Lenihan Dam Outlet Tunnel Celebrates Completion
The Lenihan Dam Outlet Modifications Project celebrated project completion in October 2009 with a “first flow” commissioning ceremony. The new outlet system had actually been in use for over a month at the time of the ceremony, but project completion could not be declared until the old outlet works were abandoned. On hand to celebrate the project completion were project team members, the Santa Clara Valley Water District board members, local news media, and a fifth grade class from a nearby elementary school. The event’s activities included speeches from board members, a presentation from the students, and the opening of the valves, permitting the near maximum flow of 360 cubic feet per second (10.19 m3/sec).
Jacobs Associates provided tunnel design and construction support services for the 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. Construction of the project began in September of 2007, and consisted of the installation of a 4.6-foot-diameter (1.4 m) steel pipe within a 13-foot (4 m) cast-in-place concrete horseshoe tunnel. 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. Outlet works consist of a downstream structure housing isolation and outlet control valves, and energy dissipation devices.
Completion of the project required that the existing steel-lined outlet pipe running underneath the dam be entirely filled with grout and cellular concrete to prevent future collapse and to stop the seepage of water. All access to the 1,400-foot-long (427 m), 50-inch-diameter (1,270 mm) pipe was through a 12-foot-diameter (3.7 m) vault at the downstream end of the pipe. Approximately 950 feet (290 m) of the pipe was backfilled with a low-strength cellular grout in four reaches. The remaining 450 feet (137 m) of pipe, in two reaches, was filled with a neat cement grout and later contact grouted to ensure all voids were completely filled. The contractor faced numerous challenges, including pumping grout over long distances, constructing strong and leak-proof bulkheads, and working in a very tight space. However, after approximately two months, the outlet pipe was completely filled and leak free.
Construction was completed on time and below the contractor’s bid amount of $39 million. The project was recently selected as the San Francisco ASCE Section’s 2009 Outstanding Project in the Large Project category.