| Three quarters of Nanjing is in the flood plain area of the Yangtze River and the Qinhuai River,the soil layer distribution has typical binary structure characteristics,rich groundwater has great impact on foundation pit excavation.Compared with a single foundation pit,the construction period of foundation pit group is longer,the volume is larger and the working conditions of synchronous or cross dewatering construction are more complex,which greatly increases the difficulty of seepage control of confined water,in the process of dewatering,it is faced with significant problems of surrounding environmental safety and foundation pit interaction.This paper,which is based on the foundation pit group engineering of a subway tunnel in Nanjing,processed the pumping test and settlement monitoring data of the test with Aquifer Test and Abaqus software,analyzed the impact of the design scheme on the surrounding environment of the foundation pit group,and studied the optimization effect of different groundwater seepage control measures.The main work of this paper is as follows:1.Analyzed the pumping test data,calculated the permeability coefficient,water storage coefficient,water storage rate and influence radius of the pumping test of the confined aquifer(3)silty sand,silty fine sand layer and(4)medium coarse sand layer under the foundation pit by method of the steady flow formula and unsteady flow analysis respectively,studied the hydraulic connection between the underlying aquifers in the floodplain area of the Yangtze River.2.By establishing the finite element model of group pumping test to verify the parameter calculation results of the analytical method,the results show that the difference between the simulated value of groundwater level in the observation well and the measured value is less than 10%,which meets the needs of engineering design and can be used as the basic parameters of numerical simulation of the foundation pit group project.The calculation results of group pumping test parameters are as follows:the horizontal permeability coefficient of layer(3)is k_h=28.5m/d,the vertical permeability coefficient is k_v=3.85m/d;And for layer(4),k_h=72.5m/d,k_v=11.6m/d.3.By analyzing the settlement and drawdown data,it can be seen that the variation trend of layered settlement caused by dewatering with time is consistent with the ground settlement and basically consistent with the variation trend of confined aquifer water level;After the pumping test,the foundation soil did not rebound,and the settlement of some foundation soil near the pumping well increased slightly due to consolidation;The hyperbolic method is used to predict the long-term settlement of pumping test,and it is concluded that the settlement correction coefficient of phreatic layer is 50%and that of confined aquifer is 10%.4.According to the dewatering design scheme of the foundation pit group,established the finite element numerical model.The results show that the change of groundwater level around the foundation pit group is mainly caused by the dewatering of the foundation pit in the standard section and the south section,while the change of water level within 120m outside the middle section and the north section is mainly caused by the dewatering of the foundation pit in the middle section and the north section.During the dewatering of the foundation pit group,the maximum drawdown of the target aquifer(3)-1 silty sand layer outside the pit is about 7.5%of the drawdown inside the pit.5.According to the results of numerical simulation,the deeper waterproof curtain is,the better groundwater seepage control effect there will be.It is appropriate to increase the depth of waterproof curtain around the foundation pit group between mileage K8+872 and K9+024.811 by 4m to isolate aquifer(3)-2;The depth of the waterproof curtain inside the foundation pit group can be reduced by 8m to take advantage of the superposition effect of dewatering between adjacent foundation pits.It can significantly reduce the drawdown and the settlement outside the foundation pit by setting recharge wells 10m away from the edge of the foundation pits with an interval of 20m and the depth of the filter pipe is 27m～43m,the recharge volume of single well is 120～300m~3/d.The simultaneous dewatering of adjacent foundation pits and large-scale foundation pits shall be avoided during the divisional dewatering of foundation pits;The relationship between the drawdown around the foundation pit group under different dewatering well layouts is as follows:the layout inside the foundation pit>the layout evenly in the foundation pit>the layout at the edge of the foundation pit;Additional dewatering wells can reduce the drawdown outside the pit,but the impact is small;On the premise of meeting the dewatering needs,reducing the length and depth of the filter pipe can improve the dewatering effect and reduce the influence of foundation pit group dewatering;The RJP construction method is used to seal the bottom of the foundation pit in the north,middle and south sections of the foundation pit group to form an impermeable layer,which can cut off the hydraulic connection between the internal and external groundwater and reduce the impact of dewatering. |
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