Moccasin, CA

Mountain Tunnel Improvements

San Francisco Public Utilities Commission (SFPUC)


The Hetch Hetchy Water System delivers an average of 222 million gallons (840 ML) of drinking water per day to 2.6 million people in the San Francisco Bay Area. The Mountain Tunnel, originally built in 1925, carries water from the Early Intake Reservoir to Priest Reservoir. More than 60% of the tunnel is concrete lined, and mostly horseshoe-shaped of varying dimensions from 10 to 15 feet (3.0 to 4.6 m). The tunnel has been in service for nearly a century and is approaching the end of its original design life.

Fast Facts
  • 19 miles (30.6 km) of hard rock tunnel
  • Tunnel at end of 100-year design life
  • Tunnel access improvements

Scope of Work

Delve Underground has led multiple repair projects and participated in several outages over 50 to 65 days associated with improvements to Mountain Tunnel. We completed the design and contract documents for the Mountain Tunnel Improvements Project, managing a team of 16 subconsultants. The improvements include completing the tunnel lining repairs and contact grouting, constructing a new 150-foot-deep (46 m) flow control facility and new Priest access adit, building a new siphon extension for improved water quality, making significant mountain roadway improvements to the adits, and finalizing the adit improvements. Delve Underground is currently providing engineering support services during construction.

Challenges & Innovations

To meet water delivery goals, the Mountain Tunnel must be capable of returning to service within three months if water service is interrupted. To ensure timely access into the tunnel by construction crews and equipment in case of emergency, we developed an Emergency Restoration Plan that addressed preliminary requirements for improving tunnel access through the adits.

Delve Underground performed inspections and condition assessment of the 19-mile-long (30.6 km) tunnel in 2008 and 2017. The SFPUC can only take the Mountain Tunnel out of service for short intervals during the winter months when water demand is lowest. Inspections were performed with highly qualified and experienced staff to accommodate the SFPUC’s shutdown requirements.