Performance of Large-Diameter Circular Diaphragm Walls in a Deep Excavation: Case Study of Shanghai Tower

Abstract

In this study, the performance of large-diameter circular diaphragm walls in a deep excavation in Shanghai soft clay is investigated, which has a diameter of 121 m and a maximum excavation depth of 31.1 m. Through a comprehensive field monitoring program, the distribution and development of total lateral pressure, including earth pressure and water pressure, are discussed in association with the dewatering process in the confined aquifer. The lateral deflections of the circular diaphragm walls are analyzed and compared to similar cases in the literature. Moreover, comparisons of hoop force and hoop stiffness between the diaphragm walls and ring beams are presented. Results indicated that (1) because of the symmetrical excavation scheme in this case, the circular diaphragm walls maintained in a small eccentric compression state throughout the excavation; (2) the computed lateral pressures using lateral pressure coefficient at rest agree well with the measured lateral pressure at both initial as final excavation stages if the effect of groundwater drawdown on the lateral pressure was considered; (3) due to the decrease of the lateral pressure outside of the excavation and increase of the internal supporting system stiffness, the maximum deformation occurred at a depth of about 10–15 m above the final excavation surface; (4) based on the investigation of 14 circular diaphragm wall cases in soft soil area, a reciprocal relationship between normalized wall deformation and hoop stiffness is proposed; and (5) the ring beams behaved more stiffly than the diaphragm wall panels because they were casted integrally.