excavation areas and confined site logistics were further compli- cated by having to maintain six lanes of traffic on Route 9A at all times. The efforts were so intertwined that, well into the underpass construction, the Port Authority Construction Department decided to merge a portion of that work into the WFC redevelopment. Historical Context The west side of Lower Manhattan, in the vicinity of the WTC, is pre- dominantly land reclaimed from the Hudson River. For two centuries, successive generations of bulkheads and piers were constructed into the river with the areas behind them filled with random fill and other debris. The most recent, and probably last generation of this filling, is the area now known as Battery Park City (BPC). The WFC is a high-end cluster of office towers that makes up the central business core of BPC and is immediately west of the WTC site. Beneath Route 9A, workers encountered two old bulkheads. One was a rock-filled, timber crib and the other, a historic, granite faced, concrete seawall. The timber crib did not present any significant technical difficulties. The granite seawall was more problematic. Removing 50 ft (15.2 m) of seawall required approval from the NY State Historic Preservation Office. This approval was granted, with the condition that the seawall be disassembled rather than demolished, and this work be performed in the presence of an archeological team to document any significant findings. In addition to old bulkheads, there are two, 100- year-old, cast- iron subway tunnels (PATH) traversing Route 9A. One of these tunnels is situated immediately north of the pedestrian underpass. The proximity of the tunnel is complicated by the manner in which the original WTC slurry walls were constructed around it. In order to ensure a proper seal around the tunnels, special slurry wall projections were built to intersect the tunnels at the point where the mid-height of the tunnel rose above bedrock surface. This allowed the excavation buckets to remove the materials above the tunnel and socket into the adjoining bedrock. The projection structure immediately north of the underpass presented an opportunity and a challenge in the underpass construction. Subsurface Conditions The surficial soils are miscellaneous fills and extend to a depth of from to 30 to 35 ft (9.1 to 10.7 m). Underlying the fill is a deposit of river sediments. These consist of slightly organic, clayey silts. These soils, traditionally quite soft, have been significantly consolidated due to the fill loading, and also because there has been a general lowering of the groundwater due to decades of leakage into the adjacent WTC basement. Underlying the river sediments are glacial outwash deposits. The deposits are from 10 to 15 ft (3 to 4.6 m) thick and consist of dense sands, with occasional cobbles and boulders. Beneath the glacial deposits is a relatively thin layer, typically less than 5 ft (1.5 m), of decomposed bedrock. Bedrock is predominantly mica schist (Figure 2). Placement of rebar for tunnel base slab The multitudes of utilities that cross the tunnel alignment further complicated construction and presented challenges. There are at least 15 utilities traversing the excavation, the largest being a 78 in (2 m) sanitary sewer. This sewer was treated with great deference. In addition, there are two, very large, ~ 10 ft (3 m) communication duct banks crossing the excavation. These duct banks were also treated with great care. DEEP FOUNDATIONS • SEPT/OCT 2012 • 11 Groundwater level varies from east to west. At the seawall, it mimics the tidal variation of the river. While adjacent to the WTC slurry wall, groundwater levels are depressed as much as 15 ft (4.6 m) below sea level. Figure 2. Soil conditions Construction The original plan for the underpass construction envisioned all the work progressing in five separate stages across Route 9A using bottom-up technology (Figure 3). The excavation support was primarily secant piles, with jet grout closures beneath large utilities and at juncture points with existing structures. In addition, the project was able to make use of approximately 150 ft (46 m) of the existing WTC slurry wall for excavation support. All stages of the excavation were carefully choreographed with lane shifts and temporary bridging of Route 9A.
