In the late 1920s and early 1930s, miners used drill-and-blast techniques to construct the original 3.5-mile Irvington Tunnel between Sunol and Fremont, California. A vital part of San Francisco’s Hetch Hetchy Regional water system, the tunnel supplies 85% of the City of San Francisco’s water as well as supplying other clients in the Bay Area.
But that water supply is under threat. According to Julie Labonte, director of the City’s $4.6 billion Water System Improvement Program, “Although the Hetch Hetchy water system is truly an engineering marvel, some critical elements are seismically vulnerable. It’s not a matter of if, but when a major earthquake strikes — we are truly in a race against time.”
Because the transport of water from through the tunnel to the Bay area is critically needed, the tunnel has not been dewatered and taken out of service for a physical inspection since 1966.
A new tunnel was needed to provide redundancy, but gassy ground conditions, groundwater inflows, and seismic zones with squeezing ground made the $250 million New Irvington Tunnel very challenging. “The major construction challenges will be due to groundwater,” said David Tsztoo of San Francisco Public Utilities (SFPUC) in 2011. “We have to mine into zones where we will encounter 300 static feet of groundwater.” A pre-excavation grouting program was used throughout the tunnel excavation, with over 7.8 million pounds of cement injected into the ground to reduce the groundwater inflow before the ground could be excavated.
The New Irvington Tunnel was designed to parallel the original tunnel. It will be approximately 3.5 miles long, with a horseshoe shape and excavated dimensions of approximately 12 by 14 feet and a final finished inner diameter of 8.5 feet with a steel liner.
Mott MacDonald was retained as the lead consultant construction manager for the construction phase of this project. We assisted the SFPUC in reviewing contractor prequalifications and constructability for the bidding documents, and helped prepare the final bidding documents. We were also responsible for:
- Field quality assurance inspection and testing
- Environmental compliance monitoring and biological resource specialist monitoring
- Field contract administration
- Estimating, scheduling, change order, and claims management
- Tie-in and disinfection work coordination
- Groundwater management plan including preconstruction mitigation measures
The new tunnel was excavated using roadheader and drill-and-blast methods. Because of the number of fault zones, a tunnel boring machine would have run the risk of getting stuck underground in the squeezing ground normally associated with these fault zones. “The tunnel design engineers felt that there were seven zones, so seven possible chances to get stuck,” said Tsztoo.
North American Tunneling Journal reported that the project needed “old-school miners who could adapt quickly to changing ground conditions.” According to Dan McMaster, our construction manager, “There have been days when we have started with drill and blast, and by that afternoon we have been into softer Class III and Class IV ground.”
In June 2012, the New Irvington Tunnel achieved its first hole-through, with teams from the Irvington Portal in Fremont and the Vargas Shaft meeting underground. “Prior to the hole-through,” reported Underground Construction, “the crews stopped within nine feet of each other. The road header from Vargas then advanced until its spinning, titanium-studded knuckle broke through to the other side. Miners then made their way around their machines to shake hands and congratulate each other.”
On October 8, 2013, the contractor holed through the remaining 14,400 feet between Alameda West Portal and the Vargas Shaft: a monumental occasion for the City and the contractor’s very determined tunneling crews.
This completed the excavation of the entire 3.5 miles of new tunnel by conventional tunneling using roadheaders and drill-and-blast methods. Conventional mining techniques were successfully and safely used to excavate the 3.5 mile New Irvington Tunnel with a lower recordable incident rate than the industry average.
“It was a real team effort as well as having multiple tools in the tool box to use to excavate and deal with the highly variable ground and groundwater that was encountered,” said Daniel McMaster.
Next, workers would clean up the tunnel invert and install panning to divert groundwater inflows into a collection of drainage pipes. Welded steel pipe with a diameter of 103 inches would be installed in 50-foot lengths, and low-density cellular concrete would be pumped in lifts to fill the annulus between the pipe and the tunnel walls. Contact grouting and mortar lining of the pipe would be completed before the tunnel was connected to outside pipe work and finally commissioned.
The 3.5-mile New Irvington Tunnel will carry drinking water to 2.6 million customers in San Francisco, San Mateo, Santa Clara, and Alameda Counties. Construction was substantially completed in September 2015.
Once the new tunnel is commissioned and in operation, the San Francisco Public Utilities Commission will have the opportunity to drain the old tunnel in order to inspect its conditions and carry out emergency repairs.