FOUNDATION CONSTRUCTION CHALLENGES AT 100 11 TH AVENUE INMANHATTAN - OSTERBERG CELL LOAD TEST IN MICA SCHIST

摘要

This paper is the second in a series of two describing the design andconstruction of a foundation for a major high-rise real estate development onthe west side of Manhattan, N.Y. The foundation members consist of large-diameterrock-socketed caissons, driven closed-end steel pipe piles bearing onrock, and a secant-pile retaining wall surrounding the entire site. This paperfocuses on the Osterberg cell (O-Cell) load test completed for determining theallowable side-shear resistance between the concrete and the sides of the rocksocket. Different qualities of mica schist bedrock, demonstrated by thepenetration of rock with the soil auger or drill casing, were encountered.Accordingly, an additional subsurface investigation consisting of test borings ateach caisson location was completed. Unconfined compressive strength testswere performed on representative rock samples. The “soft” mica schist wasobserved to have vertical foliations and high mica content. For the first time inManhattan, an O-Cell load test was performed to determine the allowable sideshear resistance for both the soft and hard mica schist bedrock. The side shearresistance obtained from the O-Cell load test is compared with the side shearresistance allowed by the New York City Building Code and empiricalcorrelations using unconfined compressive strength values for rock. The rocksocket design for the caissons was refined based on the O-Cell load test results.