The 3.5-mile-long, 18-foot diameter water transmission tunnel is parallel to the existing Hetch-Hetchy Aqueduct in Fremont, California. The tunnel provides a seismically-designed connection between water supplies from the Sierra Nevada Mountains and the Alameda Watershed to Bay Area water distribution systems.

Construction excavation occurred between 30 and 700 feet underground, utilizing conventional mining methods including mechanical or road-header excavation, and controlled detonation where needed for hard rock removal. Extensive dewatering and tunnel reinforcement were used for groundwater inflows and unstable ground along the tunnel alignment.