Characteristics of a large and deep soft clay excavation in Hangzhou

A 14.85 m~17.35 m-deep multi-strutted soft clay excavation in Hangzhou has been monitored. The excavation is supported by contiguous piles in row. The monitoring covers wall and soil deflections, surface ground settlements, axial forces of concrete struts, earth pressures and pore water pressures. It is shown that the maximum horizontal deformations of the large scale excavation are far larger than those narrow excavations. “Creep” deformation occurs apparently during the curing of the bottom slab of the basement. The excavation depth, space effects, partition wall and a basement adjacent to the excavation are the important factors that affect the deflections of the excavation. The distributions of the surface settlements seem to be parabolic, and the settlement influence zone can reach a distance of about 2.5H, where H is the final excavation depth, and the location of the maximum settlement is at 0.67H away from the retaining wall. The relationship between the maximum ground settlement and the maximum horizontal deformation can be summarized as . The distribution of the longitudinal settlements is in a saddle shape, and the longitudinal surface settlement trench is wider than the pit. The axial forces of the struts change dynamically during the excavation and construction, and the magnitude of the axial forces of the second-level struts seems to be larger than that of the other two level struts. “Soil arching” phenomena are found at the depths where the struts are located, which result in the nonlinear distributions of the earth pressure on the wall. The measured pore water pressures decrease during the excavation. The negvative excess pore water pressures occur in the ground due to the unloading on the excavation boundary.