Lenihan Dam Outlet Modifications

Jacobs Associates provided tunnel design and construction support services for a new outlet tunnel under the abutment of the existing Lenihan Dam. The modification provides the Santa Clara Valley Water District with an improved system to regulate flow through the dam to Los Gatos Creek, while meeting operational requirements of the California Department of Water Resources, Division of Safety of Dams.

The project involves installation of a 4.6-foot-diameter (1.4 m) steel pipe within a 13-foot (4 m) cast-in-place concrete horseshoe tunnel, connecting a drop shaft and inlet works with a downstream outlet structure housing energy dissipation valves. The seismically active San Andreas fault, approximately 1.3 miles (2.1 km) from the site, is a known source of severe ground shaking.

The tunnel cuts through Franciscan mélange, consisting of greywacke sandstone, serpentinite, and greenstone embedded in a weak clayey shale matrix (mélange). Such a contrast in rock composition resulted in a multifaceted excavation approach. A tunnel boring machine (TBM) was not economical for excavation of the 1,968-foot-long (600 m) tunnel. Therefore, a roadheader was used in conjunction with drill-and-blast methods to excavate through the more massive, stronger rock. Mining through the shale matrix required monitoring and control of ground movement to maintain tunnel stability. Weak matrix zones were initially stabilized by spiling and steel ribs with the later application of shotcrete lagging.

A five-level, multiport sloping intake provides variable inlet control based on reservoir operations and future sedimentation projections. The intake gates and tunnel are connected via a drop shaft constructed during a reservoir drawdown. Construction was completed in the fall of 2009.