Building Lenihan Dam Outlet Modifications
At the foot of the Santa Cruz Mountains, the deteriorating outlet structure for the 50-year-old Lenihan Dam—a 1,000-foot-long earthen barrier holding water stored at the Lexington Reservoir in Los Gatos, CA was replaced. The project is a seismic upgrade from a 50-inch steel pipe that was restricted to a maximum outlet of 70 percent capacity to a 54-inch pipeline that allows maximum outflow of the reservoir in the event the reservoir needs to be lowered to prevent the failure of the dam during an earthquake.
The bid-build project included construction of a 2,000-foot-long outlet structure approximately 14-feet wide by 13-feet tall through Saint Joseph Hill. The tunnel begins near the existing outfall pipe, terminates on the reservoir side of the dam and connects to a 15-foot-diameter intake facility by way of a 35-foot vertical shaft.
The new outfall building has three sets of valves to control the discharges into Los Gatos Creek. The new intake structure consists of 450 feet of 54-inch welded steel pipe, with four 42-inch intake gates and a new building to house the hydraulic control unit and various monitoring equipment. During construction, a road header was used to bore through soft material and explosives were used to fracture harder rock. After excavating the material, an arched traveling form was used to pour 60-foot-long sections of the tunnel. Ventilation and low-flow pipes were hung from the tunnel ceiling and a 5-foot welded steel outtake pipe was installed with permanent cast-in-place supports.
Originally, the shaft required excavation and grout around its base, which was very time consuming and costly. An innovative water-tight pile secant wall was proposed for the shaft excavation, allowing the project to be built quickly, reducing cost and decreasing environmental disturbances by keeping grout from entering the reservoir. The value engineering solution additionally improved water-tightness around the shaft. During construction, a detour for the highly used Los Gatos trail was created to ensure safe public recreation. The project was completed nine days early and $1.4 million under budget.
Construction of a new 2,000-foot-long outlet structure
A new 15-foot-diameter intake facility connected by a 35-foot vertical shaft
2,100 feet of new 54-inch steel outfall pipe