The Mormon Island Dam is the largest zone fill earthen saddle dam at the Folsom Dam complex. Although the dam was founded on competent bedrock, the downstream toe was resting on dredged alluvium up to 72 feet deep. Further exacerbating the poor foundation was the regional groundwater level, which at times was just a few feet below the downstream ground surface. The scope of this project included constructing a 900 feet long by 55 feet wide by 40 feet deep lean concrete key block anchored 8 feet into the amphibolite schist bedrock. The concrete key block was then backfilled with select backfill up to the existing downstream grade.
To reach the final foundation depths of up to 80 feet, Shimmick constructed a design-build deep shoring system utilizing over 1,000 reinforced secant piles with up to 6 levels of internal bracing. The system was designed to withstand the full liquid head of groundwater, soil pressures, the weight of the existing dam and reservoir, anticipated earthquake loads. In addition to the design and construction of the deep shoring, the project involved 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.
This project won the 2013 ENR Award for Best Project.