Investigation Of Environmental Impact Induced By Pumping During Excavation In Multi-aquifer System

This study was undertaken to evaluate the groundwater seepage behavior of aquifer caused by pumping with waterproof curtain in the foundation pit.Model test,numerical simulation and back analysis of measured data of field cases were adpoted.Based on the model test,variation of groundwater level induced by dewatering-excavation,wallpumping well distribution is analyzed so that the effective filter length,the effective depth of waterproof curtain are obtained.The method to calculate water level difference between the two sides of the waterproof curtain for partially penetrating well is modified.A three dimensional groundwater seepage model with 1D consolidation model is built to discuss the groundwater seepage and settlement caused by the excavation-dewatering.The concluding remarks of this study are summarized as follows:(1)Laboratory investigation of water level distribution in confined aquifer during dewatering.With the similar material in the dewatering model test,the effect of excavation-dewatering,monitor length,and buried depth of waterproof curtain are investigated.Test results show that: i)When the water level keeps stable during dewatering process,with a constant flow rate,water level will be upwards with the excavation of soil in the foundation.The value of water upwards size depends on the depth of excavation.ii)When filter length increases,the inlet section of dewatering well increases,and the water outside the waterproof curtain will flow into easily.Water level difference on both sides of the waterproof curtain becomes smaller,which will strengthen the environmental effects of surrounding soils.iii)When the buried depth of waterproof curtain increases,the cross section of water decreases and the seepage path increases.Water level difference on both sides of the waterproof curtain becomes larger,which will decrease the influence of the surroundings.(2)Investigation of the reasonable buried depth of waterproof curtain and the optimal length of filters in confined aquifer.Based on the model test,a 3D groundwater seepage model with 1D consolidation model is utilized to analyze the environmental effect caused by foundation pit dewatering.Results show that: i)During dewatering process,the final drawdown for the grading precipitation and one-time precipitation are the same.The settlement caused by excavation accounts for about 24.8% of the total settlement;ii)In order to control the hydraulic gradient increment,filter length should not exceed 53% of the depth of the aquifer;If the settlement is considered as the control target,the optimal filter length should be 35% thickness of the aquifer;iii)In order to control the hydraulic gradient increment,the depth of waterproof curtain should be larger than 53% thickness of the aquifer and the optimal depth is 90% thickness of the aquifer;If the the settlement is considered as the control target,the optimal depth should be 73% thickness of the aquifer;(3)A modified method based on Wu's method to calculate water level difference between the sides of the waterproof curtain for partially penetrating well is proposed,in which anisotropy of aquifer is considered.A three-dimensional finite difference model(FDM)was established.Rusults show that: i)buried depth of waterproof curtain,filter length,anisotropy of confined aquifer,different positions in depth direction of filters are all have an effect on the distribution of drawdown in the depth direction of confined aquifer;ii)different positions in depth direction of filters have no effect on water level difference between the sides of waterproof curtain in the longitudinal direction.Only have an effect on the water level difference in the transverse direction.(4)Analysis of foundation pit dewatering of case histories.Based on measured data of a metro station in Ningbo and a metro station in Shanghai,effect on groundwater seepage and surrounding environment caused by dewatering is analysed.3D groundwater seepage model with 1D consolidation were utilized in the analysis.First,back analysis of the hydraulic parameters with the observed data;Secondly,based on the hydraulic parameters,effects of buried depth of waterproof curtain and filter length on groundwater seepage and surrounding environment are analyzed;Lastly,the optimal buried depth and the optimal filter length are obtained,furtherly verify the correctness of the model tests.Results show that: i)In order to control the hydraulic gradient increment,filter length should not exceed 52% of the depth of the aquifer;If the settlement is considered as the control target,the optimal filter length should be 36% thickness of the aquifer;ii)In order to control the hydraulic gradient increment,the depth of waterproof curtain should be larger than 53% thickness of the aquifer and the optimal depth is 90% thickness of the aquifer;If the the settlement is considered as the control target,the optimal depth should be 73% thickness of the aquifer.