The 27,700 sq ft of slurry wall wedged between a school and New York City Transit elevated subway tracks in Queens required the extraction of two levels of tiebacks at the edge of another slurry wall that Treviicos Corp., Englewood Cliffs, N.J., built four years ago, says di Cervia, the firm's director. There are two more slurry walls in the immediate vicinity that the firm built, he says.

TOUGH SITE Slurry wall segments were placed in tight working conditions.

Now Treviicos is wrapping up a $3.5-million subcontract to the Park Ridge, N.J., office of Kiewit Constructors Inc., which has the $16-million contract for excavation and earth support systems that lays the base for New York City's $4-billion East Side Access project that will send Long Island Rail Road trains to Manhattan's Grand Central Station. The network of walls, sheet piles and 800-kip tiebacks will allow future crews to set up tunnel operations to Manhattan.

The slurry wall now is receiving the last of 21 panels socketed 6 in. into hard rock and chemically grouted, says Henry Riley, Treviicos superintendent. About 3,510 cu yd of concrete will be poured.

The subcontract, begun in December 2002, was slated for nine weeks, but "contractually it was foreseen that delays might be possible due to boulders," says di Cervia. Boulders of 3 to 4 ft across and up to 5 ft high were found. Work also was slowed because of requests by the transit authority not to swing the 250-ton cranes at an angle facing the tracks. This caused picking and placing of the asymmetrical L-shaped rebar cages at a more difficult angle.

The wall, up to 48 in. thick, goes down to 83 ft through a geological "crazy quilt," facing the 63rd St. tunnel tubes built with two levels of tracks. The lower level will accommodate LIRR trains that will eventually serve Grand Central.

The wall is being placed in an irregularly shaped semicircle. When complete, Kiewit will excavate a pit to allow a contractor to assemble a tunnel boring machine and move it through the existing tunnel to Manhattan where a new tunnel will be driven, says Dwight Metcalf, Kiewit project manager. Existing bulkheads on the existing tubes will be broken out to start the tunnel.

Another wall will be built across the street "like this, only five times bigger," Metcalf says. That job is set to go to bid in early May. It will support an open-cut excavation to provide access for an earth pressure balance tunneling machine that will work in the opposite direction, to railyards and a connection to existing LIRR tracks.