This past winter we completed the following two projects to ensure continued operation of the Mountain Tunnel in the Sierra foothills. The tunnel has transported water from Kirkwood Powerhouse to Priest Reservoir since 1925, and is a critical link in our Hetch Hetchy Regional Water System.
|The original Mountain Tunnel construction began in 1918.
||The Tunnel underwent a thorough inspection in early 2017.
Mountain Tunnel Adits and Access Improvements
To meet water delivery goals, the Mountain Tunnel must be capable of returning to service within three months in the event that water service is interrupted. Since quick entry into the tunnel by construction crews and equipment would be essential, we widened and improved certain adits (connecting entry tunnels) and access roads to minimize the time required to return the tunnel to service. An emergency restoration plan is now in place, and we installed new instruments to enhance the existing monitoring system that continually checks for changed conditions in the tunnel.
Mountain Tunnel Inspection and Repairs
In early 2017, we made a thorough inspection of the 19-mile tunnel to update a previous 2008 condition assessment, and our team repaired approximately 7500 linear feet of concrete lining containing large and deep defects that required urgent repair.
In early 2019 another winter shutdown of the tunnel is scheduled to complete other urgent repairs that time did not permit during the last shutdown. The urgent lining repairs are necessary to keep the most significant defect from continuing to worsen and possibly precipitate a lining collapse that would compromise flow and water quality in the tunnel before long-term Mountain Tunnel Improvements are completed in 2026.
Mountain Tunnel Long-Term Improvements
Planning for long-term improvements has been completed, and a decision was made in Summer 2017 to forego a new bypass tunnel and make permanent lining repairs to the existing tunnel instead. The 2017 inspection and condition assessment findings determined that the existing tunnel is still in sound structural condition, despite the numerous lining defects. The City's selection of the Rehabilitate Alternative as the Preferred Project for implementation affirms the feasibility of simply repairing the existing tunnel for another 100 years of service life. Project design and environmental review has commenced, and both phases are anticipated to be completed at the end of 2019. Construction is scheduled for 2020 through 2026.