Cherry Tunnel Rock Trap Rehabilitation
Tuolumne County, California
Cherry Tunnel serves as both a power generation and water delivery tunnel. In order to clean out the unlined section of the tunnel, this effort involved a planned power outage during a period of low demand.
Work consisted of removing rock and gravel from the existing rock trap (and restoring to its original dimensions) in order to keep debris from the turbines at the powerhouse. The rock trap was approximately 8,000 LF upstream from the penstock.
Mitchell pulled out the existing rollout section of the rock trap, built a ramp into the trap from the penstock, and removed approximately 2,000 yards of rocks and debris from the rock trap using four 2-yard and 2.5-yard muckers.
The rollout section was then replaced.
Work was finished ahead of schedule, so Mitchell continued up the tunnel, removing more rocks that were obstructing water flow. Upon completion, this work improved power production from the powerhouse by 10 percent.
Interestingly, though built in the 1950’s, this tunnel had received no maintenance until Mitchell Engineering began its work.
Owner: San Francisco Public Utilities (Hetch Hetchy)
