Mountain Tunnel: Repairing a 100-year-old Water Supply Tunnel

Project Update

The San Francisco Public Utilities Commission (SFPUC) supplies drinking water to 2.7 million residents in San Francisco and four Bay Area counties. Water is collected from the Hetch Hetchy Watershed in Yosemite National Park and travels through the Hetch Hetchy Regional Water System, a series of tunnels and pipelines constructed in the 1920s. As part of that system, the 18.9-mile (30.4 km) Mountain Tunnel extends between the Kirkwood powerhouse to the east and Groveland to the west in Tuolumne County, CA.

Delve Underground began assessment of the Mountain Tunnel in 2016. Following condition surveys, it was determined that the tunnel was in serviceable condition and repairable, despite numerous lining defects. With the goal of addressing water delivery reliability, design alternatives were developed and after analysis, a final scope was determined. The design phase continued until October 2019, when the 100% plans and specifications were finalized.

The approved design includes work to be performed during planned tunnel outages and work that could be performed during non-outage periods. The non-outage work consists of a new 650-foot (198 m) siphon extension for improved water quality through a fractured rock zone, a new 1,000-foot-long (305 m) access adit and bulkhead doorway to accommodate equipment entry, significant mountain roadway improvements, and a new large Flow Control Facility (FCF) and associated bypass tunnels with 72-inch (1,830 mm) and 107-inch-diameter (2,720 mm) steel piping at the downstream end of the tunnel. The new FCF facility will contain 72-inch flow control valves to address concerns that liner has been exacerbated by turbulence and surge associated with fluctuations between full and open-channel flow. The facility is housed in a new 162-foot-deep (49 m), 55-foot-diameter (17 m) shaft built offline as a bypass to the tunnel. The work during outages consists of tunnel lining repairs via patching and continuous contact grouting along the 12 miles (19 km) of lined tunnel and tie-ins at the siphon extension and FCF.

After completing the design, Delve assisted the SFPUC through two rounds of bidding from November 2019 through August 2020. Following the award of the construction contact to Michels Construction, a Notice to Proceed was issued on January 29, 2021. Since that time Delve has provided Engineering Services During Construction (ESDC) services and assisted the Construction Management team on-site through the construction phase. Within the past two years much progress has been made towards project completion.

Site preparations and the drill-and-blast excavation of the FCF Shaft, initial bypass tunnels, and new adit were completed by August 2022. Subsequently, steel piping was installed through the shaft and bypass tunnels and the bulkhead door was installed in the adit prior to a planned tunnel outage from January to March 2023. During this tunnel shutdown, tie-ins to the tunnel were made at the bypass tunnels and adit, additional piping was installed in the bypass tunnel, the bypassed section of tunnel was backfilled, a new rock-trap was installed, and some repairs to the existing tunnel lining were made. During future outages, more tunnel repairs will be made, contact grouting will be performed, the siphon work will be completed, and control valves will be installed in the FCF.

During future non-outage periods, a final concrete lining and surface building will be installed at the FCF, the final lining of the adit will be placed, and roadway improvement work will continue. Final completion is scheduled for 2026–2027.