Camino Penstock Stabilization, El Dorado County, California

After 35 years of chronic slope movements and several engineering studies by others, SMUD contracted with Layne Christensen Company (LCC) as the construction contractor and Harza Engineering Company as the design engineer for an $8 million, design-build stabilization of the Camino Penstock. The penstock, which is 1,500 ft. long and 10.5 to 12 ft. in diameter, culminates in a powerhouse containing two 75 MW turbines. Over the years, SMUD has made continued adjustments to the penstock to accommodate downslope movements, including both toppling and sliding of the foliated metamorphic bedrock. Sections of the penstock have moved downslope about 7-inches since construction in 1961. The selected stabilization system consists of a grouted "gravity structure" along the lower 250 feet of the west side of the penstock, constructed by grouting and doweling the inplace bedrock. The toppling movement will be stabilized by tying this "gravity structure" and bedrock upslope of the "gravity structure" with forty-three 400-kip tendon anchors, spaced approximately 10 feet apart, reaching about 120 feet into the bedrock. The stabilizing forces required to be provided by the tendon anchors were determined using a rock toppling analytical model developed after Goodman and Bray and Ke, Goodman and Thapa. Additionally, three of the ring-girder foundations will receive anchor block support using tendon anchors to stabilize downslope sliding movements.