4.2 Spatial discretization and boundary conditions

To overcome the capriciousness of results caused by the uncertainty of the boundary, according to the principle that boundary of constant head should be far away from the source and sink term, this calculation takes furthest border points in the east, west, south and north of the entire foundation as starting points by using a trial-and-error procedure, and expands outward about 400 m in each direction, namely the actual plane size is 1060 m2×870 m2 , all around are treated as boundary of constant head, and the sea level of the foundation center is taken as the origin of coordinates. According to the hydrogeologic characteristics of the study area and in order to meet the requirements of the retaining wall of foundation pit shield and the filters of pump wells, quaternary loose sediments entirely is pided into 10 layers vertically, and partition in the horizontal direction becomes sparse gradually from the foundation center to the outward, which can be seen in Fig.2. The finite element mesh is subpided into a total of 16280 nodes and 13980 units.

4.3 Identification and verification of model

The water level descending period of six pump wells labeled as W23, W24, W25, W26, W27 and W28 was selected for the model identification, which was from 21:15 on January 24, 2006 to 19:38 on January 29, 2006, and the recovery period was for the model confirmation, which was from 19:38 on January 29, 2006 to 14:06 on February 1, 2006 when pumping water was stopped. The whole process was pided into five stress periods, and every one was also pided into several steps. Other layers except Layers1, 2 and 10 had water lever observation wells used for fitting, the filters of water level observation holes outside the foundation pit, namely Q11, Q9, Q7, Q5, Q3 and Q1 were located in the Layers 3, 4, 5(6), 7, 8 and 9 of the model separately, and the filters of water level observation holes W17, W19, W21, W22, B5 and B6 in the foundation were located in the Layers 5(6). Figure 3 shows the plane distribution drawing of pump wells and observation wells. Based on the water level fitting of the twelve observation holes mentioned above, the hydrogeologic parameters of various aquifer are obtained. Layers 5 and 6 where the pump well filters are set are in the same parameter zone, which is pided into seven parameter zones respectively, the Layers 4 and 8 are pided into two parameter zones respectively, and the Layer 7 is into four, while others are in a parameter zone respectively. The parameter values of every parameter zone can be seen in detail in Table 1. The fitting precision for groundwater table of the observation holes W17 in the foundation pit and Q11 outside the foundation pit is shown in Fig.4. The hydrogeologic parameters zoning map of Layer 5 is shown in Fig.5. According to the fitting results, the general changes of the calculated head tends to be in accordance with those of the observed head. Therefore, this model can be used to simulate and forecast.

  4.4 Model forecast

According to the above mathematical model after identification and verification, the optimum design of dewatering of the forth subway of Dongjiadu tunnel repair foundation pit in Shanghai was performed. In view of economy and technique, the final optimization program of dewatering was obtained by simulation and comparison analysis for different programs, which shows that a total of 25 wells will be required, the distribution of wells can be seen in Fig.6, and the pump discharge of each well can be seen in Table 2. The filters of the

 pump wells are set between 44 m to 59 m, the retaining wall of foundation pit shield goes deep to 65 m (see Fig.7). After 30 d of dewatering, the water level in foundation pit will fall down to below − 38.8 m (42.25 m in the burial depth), so that the demand of dewatering can be met, and the maximum decrease of water level outside the foundation pit will be 3 m, which is incapable of causing destructive land subsidence based on the existing experience in Shanghai. Figure 8 shows the plane water level isogram map of the bottom of the foundation pit (Layer 4 in the model) after 30 d of dewatering, and Fig.9 gives the section water level isogram map at the center of the foundation pit in the Y direction after 30 d of dewatering. By the verification of follow-up project, it is shown that the above optimization program and the practical case are consistent with each other, so it is effective to control the land subsidence and geologic disasters around the foundation pit. According to observation by the follow-up project, the maximum land subsidence outside the foundation pit is only 3.5 mm, which guarantee the security of the Linjiang Garden Building effectively. 三维有限元数值模拟深基坑防渗降水英文文献和中文翻译(3):http://www.751com.cn/fanyi/lunwen_54797.html