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UBC Theses and Dissertations

Integrated use of terrestrial laser scanning and advanced numerical methods for a total slope analysis of Afternoon Creek, Washington Strouth, Alexander Brian


On November 9, 2003 a rock slide, involving approximately 750,000 m³ of jointed, orthogenesis occurred at the Afternoon Creek ridge adjacent to the North Cascades Highway in northwest Washington. Most of the failed blocks slid into the shallowly-sloping Afternoon Creek, and did not reach the highway. However, a small fraction of the failed volume traveled down the west side of the ridge, and entered the steep, bedrock-exposed Falls Creek. This debris fell more than 600 meters and impacted the highway, destroying a section of the roadway. There is potential for future rock slope failures to occur at this site, thereby presenting a hazard to the highway below. The purpose of this research was to investigate the November, 2003 rock slope failure and post-failure motion, and use the results of the investigation to assess the location, volume, and effects of future slope failures at Afternoon Creek. A Total Slope Analysis methodology was followed, which linked the failure initiation analysis with runout analysis. The structural and topographic controls to failure were analyzed with limit equilibrium, and the 2-D and 3-D distinct element codes, UDEC and 3DEC. Runout was analyzed with the 3- D dynamic analysis code, DAN3D. A data collection methodology designed specifically for numerical modeling was implemented. A laser scanner and field survey were used to collect discontinuity characterization data at the inaccessible, hazardous site. The analysis suggested that the past event involved a single-stage, extremely-rapid translational failure on a highly-persistent, moderately-dipping joint set. Material that entered Falls Creek was most likely caused by secondary rock fall immediately following the slide. A future rock slope failure up to 300,000 m³ emanating from the existing failure scarp is possible; however runout debris from an event of this volume is not expected to reach the highway at Afternoon Creek. In contrast, retrogression of the failure scarp crest by sliding or toppling of individual columns may cause rock fall to enter the steeper Falls Creek travel path where it will most likely impact the highway embayment.

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