Environmental assessment section on physical resources, 2001

NYC 9/11 Public Portal Document

was severely affected; after several months, some residents have not returned to their homes. Four area post office stations, on Bowling Green, Wall Street, Church Street, and Peck Slip, were closed (FEMA Situation Report #5 and #14).

3.1.1 TOPOGRAPHY, GEOLOGY, AND SOILS The WTC site is located in New York County, New York on the Jersey City N.J. - N.Y. 7.5 minute topographic quadrangle map (U.S. Geological Survey [USGS], 1981). The WTC impact area covers approximately 16 acres. The ground surface elevation in the vicinity of the WTC ranges fi'om less than 10 feet near the shore to 30 feet above sea level. National Geodetic Vertical Datum (NGVD) {Figure 3-2).

The New York City area is underlain by a sequence of Proterozoic to lower Paleozoic crystalline rocks, consisting of schist, gneiss, and marble (highly metamorphosed sedimentary and igneous rocks). In the lower Manhattan area, bedrock is encountered at least 50 feet below ground surface and typically is described as schist. Ridges of erosion­ resistant schist and shallow valleys of weaker marble trend north-northeast in lower Manhattan. The topography of the WTC area has been extensively altered by urban development and much of the soil covering the area is actually fill material. The WTC area is almost entirely developed, except in parks and other open spaces. Major surface water features are the Hudson River and the East River, which is a tidal strait between Long Island Sound and the New York Harbor. Battery Park City is a built-up area along the Hudson River created from the fill material (“landfilled”) generated during the construction of the WTC complex {Figure 3-2). Substantial sections in lower Manhattan have been graded, filled and landscaped. Soil boring logs in the area of the WTC maintained by the New York City Department of Design and Construction (NYCDDC) were examined to determine the depth of fill and shallow groundwater elevations. Fill materials, including clay, silt sand, and gravel deposits as well as bricks, wood, glass and concrete, range in thickness from 40 to 70 feet (WfCDDC 2001).

There is an undergroimd retaining wall (slurry wall) beneath the WTC site that was constructed in the 1960s to prevent the flooding fi-om the Hudson River and shallow groimdwater. This retaining wall is 3,000 feet in circumference and extends down to bedrock, 60 to 70 feet below ground level. The retaining wall was supported from the inside by the concrete floors of the WTC’s six-level basement, large portions of which were destroyed. Approximately 40 of the anchors, each consisting of 18 steel cables wrapped together and driven into bedrock, have been constructed along the southern edge of the WTC site near Liberty Street, in an area that was inunediately compromised as a result of the collapse of the WTC. As many as 1,100 of the anchors, known as tiebacks, will eventually be connected to the ictaining wall. The retaining wall, nicknamed the bathtub, was built to lend stability to the soil around the WTC site and keep groundwater from flooding the area {The New York Times 2001a).

The New York City area is characterized by little seismic activity. The faults mapped in the city appear to be related to ancient mountain building and plate tectonic events of the Paleozoic Era (greater than 225 million years before present). An earthquake with a

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