Workers just finished stabilizing foundation walls of the 1,000x500-ft basement of the former World Trade Center. With that accomplished, the engineer for the work has revealed that damage to a 110-ft-long section near the south end of the east wall was more severe than first observed, when debris pinned it and blocked it from view.

When the 110-story Two WTC collapsed last Sept. 11, it not only crushed the uppermost 20 ft of the reinforced concrete section of wall, it caused the lower 50 ft to bulge in toward Ground Zero, as much as 4.5 ft at the wall’s top.

The bulge was discovered as debris, which was supporting the 3-ft-thick wall, was removed in preparation for the installation of rows of tieback anchors. The first anchors in the crushed zone went in in March, from the top down (ENR 4/1 p. 14).

As crews excavated a temporary equipment ramp, it became clear that the condition below the crushed section was just as bad as it was at the top. "The wall took such a shot vertically that the reinforcing steel buckled out," says Andrew Pontecorvo, an engineer with Mueser Rutledge Consulting Engineers, New York City, which designed the fix. "Between the steel is crushed concrete," he adds.

BIG BULGE FIX Damaged section of wall meeded permanent liner wall, not just tieback anchors. (Photo above and top courtesy of Mueser Rutledge Consulting Engineers)

The condition was so poor that the engineer decided to add a 110-ft-long permanent liner wall, 70 ft tall, up against the bulge. The liner, 2.5 ft thick at its base, widens to 6.5 ft at the top to include the "cap" for the missing section of wall.

Three rows of stressed tieback anchors supported the damaged wall until crews from Bronx, N.Y.-based Civetta Cousins Joint Venture built the liner and cap. The liner, done in several lifts, only needed a one-sided form because it was cast up against the damaged wall. At the cap, a conventional two-paneled form was used. "It’s very complicated," says the engineer. "It’s really a gigantic Band-Aid."

Formwork for the liner was constructed with tieback sleeves containing extensions of tendons already installed, 10-ft-long "pigtails" initially free of stress. After crews cast each liner lift, workers from the Cuddy, Pa., office of Nicholson Construction Co./ Hayward Baker, A Joint Venture, would stress the pigtails and transfer the resistance from the damaged wall to the liner.

Each of the 18 strands for each tieback was stressed up to 40 kips to relieve the load from the damaged wall and transfer it to the liner wall. Workers pulled a strand at a time.

The liner will be incorporated into the permanent basement of the future below-grade structure of the rebuilt WTC.

At Ground Zero, a total of 976 tiebacks have been installed. "Everything went relatively smoothly," says Christopher S. Hynes, senior project manager for Nicholson/Hayward Baker.

The "bathtub" stabilization is done for now. But there may be some future work, says Hynes, if intact parts of lower levels of Six WTC are removed or if the new tiebacks interfere with the foundations for the new Seven WTC.