Field tests on performance of diaphragm wall for an ultra-deep excavation in Shanghai soft ground

Based on the field observation, the spatial characteristics of the diaphragm wall for a 31.5 m-deep excavation of a passageway project in Shanghai downtown under the coupling effects of excavation and dewatering are investigated. The results show that: (1) With the small length-depth ratio and length-width ratio, due to the corner effects, the lateral deflection of the diaphragm wall exhibits dramatical spatial effects, and the induced flexure along the length is great. The local deflection-span ratio (DSR) near the pit corner exhibits larger volume than that in the middle, with the average local DSR about 0.15δ_h/H_e to 0.23δ_h/H_e for the middle-span position and 0.32δ_h/H_e to 0.56δ_h/H_e for the position near the corner. (2) The maximum wall deflection increases non-linearly with the excavation depth H, and its rate of change raises especially when the excavation depth exceeds 12 m. (3) The excessive pre-dewatering dramatically enhances the deformation of the diaphragm wall, leading to a significant increase in the cumulative deformation of the subsequent excavation. Consequently, during the excavation, the maximum wall deflections and the maximum vertical displacement of wall top are 0.7%H_e and 0.1%H_e (uplift), respectively, while the uplift of column is 0.2%H_e, twice to the wall uplift. All of them are significantly larger than the statistics of normal Shanghai metro stations (excavation depth of 16 to 20 m). Thus the unloading effects of pre-dewatering should be paid great attention to during the construction of ultra-deep excavations, which should be substituted by step dewatering.