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FINITE ELEMENT NUMERICAL SIMULATION OF THREEDIMENSIONAL SEEPAGE CONTROL FOR DEEPFOUNDATION PIT DEWATERING Abstract: For deep foundation pit dewatering in the Yangtze River Delta, it is easy to make a dramatic decrease of the underground water level surrounding the dewatering area and cause land subsidence and geologic disasters. In this work, a three-dimensional finite element simulation method was applied in the forth subway of Dongjiadu tunnel repair foundation pit dewatering in Shanghai. In order to control the decrease of the underground water level around the foundation pit, the foundation pit dewatering method was used to design the optimization project of dewatering ,which was simulated under these conditions that the aquifers deposited layer by layer, the bottom of the aquifers went deep to 144.45 m, the retaining wall of foundation pit shield went deep to 65 m, the filters of the extraction wells were located between 44 m to 59 m, the water level in the deep foundation pit was decreased by 34 m, and the maximum decrease of water level outside the foundation pit was 3 m. It is shown that the optimization project and the practical case are consistent with each other. Accordingly, the three-dimensional finite element numerical simulation is the basic theory of optimization design of engineering structures of dewatering in deep foundation pit in such areas.51234
Key words：Yangtze River Delta, land subsidence, deep foundation pit dewatering, seepage control
1. Introduction
The Yangtze River Delta is one of the most economically developed areas in China. In recent
years, with the rapid development of the national economy and the continuous expansion of urban construction, the development of underground space has received more and more attention. Quaternary loose sediments with great thickness distributes in the Yangtze River Delta, in which form a few pore-confined aquifers with large thickness. The aquifers are pided by the clays with weak permeability, but there is close hydraulic relation among these aquifers. Because the thickness of the aquifer is large, the burial depth of the impervious base is deep, the water level is high, and the quantity is abundant, it is difficult for the retaining wall of foundation pit shield to reach the base, and the foundation pit dewatering is exceedingly easy to make a dramatic decrease of the underground water level surrounding the dewatering area and cause land subsidence and geologic disasters, even major security incidents if not handled properly[1,2]. Therefore, based on the characteristics of three-dimensional geological body, it has become a top priority to design rationally the entire engineering structures of dewatering if economic and technological conditions are available, so that the demand of the decrease of the underground water level in deep foundation pit can be met and meanwhile the decrease of the underground water level around foundation pit is incapable of causing land subsidence and geologic disasters. The forth subway of Dongjiadu tunnel repair foundation pit dewatering in Shanghai was used as an example, this work aims to elaborate the theories and methods of three-dimensional finite element simulation[3] of deep foundation pit dewatering based on the control of land subsidence in such areas. According to the numerical simulation of different engineering structures of dewatering, the optimal program of dewatering is determined, and the practice has proved that this program is correct and reliable and can control effectively the land subsidence around the foundation pit. Accordingly, three-dimensional finite element numerical simulation is the basic theory for designing engineering structures of deep foundation pit dewatering in such areas.
2. Mathematical model of groundwater seepage flow
2.1 Governing equation
By taking the direction of the main infiltration corresponds with the coordinate axis direction in anisotropic porous medium, the governing equation for three-dimensional transient flow is[4]