Contractors on the centerpiece of San Francisco’s $4.6-billion upgrade of the Hetch Hetchy water system, a $215.3-million tunnel under San Francisco Bay that will replace the 1920s-era pipeline structure, were told to start work on April 1. Officials say the seismically enhanced structure and others planned will better protect the city water supply in an earthquake, such as the Southern California temblor on April 4.

Photo: San Francisco Pulbic Utilities Commission

Image shows underwater pipeline and tunnel, which will have new seismic protections.

The five-mile-long tunnel will replace old structures with a 9-ft-dia welded steel pipeline bored as deep as 100 ft under the bay’s floor. A tunnel-boring machine will minimize environmental impact. Awarded the construction contract in January is the joint venture of Michels Corp., Brownsville, Wis., Jay Dee Contractors Inc., Livonia, Mich., and Frank Coluccio Construction, Seattle, the team now building the latter city’s Sound Transit tunnel.

Bidding was competitive, with three of four teams submitting bids under $225 million, says a city official. A JV of Obayashi Construction and Kenny Construction, which narrowly lost with a $215.4-million bid, had protested, challenging the winner’s “local business enterprise” subcontracting. But the protest was rejected as without merit.

The contractor will begin work this summer boring the 14-ft dia main tunnel, with excavated soil to be used in a marshland restoration project. The project is planned for completion in 2015. Jacobs Engineering Group is project-managing the dig and work on two connecting pipelines, seven and nine miles long, that both came in almost 30% below estimates.

Also part of the water system upgrade are two other tunnels. The joint venture building the $55.7-million, 4,200-ft Crystal Springs Bypass tunnel said last month that excavation was complete. Next month, officials will award a contract for the $338-million, 3.5-mile Irvington tunnel, running parallel to an existing tunnel between the Sunol Valley and Fremont, Calif. It will provide a seismically designed link from water supplies in the Sierra Nevada Mountains and the Alameda Watershed to the Bay Area. In all, the upgrade will involve 34 projects and more than 100 miles of pipelines, water treatment plants and dam improvements.

Timing is very important, says Tasso Mavroudis, senior project manager for the San Francisco Public Utilities Commission, noting key milestones to be met for needed demolition projects. “The expansion of the stilling basin is the biggest challenge,” he says. “It is a very tight deadline for environmental and operational reasons.” Mavroudis notes that any water spilling into the basin before it is complete could undermine the dam’s integrity. He anticipates the need for multiple construction shifts to move more than 22,000 cu yd of soil as well as install 11,000 cu yd of concrete and a 10-ft compacted riprap bed by Nov. 15.