RUBY CREEK ROCK SLOPE STABILIZATION

Located on State Route 97 near Blewett Pass in the Cascade Mountains of Washington State, the Ruby Creek Site consists of a 200-ft high rock cut originally constructed in 1959. The complex geologic setting for the project site includes slates or argillites, tectonically emplaced greenstone blocks and basalt. As part of its Unstable Slope Management Program, the Washington State Department of Transportation had rated this site as a high priority based upon the hazard to the highway and the potential economic impacts if a slope failure were to occur. The significant hazards included a 5,000-cu yd mass of rock susceptible to a planar mode of failure, an 11,500-cu yd rock mass susceptible to a wedge mode of failure and numerous small-scale features contributing to rockfall. Site investigations included aerial reconnaissance, detailed surveying of the location of major fault structures, joint population mapping including multiple traverses down the slope face on ropes, shear strength testing of fault gouge material and testing for intact rock strength. Stereographic analyses and computerized stability analyses led to the conclusions that the planar mass should be removed by controlled blasting and that the wedge mass should be stabilized through reinforcement. Design criteria and specifications were provided for blasting and scaling, high capacity rock bolts, low capacity rock bolts, untensioned dowels and shear pens. Shotcrete and extensive application of cable netting were recommended to control ongoing rockfall. The blasting portion of the project was subject to stringent timing constraints to accommodate summer holiday traffic and to safeguard biological resources in the area. These constraints required the consideration of multiple blasting strategies to remove the 5,000-cu yd planar block. The adopted plan consisted of a single shot drilled from the top of the block. The results of the blast provided valuable lessons and specific data on the issues of access for drilling, hole deviations in 160-ft long angled blast holes, muck pile run out, flyrock trajectories, blast cleanup methodology, and traffic interruption. The scaling, bolting and cable net installations also yielded practical information on the challenges of converting design intention to construction reality.