Best Water/Environment Project, Northern California: Folsom Dam - Mormon Island Auxiliary Dam Key-Block Construction Project, El Dorado Hills
Built in 1956 by the U.S. Army Corps of Engineers, the 105-ft-tall, earthen-saddle Mormon Island Auxiliary Dam had a foundation that was deemed at high risk of liquefaction during an earthquake. As a remedy, this $43.6-million project included full-depth removal of up to 72 ft of dredged alluvium and excavation into 8 ft of the underlying amphibolite schist bedrock in a 900-ft-long, 55-ft-wide excavation constructed in 150-ft-long segments.