The San Francisco Public Utilities Commission (SFPUC) announced March 4 that it has completed construction on the $340 Million New Irvington Tunnel Project, a 3.5 mile-long seismically-improved tunnel to deliver water to 2.6 million people in the San Francisco Bay Area.

The project which began in August of 2010, completes the last of three new tunnels creating a "water lifeline able to withstand earthquakes on the Hayward, Calaveras, and San Andreas faults," says the SFPUC. Located between the Sunol Valley and Fremont, the tunnel is part of the agency’s $4.8 billion Water System Improvement Program (WSIP)

“The seismically-resilient New Irvington Tunnel is a project that strengthens the backbone of the Hetch Hetchy Regional Water System,” said SFPUC General Manager Harlan L. Kelly, Jr. in a recent press release.

SFPUC says it has not been able to take the tunnel out of service to inspect it since 1966 due to customer water demands. The tunnel lies between the Calaveras Fault to the East and the Hayward fault to the West and transmits the majority of the drinking water the SFPUC provides its customers.

The tunnel was designed by URS (AECOM) and Jacobs Associates, with Hatch Mott MacDonald serving as construction manager and the Southland/Tutor Perini Joint Venture serving as contractor.

The 8.5 ft diameter New Irvington Tunnel was constructed parallel to the existing Irvington Tunnel, which was completed in 1932. A secondary tunnel allows the SFPUC to take either tunnel out of service for maintenance and inspections. The goal of the project is to restore water deliveries within 24 hours after a major earthquake in the Bay Area.

Project officials say the job has not been without its challenges. With the anticipation of complex geology, the new tunnel was constructed using conventional mining techniques rather than a tunnel boring machine. The crews mined from four separate headings using road headers and controlled detonation. The miners experienced frequent ground class changes and extreme ranges of rock hardness during construction.

The excavated tunnel was lined with steel pipe in 50 ft sections. The space between the steel pipe and the excavated tunnel wall was filled with cement grout. During the final stages, the interior of the steel liner was lined with a cement mortar lining to protect the liner from corrosion.

In addition, heavy groundwater inflows and “squeezing” ground needed to be addressed along the way. In 2011, CalOSHA reclassified the tunnel as “gassy” requiring more rigorous measures to monitor for explosive or toxic gas, and enhanced protective measures for worker safety.

Tunneling began in March 2011 and crews achieved their first "hole through," where the two tunneling teams met underground, on June 12, 2012. The second hole through on October 8, 2013 marked the completion of tunnel excavation. Currently, the Water System Improvement Program, which is comprised of 83 projects, is more than 85% complete.

In the coming weeks, crews will take the existing tunnel out of service for inspection. Meanwhile, the project team will work to restore above ground facilities around the new tunnel. This above ground work will last through fall of 2015.