Structural Behaviors of Large Underground Earth-Retaining Systems in Shanghai. II: Multipropped Rectangular Diaphragm Wall

Abstract

Via analyzing the field instrumentation data, this study examines the structural behaviors of the multipropped diaphragm wall for excavation of the outer rectangular pit. The circumferential stresses in waler beams as well as boundary beams were closely related to the corresponding wall deflections. Braced struts might lead to stress-concentration in the retaining structures. Because of the uneven vertical movements of floor slabs, boundary beams suffered bending stresses. The stresses in interior columns were governed by the weights of cast floor slabs and rebounds of basal soils. The significant upward movements of interior columns and heavy floor slabs indicate that the released stresses due to soil removal outweighed the heavy weights of the retaining structures. The excavation-induced lateral loads against the retaining wall were mainly carried by the rigid floor slabs and concrete beams, which were up to 10 times or more those carried by the temporary propped struts. The comparison between the field data and the design analysis results shows that the theoretical beam-on-elastic-foundation design models can make a relative reasonable estimation on the wall deflections for the unpropped circular diaphragm wall, but highly underestimated the wall deflections of the multipropped rectangular diaphragm wall. The potential adverse effects resulting from structural deficiencies (e.g., openings on floor slabs for soil discharge) and pit-size effects on weakening the capabilities of cast floor slabs or braced struts to constrain wall deflection should be taken into account in design analysis of top-down excavations.