Completed in 1955, Folsom Dam is located on the American River in northern California, approximately 40 km northeast of the city of Sacramento. The 100 m (340 ft) high and 430 m (1,400 ft) long dam creates the Folsom Reservoir that has a maximum possible water area of 4,830 ha (11,930 acres). The Folsom Power Plant has three Francis Turbines with a total output of 198.72 MW.
In order to significantly lower the probability of floods and to double the amount of water the dam can hold in the event of flooding, the U.S. Army Corps of Engineers and the Central Valley Flood Protection Board cooperated with the state of California and the Sacramento Area Flood Control Agency to undertake the "Joint Federal Project" (JFP).
New spillway with six submerged tainter gates
The JFP includes a new spillway with six submerged tainter gates located in a new control structure. This structure is an additional dam that prevents water from entering the spillway when the gates are closed. The project consists of five phases.
In phase IV of the construction, a 335.3 m (1,100 ft) long approach channel is being built for the new spillway on the reservoir side. Essentially, this includes a second dam, the 922.6 m (3,027 ft) long spillway chute and the stilling basin on the downstream side. The stilling basin works as an energy dissipater to slow the water as it enters the American River. In this phase, the soil for the additional dam and part of the spillway chute is excavated, and the foundation of the control structure is built.
For this purpose, DSI Construction USA supplied a total of 316 rock anchors consisting of 168 t of galvanized tendons that were installed in the approach slab of the spillway chute. Furthermore, DSI supplied tendons to extend the 30 rock anchors that were installed in phase III as well as the complete stressing equipment and a grease pump. The technical challenge was to develop four different customized anchor geometries to extend just 30 anchors.