Fulton Street Transit Center, New York City - Ground Movements Assessment

Note: Several images and text have been taken from:

Appleby, J., et al., (2011), "Fulton Street Transit Center – Foundation Design and Construction in a Dense Urban Environment," Proceedings, 2011 Pan-Am / Canadian Geotechnical Society Conference, Toronto, Canada, 9p.

Arup (2004), CM-1252 Fulton Street Transit Center, Ground Movements Report - Stage 2, Prepared for MTA - New York City Transit, Ove Arup and Partners, New York September 2004.

The Fulton Street Transit Center (Fulton Center), in Lower Manhattan, is a recently constructed transportation hub located in one of the most congested parts of New York City. The project included significant station upgrades as well as new connections between several subway lines and eventually linking with the redevelopment of the World Trade Center. The centerpiece of the project is the new, glass-clad Transit Center situated on the corner of Broadway and Fulton Street. The excavation for the substructure required the underpinning of the existing buildings that abut the site on two sides, including the Federally-listed Corbin Building to the south. On the other two sides are existing subway structures. The excavation was supported by internally braced stiff secant pile walls to reduce the ground movements. Monitoring was carried out on all surrounding structures and extensively for the Corbin Building. Once at full depth, the 5ft thick raft foundation was cast directly on the local interbedded silt and sand soils and the secant piles were broken down to open the structure to the existing subway lines and the underpass beneath the 4/5 line that was constructed under an earlier construction contract.

Plan view of the Transit Center site showing locations of adjacent subway stations. 

Table of Soil Stratigraphy, Fulton Center

A preliminary (Stage 2) ground movements assessment was performed in 2004 using the commercial 2D Finite Element software package PLAXIS. This Stage 2 assessment did not explicitly consider the surcharge effects of the nearby structures, particularly the historic Corbin Building to the south of the Transit Center site. The results of a trial 35 ft. deep excavation greenfield assessment suggested a maximum lateral wall movement of 0.11 ft. (1.4 inch, or 35mm), which was estimated to occur at the base of excavation level.

In reality, to accommodate a new pedestrian escalator from the Dey Street Concourse level and through the basement of the existing Corbin Building, the structure's inverted brick arch shallow foundation (no piles) was underpinned using traditional hand-mined pit underpinning techniques, as shown below, along its northern and southern boundaries. Furthermore, jet grouting was implemented beneath the northern boundary of the Corbin Building (at the interface with the Transit Center to the immediate north), but not beneath its southern boundary.

Schematic section showing the underpinning of the Corbin Building. Facing north.

Hand excavated underpinning pit. 

The escalator pit beneath the Corbin Building, facing west toward Dey St. Concourse, showing inverted arch shallow foundations

Aerial view of Transit Center site, October 2009 (facing north from Corbin Building)

Construction of the Transit Center basement was carried out by Skanska USA Civil Northeast and commenced in March 2009 and was substantially completed in August 2010. Following the demolition of the existing structures on site and the underpinning of the buildings to the east with hand dug concrete piers, the first major activity was ground improvement in the form of jet grouting. Ground improvement was required in the contract primarily to stabilize the sensitive soils on site. Previous experience indicated that the vibrations due to the installation of the secant piles were sufficient to cause settlement within the stratified silts and fine sands beneath the site. These fine grained deposits, known locally as the ‘Bull’s Liver’, quickly lose strength when disturbed or wetted and were of particular concern both during the piling and the excavation works. Jet grout columns were formed at each secant pile location, through the stratified silts and sand and down to the Glacial Till below, to bind the soil and minimize settlement. The use of jet grout had the advantage of strengthening the ground, reducing the loads on the secant pile wall by effectively underpinning the surrounding buildings as well as reducing the permeability of the ground. The installation of the secant piles followed shortly after the jet grouting.

During design, a series of finite element analyses were carried out as proof of design and in order to predict the likely wall deflections during the excavation. As the temporary bracing system was contractor designed, Skanska was required to carry out their own analysis to demonstrate that their proposed system would perform within the contract tolerances. The results of these analyses are both shown below, along with the actual wall movements at the center of the south secant pile wall, measured by inclinometer. The south secant pile wall (adjacent to the Corbin Building) was the most heavily surcharged, as well as being adjacent to the deepest part of the excavation and therefore showed the highest deflections in the model, as well as during construction. It should be noted that as the bracing system was to be contractor designed, the designer’s (Arup's) analysis assumed a bracing system that proved different from the system finally used. The contractor’s analysis predicted approximately 1inch (25mm) movement, with maximum movement occurring at elevation +98ft. The actual deflection was slightly more than 1 inch with maximum movement at +93.5ft. Although the elevation of actual deflection was different, the magnitude of deflection was predicted relatively close to the observed value.

Plot showing the predicted and actual wall deflections for the center of the southern secant pile wall. Note that the movements predicted by the designer assumed a different bracing system to the one used during construction

The settlements of the structures on all four sides of the Transit Center site were monitored throughout the excavation and construction of the substructure. The buildings to the east of the site and subway structures to the north and south were monitored through the installation and monitoring of survey points using manual techniques. Due to its sensitivity and significance the historic Corbin Building to the south was monitored more heavily. Liquid levels and tiltmeters were installed at the base of each of the building columns in the sub-basement throughout the construction. In addition, during the underpinning and excavation beneath the building, prisms through the length and height on the exterior of the building were continuously monitored using an Automated Total Station. All of the automated monitoring was also verified with manual surveying. Settlement results for a sample point on the north wall of the Corbin Building and a sample point on the south are shown below. These points represent the highest movements observed at the north and south faces of the structure.

 The north wall of the Corbin Building, immediately adjacent to the Transit Center site shows approximately 0.6 inch (15 mm) of settlement before June 2009. This movement was due to the demolition of the existing buildings on the site (approx 0.475in, 12mm), and the installation of the jet grout columns beneath the wall (approx. 0.125in, 3mm). Once the jet grout was installed, the settlement of the wall due to the installation of the secant piles, excavation of the Transit Center site, construction of the underpinning piers and excavation of the escalator pit beneath the Corbin Building was relatively small, when compared to the south wall, only experiencing a further 0.4 inch (10 mm) of vertical movement. The south wall of the Corbin Building is approximately 25ft further from the Transit Center site and was not underpinned by jet grout. It experienced little settlement due to the demolition of the existing buildings on site, and the installation of jet grout and secant piles to the north (0.25in). However, the south wall continued to settle throughout the remainder of the construction works, most significantly during the dewatering and excavation of the Transit Center site and of the Corbin Building site.

Plot showing the settlement for two of the columns of the Corbin Building, one in the north wall and one in the south. The Corbin Building north wall was underpinned with jet grout prior to the excavation of the Transit Center site

The dewatering and excavation of the Transit Center site took place from September through November 2009, during that time the south wall of the Corbin Building settled by approximately 0.9in. By comparison, the north wall settled by less than 0.1in in the same time period. During the same period, a jet grout curtain was installed to create a water tight bath tub beneath the Corbin Building, as this was located away from the building footings the effect was not expected to be significant and the majority of the movement of the south wall during this period is assumed to be due to the main site works. The second significant movement of the south wall occurred during the dewatering and excavation of the escalator pit beneath the Corbin Building itself, approximately 0.75in. Daily monitoring at the time indicated that the excavation, construction and the transfer of load onto the underpinning piers caused about 0.125in of this, the remainder being due to the dewatering and excavation of the escalator pit itself. Once the escalator pit was completed, settlements decreased to effectively zero. The final settlement of the Corbin Building was approximately 1 inch (25mm) at the north wall (with jet grout underpinning) and 2 inches (50mm) at the south wall (without jet grout underpinning). This 1 inch of differential distortion corresponds to a tilt of approximately 1/300 to the south (away from the excavation). Tilting of the Corbin Building to the south was predicted in the Stage 3 ground movements assessment, which explicitly considered building surcharges and some degree of pit underpinning.

The buildings to the east of the site and subway structures to the north and west showed little appreciable movement throughout the construction. 

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