Failure Investigation at a Collapsed Deep Excavation in Very Sensitive Organic Soft Clay

Collapse of a 15.7-m-deep excavation in very sensitive organic soft clay in Hangzhou, China, is presented in this paper. The collapse of the excavation caused the break and/or inclination of the retaining walls, the wracking of the supporting system, significant sinking of a major road adjacent to the excavation site, and also the break in a water main beneath the road. After the collapse, the ground disturbance due to the collapse was investigated by cone penetration tests (CPTs) and field vane shear tests (VSTs). A simplified method, based on the CPT and VST test results, was used to evaluate the strength reduction ratio (SRR) of the in situ soils. According to the SRR values of the soils at different depths, the lower boundaries of the severely disturbed soils were determined and a possible slip failure surface passing through the bottom of the severely disturbed soils was justified. The factors of safety (FOSs) against basal heave of the excavation calculated from various methods were 1.05, 0.74 and 0.89, respectively. These computed values were much smaller than the corresponding recommended values. The stability number N of the excavation was calculated to be 7.4. The overall FOS was studied using the shear strength reduction technique based on a finite-element method (FEM) program. The FEM analysis results indicated that the FOS of the excavation was 0.97, and the computed slip failure surface fit well with that from the soil disturbance investigation. The failure mechanism of the excavation was then justified to be a basal heave. The main reasons leading to the collapse and lessons learned from the collapse are discussed. The investigation of the failure provides experiences and lessons for deep excavation design in very sensitive organic soft clay.