Residents in the area of Anderson Dam over the next few weeks may hear loud warning horns and explosive sounds as crews continue to excavate a tunnel under construction for the dam’s seismic retrofit project, according to Valley Water.
Water district staff say the impact to residents and passing traffic should be minimal.
Starting on Sept. 12, construction crews will begin the controlled blasting of hard rock for the Anderson Dam Tunnel Project. Scheduled detonations over the next few weeks will take place Monday through Friday, and possibly on Saturdays, from 8am-7pm, Valley Water spokesperson Matt Keller said.
Residents within several hundred feet of the tunnel entrance site, which is located off Cochrane Road at the base of Anderson Dam, may hear a series of short and long warning alarms before and after each controlled blast.
“These safety precautions are in place to protect construction crews and are similar to measures employed in residential and commercial areas for comparable public works projects,” Keller said in a statement. “Trained personnel will oversee the detonations, ensuring the utmost safety with noise and vibration monitoring, storage, handling restrictions and additional precautionary measures.”
The Anderson Dam Tunnel Project is part of Valley Water’s extensive Anderson Dam Seismic Retrofit project. The 20-foot diameter tunnel will be about 1,700 feet long when completed, running through the bottom of the dam into the floor of Anderson Reservoir.
The tunnel is designed to allow the water district to divert large volumes of water out of the reservoir when full or reaching capacity, and give the district greater control over the water body’s levels.
Construction on the tunnel started in May 2021. As of Sept. 11, crews had excavated about 775 feet of the tunnel, Keller said.
Soon after the tunnel is completed, crews will proceed to the next phase of the retrofit project—construction of a new dam embankment. That phase will be under construction until 2032, and Anderson Reservoir will remain empty until the project is complete.
Valley Water has been working on the retrofit project since shortly after state authorities in 2009 determined that Anderson Dam would not withstand a major earthquake, and the crest of the dam could slump in such an event—leaving Morgan Hill underwater within minutes.
Anderson Reservoir is Valley Water’s largest reservoir, with a capacity of 89,278 acre feet.
Valley Water will continue to update residents about activity on the seismic retrofit project on its social media pages, including on NextDoor, Facebook and the district’s website at valleywater.org. Updates will also be posted to the City of Morgan Hill’s social media channels.