Performance and Three-Dimensional Analyses of a Wide Excavation in Soft Soil with Strut-Free Retaining System

Abstract

This paper presents the results of monitoring the diaphragm wall and ground surface settlement and three-dimensional finite element analyses of the Xizhi excavation case history. The Xizhi excavation was 9.2 m deep, covering an area of about 195.7 m × 88.6 m. The soil layers above the final excavation level are dominated by a soft-to-medium clay layer (NSPT = 2–7) and a loose silty sand layer (NSPT = 2–6). Because of its large excavation geometry, this project utilized diaphragm walls, buttress walls (BWs), and partial floor slabs as a BW strut-free retaining system to reduce the construction cost and period. This project was successfully constructed with a ratio of the maximum wall deflection to excavation depth equal to .55% and a maximum ground surface settlement of 27 mm. The mechanism of the BW strut-free retaining system in controlling wall deflections was governed by two factors: the combined stiffness between the buttress and diaphragm walls and the frictional resistance between BWs and adjacent soils, where they contributed 72% and 1.5% reduction of the wall deflection, respectively. In addition, the effectiveness of the first partial floor slabs in reducing the top wall deflection depended on the length of the diaphragm wall. The shape, location, and length of BWs had only moderate effects, while the groundwater level had a major effect on the performance of the BW strut-free retaining system.