Cut-and-Cover Construction Using Floating Diaphragm Walls—A Case Study

This paper presents a case study of cut-and-cover construction of an underground subway station in Manhattan, New York, which utilized floating diaphragm walls in weak glacial silts for temporary support. Vertical loading was imposed on the diaphragm walls by a temporary traffic deck that spanned between the top of the diaphragm walls and the utilities hanging from it, including two to three levels of steel pipe struts and self-weight of the diaphragm walls. The vertical loads were resisted primarily by sidewall shear resistance since the end bearing component was small. A rotational mode of wall deflections was observed about the upper-level struts, with a kick-back at the top of the diaphragm walls and significant lateral toe displacements. Maximum wall deflections ranged from 0.17% to 0.31% of excavation height. It was noted that the upper-level struts in a two-strut system were very sensitive to the removal of the lower-level struts during the bottom-up sequence, with large increase in load transferred to the upper strut. In both two and three-strut systems, the lower struts experienced significant reduction of loads over the period of invert slab construction, indicating the sensitivity of the floating diaphragm wall deflections to imposed surcharge loads on the weak glacial silts encountered at the excavation base. The study demonstrated that interpretation of the observed lateral earth pressures resisted by the struts using conventional apparent earth pressure envelopes was difficult when soils are layered and mixed cohesive and cohesionless soils are involved.