A Challenging Emergency Rockfall Project Along The North Cascades Highway, Washington

摘要

In December, 2014 the WSDOT Geotechnical Office was requested to inspect a rock slide that occurred at a previously known unstable slope location along State Route 20 near mile post 142.7, in the North Cascades, Washington. Fortunately, this slope was located along a section of the highway that was closed for the winter. Our initial on-highway review indicated that approximately 7500 yds~3 of material failed from the slope and came to rest on the highway. Our on-slope inspection revealed that approximately 11,000 yds~3 of highly distressed phyllitic rock remained on the slope, bound upslope by a +5-ft-wide tension crack. The tension crack was located approximately 100 feet above the highway and was about 70 feet long. The detached rock mass was perched on an irregular set of joints dipping adversely out of the slope at roughly 50°. There were indications that the rock mass had moved since the initial failure, or was still moving. Through repeated ground-based LiDAR scans over several months, several feet of additional movement was measured without the occurrence of a catastrophic failure. This emergency mitigation project had significant issues to overcome that included safety and environmental concerns, contracting issues, weather, and pressure to open the highway early due to a mild winter. The slope remediation included conventional surveying and routine ground-based LiDAR scanning to monitor for slope deformation, and the use of a long-reach excavator, slope scaling, surface and trim blasting, debris removal, and slope reinforcement to stabilize the slope.