Numerical simulation of deep foundation pit dewatering and optimization of controlling land subsidence

Shanghai is a typical area of soft soil distribution, Metro railway construction is now being developed in Shanghai City on a large scale and the planning of Metro stations are often located in densely populated districts with tall buildings. Metro station constructions are mostly taking pit dewatering measures, while the dewatering of aquifer may cause soil layer compression, land subsidence, foundation's deformation, cracking and tilting of the buildings, and so on. In order to control the land subsidence effectively, the underground continuous concrete wall is often used in the deep foundation pit dewatering. The depth of underground continuous concrete wall and the filter tube position of pumping well affect drawdown outside the pit and land subsidence directly. This study refers to the deep foundation pit dewatering project of Hangzhong Road station of Shanghai Metro Line No.10. The excavation depth of foundation pit is 15.60–17.60 m, and the design depth of underground continuous wall is 28 m in the standard part and 30 or 31 m in the end well. Three-dimensional finite differences method is used to simulate the pit dewatering through the inversion of permeability parameters based on the field pumping tests. The hydraulic barrier function of the underground continuous wall is simulated at four different depths including primary design depth, increasing 3 m, 4 m and 6 m. The result of the numerical simulation indicates that the drawdown of the aquifer decreases with the increase of the underground continuous concrete wall depth. When the underground continuous concrete wall increases 4 m on primary design basis, the drawdown outside the pit and land subsidence can be controlled effectively. The monitored results indicate that the drawdown outside the pit at a distance of 1–5 m to the wall is less than 2 m, while the maximum land subsidence is 7.97 mm, which is of nearly no influence on the environment around the pit during dewatering.