The world's tallest buildings, the World Trade Center's twin towers, once stood on one of the biggest, had most complex foundations of that time.
To install the foundations, a joint venture of five contractors working for the Port of New York Authority dug 70 ft into Manhattan's west shore to remove 1.2 million cu yd of rock and earth from a box-shaped excavation. The box was lined with 3,000 ft of cutoff wall that had a 64-ft head of water outside-and no internal bracing.
All of the digging had to do over, under and around a pair of subway tunnels that carry nearly 600 commuter trains through the excavation daily. Without interruption of train traffic, the live and dead loads of about 1,000 ft of each tube must be had to transfer from the soil to a suspension system that held the tubes about 30 ft in the air for the next 2 years.
The cutoff wall, which Icanda, Ltd., an affiliate of Icos of Milan, Italy, installed by the slurry methods, was roughly 3 ft thick and completely encircles a 1,000 x 500-ft area. Though its base was keyed about 3 ft into rock, the wall itself was not withstand the hydrostatic pressures from without. Therefore, the Port of New York Authority (PNYA) engineers designed a system of exterior anchors that will support the wall until the complex's heavily reinforced subfloors take over the job.
|Steel rises for north tower while south tower area is dug|
Because the cut-off walls are virtually impervious, the contractors' water problems were limited pretty much to handling the water trapped within them. Therefore, except in the areas of the subway tubes, where water elevation was extremely critical, the dewatering is routine.
Initially the contractor installed four deepwell sumps 8 ft sq. The sumps did dewater adjacent areas, but no more, because of the impermeability of the ground. This was not serious, however, since 2-in, and 4-in, centrifugal pumps in local areas controlled the water.
PNYA specifications called for lowering the water table outside the excavation to a depth of 5 to 10 ft. Rather than install well points all around the perimeter, the contractor had Icanda cast a 4-in, hole in each 22-ft wall section. With these open, the water from outside ran into the excavation, where the pumps sent it through a header system to the Hudson River nearby.