Project Details

Owner: U.S. Department of the Interior, Bureau of Reclamation

Contract value: $35,469,300


The Mormon Island Dam is the largest zone fill earthen saddle dam at the Folsom Dam complex. Although the dam is founded on competent bedrock, the downstream toe rests on dredged alluvium up to 72 ft deep. Further exacerbating the poor foundation is the regional groundwater level which at times is just a few feet below the downstream ground surface. The scope of the current project includes constructing a 900 ft long by 55 ft wide by 40 ft deep lean concrete key block anchored 8 ft into the amphibolite schist bedrock. The concrete key block is then backfilled with select backfill up to the existing downstream grade. In order to reach the final foundation depths of up to 80 ft, Shimmick is constructing a design-build deep shoring system utilizing over 1,000 reinforced secant piles with up to 6 levels of internal bracing. The system is designed to withstand the full liquid head of groundwater, soil pressures, the weight of the existing dam and reservoir, and anticipated earthquake loads. In addition to the design and construction of the deep shoring, the project involves the excavation of 120,000 cubic yards of dredged alluvium and bedrock, processing of 60,000 cubic yards of concrete aggregate, onsite batching of 60,000 cubic yards of concrete, and placement of 60,000 cubic yards of compacted select backfill.