For the past several months, Valley Water construction crews drilled 244 holes deep into the hillside next to Anderson Dam in Morgan Hill. Inside each hole, workers placed a 120-foot-long bar, up to 2-1/4 inches in diameter. The steel bars, each weighing about 1,600 pounds, were covered with 12-inches of concrete.
This work to reinforce roughly 15,000 square feet of hillside was a milestone in an overall effort to build a new outlet tunnel at Anderson Dam. This new tunnel will allow Valley Water to draw down the reservoir reliably and quickly, providing greater control over the water levels in the reservoir and increasing public safety.
“This tunnel is a major part of getting Anderson Dam back online as a stronger facility so it can protect public safety and again serve as Santa Clara County’s largest drinking water reservoir,” said Valley Water Chair Pro Tem John L. Varela, who represents South County.
Later this summer, Valley Water will begin digging a tunnel reaching 24.5′ in diameter through the 12-inch concrete layer and hillside. Once the tunnel is complete, Valley Water will be able to release water from Anderson Reservoir five times more quickly than previously and 12 times more quickly once the whole project is finished.
“We needed to make sure the hill can withstand the tunneling process,” Valley Water Deputy Operating Officer Chris Hakes said. “The strength of the steel bars will hold the hill in place.”
The existing outlet pipe runs underneath the dam and crosses fault lines. This pipe would be unable to withstand movement caused by a large earthquake and must be replaced. The new outlet tunnel replacing the existing old pipe is designed to withstand an earthquake.
Once the tunnel is complete, Valley Water will begin retrofitting the dam embankment and spillway. That project is estimated to start in 2025 and will last about seven years. Once completed, Valley Water will again be able to fill Anderson Reservoir to capacity.
Rebuilding Anderson Dam to ensure public safety is Valley Water’s highest priority. We are working with numerous state and federal agencies to obtain the necessary permits to move forward with the project as quickly as possible.
For more information about our work at Anderson Dam, visit the project webpage at https://www.valleywater.org/project-updates/c1-anderson-dam-seismic-retrofit.