New Irvington Tunnel

OWNER: San Francisco Public Utilities Commission

ENGINEER: URS Corporation

SOUTHLAND HOLDINGS TEAM: Southland Contracting Inc.

PROJECT HIGHLIGHTS:

This project received the 2016 Project of the Year Award in the Environmental Category for projects over $75-million.

When San Francisco needed to replace the entire area’s critical clean water supply, built in 1932, to meet new earthquake-resistance standards, they turned to us. The tunnel passed seven active fault zones and encountered high-pressure ground water bearing explosive and noxious gases. Extensive dewatering, both surface and underground, was required to draw down the local water table. Due to the depth of the tunnel and the lack of surface access, tunnel excavations were sealed off by cementitious and chemical grouts as part of an extensive groundwater-control program. The area’s complex geology made for highly diverse subsurface conditions and surfaces ranging from solid hard rock to soft squeezing and running ground. Additional measures were mandated by the “Gassy Tunnel” designation from Cal OSHA’s Mines and Tunneling Unit. The project crossed environmentally sensitive and urban environments, requiring extensive traffic control as well as sound walls, view screens, wildlife exclusion fencing and other environmental mitigations. In conjunction with the Owner, we conducted extensive public outreach to ensure minimal project impact on the local community.

Located in Sunol, CA, the $255-million New Irvington Tunnel Project conveys water from the Hetch Hetchy Reservoir to the San Francisco area. A 40 ft. diameter, 140 ft. deep temporary shaft along Vargas Road provided construction access. The tunnel was excavated by road header and drill-and-blast methods 30 to 700 ft. below ground. The completed 16.5 ft. ID tunnel stretches for 3.5 miles and is lined with 84″ welded steel pipe. It is grouted in place to withstand a magnitude 7.2 earthquake and comply with earthquake-resistance requirements.