DIAPHRAGM WALL CASE STUDY #4:
World Trade Center Recovery, NY, NY

The tragic events of 9/11/01 have left few people untouched. One of the greatest "heroes" of the day has been the "bathtub" of the World Trade Center, being able to resist the tremendous forces generated by the destruction while holding back the Hudson river from entering the New York City tunnel system. The "bathtub", is actually a 3 ft thick (0.91 m) structural slurry wall that keeps water and soil out of the excavation. The term was actually incidentally used by the highly respected geotechnical engineer Mr. George Tamaro of MRCE in a PBS interview, and has ever since gained wide acceptance in the non engineering world.

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The World Trade Center (Saxena, 1974), New York, is one of the deepest early diaphragm wall projects in the US. The slurry wall was keyed into bedrock or hardpan by 8'. Six levels of rock anchors braced the excavation in most locations except at a section adjacent to a subway tunnel where only four levels were used. The wall cantilevered up to 6" before the first bracing level was installed, and was thereafter pushed back into the soil by up to 2.4" when all anchors were installed. Only, at the section adjacent to the subway did the wall moved towards the excavation (2.4") in a cantilevering mode. In that location, the first anchor level was installed under the subway, 35' below the surface.

While rock anchors provided temporary excavation support, permanent bracing of the slurry wall was provided by the original basement slabs of the WTC. When the twin towers collapsed, the falling structures demolished most of these supporting slabs. Portions of the permanent basement slabs crushed when the twin towers collapsed into the WTC basement. The result was a drastic reduction in the effective lateral support of the perimeter slurry wall. Consequently, the slurry wall experienced deformations in excess of 2 ft into the WTC basement. The slurry wall withstood both the large deformations and the large unsupported lengths. The stability of the WTC slurry wall immediately after the collapse is mainly to be attributed to the ability of poorly or non-supported individual panels to cantilever from the base, to benefit from any residual floor diaphragms, and to span adjacent panels that had adequate lateral support. Nonetheless, the stability of the slurry wall had to be ensured by proper anchoring before the recovery crews could "safely" reach the old subgrade 75 ft beneath the Hudson River. The general philosophy of the redesign was that the old anchor system had to be replicated in some fashion. Most importantly, the first level of rock anchors was prestressed with considerably excess force to account for the possibility of further collapses in the basement. Relatively small deformations occurred after the first level of anchors was prestressed. Many panels were pushed back towards the retained soil as anchor installation proceeded deeper (considering the position of the wall before the first level of anchors was installed as initial). As expected, individual panel exhibited considerably different deformations. Moskowitz and Tamaro (2002) discuss this subject in more detail.

Exhibit 1: World Trade Center Bathtub and Original Conditions (adapted from NY Times 9/18/01)

Exhibit 2: Excavation at World Trade Center Liberty & Greenwich, 1968