"We were able to measure changes the width of a human hair," says Mun Wei Leong, resident engineer with the local Jacobs Associates, the project's program-construction manager.

Community Compromise

The North Beach site for the 42-ft-sq retrieval shaft was a point of contention with area residents. Original plans called for the shaft in the median, but the community continued to resist even after the plan received environmental approval.

Just 15 months before the TBMs emerged, a "win-win" compromise was reached, according to Funghi. An abandoned building was removed and the empty lot used for the retrieval shaft. That meant virtually no impact on street traffic.

Crews are currently dismantling Mom Chung and Big Alma before they go back to Robbins for refurbishing. The extraction pit will be finished with steel.

If the city decides to expand the subway to Fisherman's Wharf, 7/10th of a mile away, the lot could be used again as an extraction site, if it is not covered over by a development of condominiums.

The launch site also had its challenges. It was located directly underneath high-traffic ramps to a freeway, so a special low overhead boom had to be procured from Soletanche Bachy, Johannesburg, South Africa, to set the slurry walls.

Crews will spend the next four years building four stations and installing rail. Contractor Tutor Perini, Sylmar, Calif., is heading up the work under an $840-million contract. Construction began a year ago in June.

Because of the space constraints, the contractor is using tangential inclined piles for some station foundations. This allows a wider pile at the deepest end, which avoids touching the walls of the tunnel 4 in. away. Pile installation is under way, with 35 of 198 installed.

"There is very little room for error," Funghi says.