Mountain Tunnel Inspection Services

The Hetch Hetchy Water System provides approximately 260 million gallons (984 thousand liters) per day of water to 2.4 million people in San Francisco, Santa Clara, Alameda, and San Mateo counties. Sierra Nevada snowmelt, stored within a reservoir in Yosemite National Park, supplies 85 percent of this water. It then flows 160 miles (257.5 km) by gravity through a series of tunnels and pipelines to the Bay Area.

Heightened awareness following the 1989 Loma Prieta earthquake and drought conditions in the 1990s led the San Francisco Public Utilities Commission (SFPUC) to undertake an upgrade and rehabilitation program for the entire water system. In early 2006, concerned about a key asset of the aging Hetch Hetchy System, the SFPUC commissioned an inspection of Mountain Tunnel, built in 1925 (and completed in the 1930s). This 19-mile (30.6 km) tunnel carries raw water from the Early Intake Reservoir, Kirkwood Powerhouse, to Priest Reservoir. The tunnel, over 62 percent lined, is mostly horseshoe shaped, with dimensions varying from 10 (3 m) to 15 (4.6 m) feet.

The SFPUC could only take the Mountain Tunnel out of service for short intervals. This required a well-coordinated inspection effort over a three-day period. Jacobs Associates provided reconnaissance-level inspection services that documented rockfall quantities and defects in the lined sections. The inspection teams also included several key members of the SFPUC’s operations, maintenance, and management staff. The collected information will assist in the preparation of repair schemes over planned outages, which ultimately will lead to the preparation of the contract documents for the tunnel repair.