM R4 5.4 1 5 5 15.2 • 23.5 18.1 21.1 11.1 SM R5 — 1 5 20 17.0 20.6 12.7 19.9 3.8 SM R6 — 5 5 30 13.5 22.1 17.1 6.9 0.4 SW-SM 1 — 5 5 15 17.2 27.4 16.4 11.6 2.4 SM 2 — — — 43 14.5 19.7 12.0 9.5 1.5 SW-SM 3 11.0 — — 19.2 13.5 26.1 11.9 20.1 9.1 SM APPENDIX B STOCHASTIC GEOTECHNICAL MODEL 127 The s t o c h a s t i c g e o t e c h n i c a l model used i n t h i s study r e p r e s e n t s the summary and refinement of ideas presented by Swanston et a l (1973), O'Loughlin (1974), Brown and Sheu (1975) and Simons et a l (1976), as developed by Simons et a l (1978). I t assumes the h i l l s i d e i s of the ' i n f i n i t e s lope' v a r i e t y . I t determines an expected f a c t o r of s a f e t y , E [ F S ] , and p r o b a b i l i t y of f a i l u r e , P. Input v a r i a b l e s are those shown in F i g u r e 3.1, each of which were d i s c u s s e d i n Chapter 3. The d e t e r m i n i s t i c model from which the p r o b a b i l i s t i c model' i s developed i s equation 3.6. The expected f a c t o r of s a f e t y i s expressed as E[FS] = L1(E[C] + E [ C r ] ) + L 2 ( E [ t a n * ] ) (1) The v a r i a n c e of the f a c t o r of s a f e t y i s formulated as VAR[FS] = L1(VAR[C] + E[C] •+ 2E[C] E[Cr] + VAR[Cr] + E [ C r ] ) + L2 (VAR[tan«s] + E[tanc*]) + 2L1 L2 E[tan*] (E[Cl + E [ C r ] ) - E[FS] (2) In equations (1) and (2), the symbols E[ ] and VAR[ ] are the expected values and v a r i a n c e s of the v a r i a b l e i n s i d e the b r a c k e t s r e s p e c t i v e l y . Assuming-uniform d i s t r i b u t i o n s f o r input v a l u e s , the expected value of a random value X i s expressed as E[X] = (Xmax + Xmin)/2 (3) and the v a r i a n c e as 1 28 VAR[X] = (Xmax - Xmin)/12 (4) where Xmax and Xmin are the upper and lower l i m i t s of the random input value r e s p e c t i v e l y . The c o n s t a n t s L1 and L2 are L i = rHsin2£(qo/rH) + [(ysat/y)M] + (rwet/r)(1-M) (5) and (qo/yH) + ( y s a t / y - l ) M + [ywet/y(1-M)] L2 = [(qo/yH) + ( r s a t / y ) M + ( y s a t / y ( 1 - M ) ) ] t a n * (6) Values of E[FS] and VAR[FS] computed from equations 1 and 2 can be used to estimate p r o b a b i l i t y of f a i l u r e . By d e f i n i t i o n , p r o b a b i l i t y of f a i l u r e i s p[FS<1] = P (7) where P i s the p r o b a b i l i t y of f a i l u r e and p[FS^1] i s the cumulative p r o b a b i l i t y that FS i s l e s s than or equal to 1.0. Ward (1976) found that a reasonable d i s t r i b u t i o n of f a i l u r e p r o b a b i l i t y i s the normal or Gaussian d i s t r i b u t i o n which allows the computation of an approximate value of P by f i r s t d e termining the value of the non-dimensional v a r i a t e U by using the equation U = ( 1 - E [ F S ] ) / ( V A R [ F S ] ) 0 5 1 29 (8) The value of U i s then used to compute the cumulative f a i l u r e ' k' by the e x p r e s s i o n k = 0.4|U| i f |U|<0.13 (9) or k = -0.01314 + 0.49494|U| - 0.15804|U| 2 + 0.01661 I U | 3 i f |U|>0.13 (10) Equations 9 and 10 are approximations with e r r o r s of l e s s than 1 percent. From U and k, the p r o b a b i l i t y of f a i l u r e P i s found as P = 0.5 + k i f U>0 (11) P = 0 . 5 - k i f U<0 (12) P = 0.5 i f U=0 (13) APPENDIX C UNIFIED SOIL CLASSIFICATION Field Identification Procedures (Excluding particles larger than 3 in. and basing fractions c estimated weights) Symbols! i s 8 « — 3 M -|J E o s ml mi lis*. |1 'MS ij -a c -Il I! 11 'Sis I So : s 8 3 t l f t i i i 1=1=1 Wide range in grain size and substantial amounts of all intermediate particle sizes Well graded gravels, gravel* sand mixtures, little or no fines Predominantly one size or a range of sizes with some intermediate sizes missing Nonplastic fines (for identification pro- cedures see ML below) Plastic fines (for identification procedures, see CL below) Clayey gravels, poorly graded gravcl-sand-clay mixtures Wide range In grain sizes and substantia] amounts of all intermediate particle sizes Well graded sands, gravelly sands, little or no fines Predominantly one size or a range of sizes with some intermediate sizes missing Nonplastic fines (for identification pro- cedures, see ML below) Plastic fines (for identification procedures, see CL below) Give typical name; indicate ap- proximate percentages of sand and gravel: maximum size; angularity, surface condition, and hardness of the coarse grains; local or geologic name and other pertinent descriptive information; and symbols in parentheses For undisturbed soils add informa- tion on stratification, degree of compactness, cementation, moisture conditions and drainage characteristics Example: Silly sand, gravelly; about 20% hard, angular gravel particles i - in. maximum size: rounded and subangular sand grains coarse to fine, about 15 % non- plastic fines with low dry strength; well compacted and moist in place: alluvial sand; (SM, Clayey sands, poorly graded sand-clay mixtures Identification Procedures on Fraction Smaller than No . 40 Sieve Size III Highly Organic Soils Dry Strength (crushing character- istics) None to slight High to very high Dilatancy (reaction to shaking) None to very slow None to very slow Toughness (consistency near plastic limit) Inorganic silts and very fine sands, rock (lour, silty or clayey fine sands with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays Organic silts and organic : ctays of low plasticity Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts Readily identified by colour, odour, sponjjy feel and frequently by fibrous texture Inorganic clays or high plas- ticity, fat clays Organic clays of medium to high plasticity . Give typical name; indicate degree and character of plasticity, amount and maximum size of coarse grains: colour in wet condition, odour if any, local or geologic name, and other perti- nent descriptive information, and symbol in parentheses For undisturbed soils add infor- mation on structure, stratifica- tion, consistency in undisturbed and remoulded states, moisture and drainage conditions Example: Clayey sill, brown: slightly plastic: small percentage of fine sand; numerous vertical root holes: firm and dry in place: loess: (ML) g (9(̂ 10 ^ g o "5 u u <8 iSJi Q j~2 Greater than 4 •few)1 Between 1 and 3 Not meeting all gradation requirements for G I F Atterbcrg limits below " A " line, or PI less than 4 Atterbcrg limits above " A " line, with PI greater than 7 Above " A " line with PI between 4 and 7 are borderline cases requiring use of dual symbols Greater than 6 <Oao>' Between I and 3 Not meeting all gradation requirements for SW Atterbcrg limits below " A " line with PI greater than 7 Above " A " line with PI between 4 and 7 are borderline cases requiring use of dual symbols 0 10 20 30 40 50 60 70 80 90 100 Liquid limit Plasticity chart for laboratory classification of fine grained soils From Wagner, 1957. * Boundary classifications. Soils possessing characteristics of two groups are designated by combinations of group symbols. For example GW-GC, well graded gravel-sand r D Al l sieve sizes on this chart are U.S. standard. xture with clay binder. These procedui Field Identification Procedure for Fine Grained Soils or Fractions e to be performed on the minus N o . 40 sieve size particles, approximately J,f 4 in. For field classification purposes, screening is not intended, simply remove by hand the coarse particles thai interfere with the tests. Dilatancy (Reaction to shakin After removing particles larger than N o . 40 sieve sire, prepare a pat of moist soil with a volume of about one-half cubic inch. Add enough water if necessary to make the soil soft but not sticky. Place the pat in the open palm of one hand and shake horizontally, striking vigorously against the other hand several times. A positive reaction consists of the appearance of water on the surface of the pat which changes to a iivery consistency and becomes glossy. When the sample is squeezed between the fingers, the water and gloss disappear from the surface, the pat stiffens and finally it cracks or crumbles. The rapidity of appearance of water during shaking and of its disappearance during squeezing assist in identifying the character of the fines in a soil. Very fine clean sands give the quickest and most distinct reaction whereas a plastic clay has no reaction. Inorganic silts, such as a typical rock flour, show a moderately quick reaction. Dry Strength (Crushing characteristics): After removing particles larger than N o . 40 sieve size, mould a pat of soil to the consistency of putty, adding water if necessary. Allow the pat to dry completely by oven, sun or air drying, and then test its strength by breaking and crumbling between the fingers. This strength is a measure of the character and quantity of the colloidal fraction contained in the soil. The dry strength increases with increasing plasticity. High dry strength is characteristic for clays of the C H group. A typical inorganic silt possesses only very slight dry strength. Silty fine sands and silts have about the same slight dry strength, but can be distinguished by the feel when powdering the dried specimen. Fine sand feels gritty whereas a typical silt has the smooth feel of flour. Toughness (Consistency near plastic limit): After removing particles larger than the N o . 40 sieve size, a specimen of soil about one-hajf inch cube in size, is moulded to the consistency of putty. If too dry, water must be added and if sticky, the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation. Then the specimen is rolled out by hand on a smooth surface or between the palms into a thread about one-eight inch in diameter. The thread is then folded and re-rolled repeatedly. During this manipulation the moisture content is gradually reduced and the specimen stiffens, finally loses its plasticity, and crumbles when the plastic limit is reached. After the thread crumbles, the pieces should be lumped together and a slight kneading action continued until the lump crumbles. The tougher the thread near the plastic limit and the stiffer the lump when it finally crumbles, the more potent is the colloidal clay fraction in the soil. Weakness of the thread at the plastic limit and quick loss of coherence of the lump below the plastic limit indicate either inorganic clay of low plasticity, or materials such as Vaolin-iype clays and organic clays whjch occur below the A-line. Highly organic clays have a very weak and spongy feel at the plastic limit. APPENDIX D RELATIVE DENSITY DETERMINATION TECHNIQUE , BLOWS „ CONSISTENCY q u (Tsf) RULE-OF-THUMB PER FOOT Very soft 0.25 Core (Height = twice the diameter) sags under own weight 0 - 1 Soft 0.25 - 0. .50 Can be pinched i n two between thumb and forefinger 2 - 4 Firm 0.50 - 1. .00 Can be imprinted e a s i l y with fingers 5 - 8 S t i f f 1.00 - 2. .00 Can be imprinted with consid- erable pressure from fingers 9 - 15 Very s t i f f 2.00 - 4. .00 Barely can be imprinted by pressure from fingers 16 - 30 Hard 4.00+ Cannot be imprinted by fingers Over 30 q i s unconfined compressive strength i n tons/sq.ft. Blows as measured with 2-in. OD, 1 3/8-in. ID sampler driven 1 f t by 140-lb hammer f a l l i n g 30 i n . See Tentative Method for Penetration Test and S p l i t - B a r r e l Sampling of S o i l s , ASTM Designation: D1586-58T. APPENDIX E TERRAIN CLASSIFICATION SYSTEM Texture SIZE mm ROUNDNESS 256 64 .062 .0039 ROUNDED BOULDERY b COBBLY k PEBBLY E ROUND OR ANGULAR SANDY s SILTY CLAYEY c ROUNDED GRAVELLY FINES f ANGULAR BLOCKY a RUBBLY r Genetic Material Anthropogenic — A C o l l u v i a l C Eolian E F l u v i a l F Ice I Lacustrine L Morainal M Organic 0 Bedrock R Saprolite S Volcanic V Marine W Bog — Fen — Swamp Qualifying Descriptor B G l a c i a l G F Active A S Inactive I Apron a Blanket b Fan f Hummocky h Level 1 Surface Expression Subdued m Ridged r Steep s Terraced t Veneer v Avalanched A Bevelled B Cryoturbated C Deflated D Channeled E F a i l i n g F Kettled H Modifying Process Karst modified K Nivated N Piping P S o l i f l u c t e d S Gul l i e d V Washed W Texture Genetic Material Modifying Process b^- F Qualifying Descriptor "Surface Expression APPENDIX F LANDSLIDE DATA AND LOCATIONS LANDSLIDES ASSOCIATED WITH ROADS ON SLOPES MANTLED WITH SURFICIAL MATERIAL SLIDE DIMENSIONS2 ' BED- NO. TYPE 1 L W D VOL. *l 6 s TCS USC ROCK M IMPACT DI DA-DF 390 10 5 9800 34° 33° fgMb GW-GM GRAN 2-3 .2 road washouts and stream s i l t a t i o n D2 DA 6 20 3 360 40° 30° rCv GM SS 3 — f i l l slope debris i n gully D3 R 50 20 2 2000 44° 44° rCb GW PHY 4 — f i l l slope f a i u l u r e causing stream s i l t a t i o n D5 SF 2 15 .5 15 35° 26° rMb GW PHY 3 — damage to road bed from organic debris deter DUb R — — .5 — 38° 36° gF Gb GP ARG 2 — f i l l slope damage and stream s i l t a t i o n D12 R 75 25 2 3750 42° 39° sF Gb SP GRAN 3 — f i l l slope damage and stream s i l t a t i o n D14 DA-DF 1500 6 1 2700 32° 35° rCb GW GRAN 3 1.0 debris flow blocked major highway D15 SC 9 100 1 900 65° 36° sF Gb SC — 3 — water diversion leading to road blockage D17 DA 15 7 1 105 — 34° sMb SW — 3-4 — damage to road bed D18 DA-DF 60 15 4 3600 45° 38° sMb SW PHY 3 . 1 road washout stream s i l t a t i o n D19 SC 6 25 1.5 225 62° 28° sMb SP-SW MONZ 5 .5 road blockage from f i l l slope f a i l u r e s D20 SC 6 6 2 72 52° 30° fsMb SW-SM ARG 3 — road blockage from f i l l slope f a i l u r e s SI R — r a v e l 75° 36° gF Gb GP — 2-3 — road blockage Al DA 40 30 .5 600 40° 40° gF Gb GW-GP PHY 3-4 — damage to road A6 SC 20 10 10 200 50° 50° rMb LANDSLIDES ASSOCIATED WITH ROADS IN STEEP ROCKY TERRAIN SLIDE DIMENSIONS2 BED- NO. TYPE 1 L W D VOL. *1 S s TCS USC ROCK MR M IMPACT D4 RF r o c k f a l l 84° 44° rCv/R GW SYE 2 Rockfa l l on road causing blockage D9 DA 90 10 1.5 1400 47° 47° rCv=R GW PHY 3 Damage to creek by blasted rock D10 RF-DA 30 10 2 600 65° 41° rCv=R GW DIOR 2 Road blockage, damage to creek D13 RF-DA — - rock f a i l u r e 66° 43° R/rCv rubb GRAN 3 Road blockage, damage to creek A5 DA 40 8 3 960 80° 39° rCv=R rubb GRAN 2 Damage to creek, road bed threatened LANDSLIDES ASSOCIATED i WITH ROADS ON COLLUVIAL APRONS WI R — discrete ravel — 54° 40° rCa GW GRAN 2 Road blockage W2 R — discrete ravel — 58° 42° rCa GW GRAN 2 Road blockage W3 R — discrete ravel — 46° 41° rCa GW GRAN 2 Road blockage 1R-Ravel DA-Debris avalanche RF-Rock f a i l u r e S F - F i l l slope f a i l u r e SC-Cut slope f a i l u r e DF-Debris flow 2 i n meters 3 6^-slope i n c l i n a t i o n at zone of s l i d e i n i t i a t i o n 8 g - i n c l i n a t i o n of entire h i l l s l o p e TCS-Terrain C l a s s i f i c a t i o n USC-Unified S o i l C l a s s i f i c a t i o n MR-Moisture Regime M-Relative height of water table with respect to the shear plane (see Chapter 3) 139

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