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

Abstract: Based on field observation, the spatial characteristics of the diaphragm wall for a 31.5 m-deep excavation of a passageway project in Shanghai downtown under coupling effect of excavation and dewatering were investigated. The results showed that: (1) with small length-depth ratio and length-width ratio, due to the corner effect, the lateral deflection of the diaphragm wall exhibited a dramatical spatial effect, and the flexure caused by that along the length was 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 0.15δh/He to 0.23δh/He for the middle-span position and 0.32δh/He to 0.56δh/He for near the corner; (2) the maximum wall deflection increased non-linearly with the excavation depth H, and its rate of change raised especially when the excavation depth exceeded 12 m; (3) it was found that excessive pre-dewatering would dramatically enhance the deformation of diaphragm wall, leading to a significant increase in cumulative deformation of subsequent excavation. Consequently, during the excavation, the maximum wall deflections and the maximum vertical displacement of wall top were 0.7%He and 0.1%He (uplift), respectively, while the uplift of column was 0.2%He, twice to the wall uplift. All of them were significantly larger than the statistics of normal Shanghai metro stations (excavation depth ranged 16 to 20 m). Thus the unloading effect of pre-dewatering should be paid great attention to during the construction of ultra-deep excavation, which should be substituted by step dewatering.