As anyone knows who has dug a hole at the beach or even the backyard, one of the challenges is preventing the surrounding earth from falling back in. Imagine the dilemma with a hole that’s 65 feet deep, half a city block wide and three city blocks long.[contextly_sidebar id=”e08c6142ac56ff9a2e7545bff6f1615f”]It is clearly a singular political achievement to raise $4.2 billion in public financing, even for a multi-generational infrastructure project backed by powerful Washington and Sacramento sponsors. But that accomplishment pales when you begin to grasp the engineering talent needed to build San Francisco’s new Transbay Transit Center.
The much-ballyhooed Grand Central Station of the West is conceived as the pivot point for the entire Bay Area’s public-transit infrastructure. The one million square-foot building is to accommodate 11 transit systems offering bus and rail service to connect the entire Bay Area and ultimately the state.
Its lowest floor, 65 feet below grade, will have six train tracks to service Caltrain and high-speed rail. Above that are a public concourse and ground levels. Bus service is 30 feet above the surface, and the building’s roof, 70 feet above ground, supports a five-acre park.
Right now, the biggest worry is keeping the enormous hole intact. Six 30-story office and condominium towers and a dozen low-rise masonry buildings line its perimeter. Their weight bears down relentlessly on the shoring that is holding the earth at bay, buttressed at four levels by three-foot diameter steel piping. The groundwater is a mere 15 feet below the earth’s surface, so it must be held back, too.
Meanwhile, Boston Properties and Hines hope to start construction this summer on the granddaddy of San Francisco office buildings. At 1,070 feet, the Transbay Transit Tower will be the tallest on the West Coast. It also will be next door to the Transit Center, and the two buildings will be under construction simultaneously for the next three years.
At the same time, the site San Francisco developer Jay Paul Co. has just acquired at 181 Fremont St. is also next to the Transbay Transit Center site. Jay Paul said March 29 that he intends to begin construction right away on a 54-story office and condominium building.
Two decades ago when the city began work to build a new Transit Center, San Francisco city leaders predicted the population would return to the region’s urban core in time, city Mayor Ed Lee told dignitaries gathered March 27 at a ceremonial groundbreaking for the 1.4 million-square-foot Transbay Transit office building. As beautiful as the new office tower will be, Lee said, the true gem is the Transit Center.
The Transit Center’s predecessor building served the city for 70 years and “was the center of progress,” Lee said. “The new Transbay Transit Center will have the same role.”
Today, five cranes, three drilling rigs and 15 earth escavators are digging the Transit Center’s huge hole and removing the dirt. Arup Senior Geotechnical Engineer Stephen McLandrich has been contemplating the transit station’s construction since July 2008. The buoyant 32-year-old University of California, Berkeley, graduate is the project’s geotechnical engineer. McLandrich’s speciality is the relationships among soil, rock, ground water and a building’s structural foundation.
The size of the excavation for the new Transit Center may be “unique,” McLandrich said. But, “most of the [engineering] techniques have been done before.”
That includes an elaborate scheme to anchor the building to the rock below it, necessary because the building ultimately will float in the groundwater that surrounds it, McLandrich said. Each of the tie-downs consists of a high-strength solid steel bar approximately three inches in diameter that is plunged 90 feet into the earth and cemented there—approximately 2,000 anchors in all.
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Photo courtesy of Arup