Large scale excavations have officially begun at the Mountain Tunnel Improvements Project. The tell-tale sign of this milestone was the appearance of the huge crane near Priest Reservoir. The specialty Liebherr crane was manufactured in Austria and carefully transported to the remote site in pieces. The crane is so large it had to be broken into 19 separate pieces and transported on 15 separate trucks to the site.

That huge crane will be in constant motion over the next couple of years. It will help excavate the 150-foot deep shaft where the tunnel’s flow control facility will be located. In addition to loads of dirt and rock, the crane will also transport all materials, supplies, and personnel into and out of the shaft during construction. When completed the flow control facility will assist in the controlling pressure inside the tunnel, help protect the tunnel lining and improve operational flexibility.

Mountain Tunnel is an approximately 19-mile-long water tunnel that has been serving the Bay Area since 1925 as part of the Hetch Hetchy Regional Water System. It transmits drinking water originating from Hetch Hetchy Reservoir from Kirkwood Powerhouse, where it generates hydropower, to Priest Reservoir downstream. Water flows entirely by gravity through this tunnel.

Constructed between 1917 and 1925, this tunnel has been in service for more than 90 years and in need of an upgrade. The Mountain Tunnel Improvements Project will rehabilitate the tunnel and make improvements to extend its useful life for the next 100 years. The project will repair damaged tunnel lining, install concrete flooring in a portion of the tunnel to improve hydraulic flow, remove debris inside the tunnel to improve flow, seal up the tunnel from groundwater intrusion, construct a new access tunnel (adit) to the existing tunnel, install a new section of tunnel (siphon extension) to reduce water infiltration, and install a new flow control valve inside the flow control facility shaft to moderate the flow of water inside the tunnel.

The project is part of a larger Hetchy Capital Improvement Program (HCIP), a multi-year capital program to upgrade or improve water conveyance, water storage, and power generation facilities in the Sierra Nevada portion of the Hetch Hetchy Regional Water System.

Construction began at the end of January 2021. The contractor has been busy making roadway improvements and preparing the sites for large scale construction. Completion of the Mountain Tunnel Improvements Project is expected at the end of 2026.