Identified by the project owner was a 45-ton rock precariously situated on a steep slope directly above the South Yuba Pipe, near Emigrant Gap, CA. The stability characteristics of the rock were largely unknown, but conditions observed from the pipe indicated that the rock had previously moved up to 16 inches downslope. Due to the potential disruption to the municipal water supply for portions of Nevada County, and a loss of hydro generation, the client proceeded with a phased approach to address the hazard by restraining the rock in place.
Members of the SAGE Rope Access Team performed a site reconnaissance at the start of the project, using rope access techniques to safely access the area above the pipe. The goals of the reconnaissance were to collect information for geologic/kinematic analyses of the hazard rock, and to evaluate potential mitigation measures. In addition, a downslope reconnaissance was performed to collect information about the pipe, pipe saddles, and saddle foundations. The results of the reconnaissance and preliminary analyses indicated that the rock could likely be restrained in place if proper methods were used to limit the risk of dislodging the rock during construction.
SAGE’s services included design and layout of a restraint system comprised of three 1-1/8-inch-diameter support ropes fitted across the downhill face of the rock to prevent downslope movement, and cable net rockfall netting attached to the support ropes with lacing wire to create a panel over the face of the hazard rock to increase the contact area of the restraint system. Members of the SAGE Rope Access Team performed on-slope layout of a mock system to ensure proper fitment when installed by the contractor. As a result of coordination/planning with the client and contractor, the restraint system was successfully and safely installed without dislodging the hazard rock. Over the course of construction, the SAGE Rope Access Team provided part time observation of drilling/grouting of anchors, installation of support ropes, and final adjustments of the system.