The secant pile wall system was used on all four sides of the site, with the north, east and west walls located directly below the shear walls and integrated into the per- manent structure. The south secant pile wall was placed outside of the property line within the sidewalk, and served as a temporary rigid support of excavation (SOE) wall to protect the NYCT subway tunnel. A cast-in-place concrete foundation wall with waterproofing was constructed along the south property line and tied at each end to the east and west secant pile walls. methods. The initial secant pile wall analysis was performed using the Continuous Beam Method (CBM), which included calculating lateral pressures and performing staged analyses to obtain the internal forces (moments and shears) in the wall and line load reactions at the supports. Using these results, the thickness of the wall, compressive strength of the concrete, size of core beam and bracing members were estimated. This method assumes the secant pile wall acts as a continuous beam, with the soil, water, and surcharge pressures preloading was implemented using flat jacks between the struts and the wales. Groundwater Control — Ground- water control and cutoff was a significant challenge. As revealed in the subsurface investigations, the permeability of the bedrock is quite high, consistent with the permeability of sand with some fines. However, because of the highly fractured and decomposed nature of the bedrock, the permeability could be even higher if the rock was excavated and left exposed. In addition, during the subsurface investi- gat ion, i t was observed that the permeability of the rock is high enough that there was communication between the boreholes, indicating that the joints and fractures can extend over a significant distance. If the rock is excavated and left exposed, confinement will be lost. Moreover, if the rock face is left unpro- tected, the water pressure from outside of the support of excavation system could slowly wash out the decomposed rock from within the rock fractures into the excavation, significantly affecting the stability of the rock mass, and negatively impacting the foundation construction. Ground-water cutoff was provided by extending the secant pile wall, including the primary secant piles, into the bedrock and below the final subgrade. Temporary lateral bracing for the secant pile wall system The secant pile walls supporting the shear walls were designed essentially as an extension of the shear walls to bring forces directly down to the soft bedrock. The axial capacity of the secant pile wall was developed in the rock socket below final subgrade by the frictional bond between the rock and the secondary piles. Lateral Bracing — Temporary lateral bracing of the secant pile walls was provided by a rigid internal bracing system consisting of wales, corner bracing and cross lot struts. Permanent lateral bracing was provided by the cellar floor slabs. The design of the secant pile wall and lateral bracing system, estimation of wall movements, and settlement of adjacent buildings was performed using con- ventional and soil-structure analytical applied outside of the wall and the internal bracing and passive soil pressure resisting the applied loads. Rankine active and passive soil and at-rest pressures were used in the initial CBM analyses. A soil-structure interaction analysis was then performed using the finite element program, Plaxis 2D, with wall and bracing parameters derived from the initial CBM analysis. The refined analysis was conducted to estimate the movement of the wall and settlement of the adjacent structures, and, in turn, to refine the secant pile wall and bracing member properties. The wall movements and settlement of adjacent shallow foundations were limited to an acceptable risk tolerance by determining prestressing loads for the bracing. In construction, the strut Preparing for an Osterberg load test 76 • DEEP FOUNDATIONS • MAR/APR 2017
