New Irvington Tunnel

Completed in 2015 by Southland Contracting, the New Irvington Tunnel Project consisted of the construction of a 3.5 miles long, 16.5 foot diameter tunnel for water conveyance, replacing the existing section of the Hetch Hetchy Water System built in 1932 and provides the City of San Francisco with a seismically sound water supply, being able to withstand a magnitude 7.2 earthquake. The tunnel excavation passed through seven active fault zones and ranged in depths between of 30 and 700 feet underground. The tunnel excavation encountered high pressure ground water bearing explosive and noxious gasses. Extensive surface and underground dewatering was utilized to draw down the local water table, however due to the depth of the tunnel and lack of surface access, and extensive groundwater control grouting program was utilized to seal off the tunnel excavations utilizing cementitious and chemical grouts. The subsurface conditions on this project were very diverse due to the complex geology of the area, with the ground conditions changing rapidly from solid hard rock to soft squeezing and running ground. The gassy nature of the ground also meant that this tunnel was classified as a “Gassy Tunnel” by CalOSHA’s Mines and Tunneling Unit. These conditions are similar to what is expected on the Regional Connector Transit Corridor. The project transverses environmentally sensitive and urban environments. Extensive use of traffic control and environmental mitigations (such as sound walls, view screens and wildlife exclusion fencing) have been put in place. Above and beyond these measures, Southland in conjunction with the Owner have conducted extensive public outreach to ensure that project impacts on the local community have been minimal.