Study On The Characteristic And Control Of Soil Deformation Induced By Dewatering In Confined Aquifer During Excavation

When the underground space exploitation is undertaken in an area rich in groundwater,the groundwater need to be treated,especially lower the confined hydraulic pressure.Dewatering will inevitably change the stress condition of soil and affect the soil deformations,which will raise new difficulties and problems to the safety construction and the environment protection of the foundation pit.Therefore,it is necessary to investigate the features and mechanism of groundwater seepage and soil deformation variation induced by pumping confined water and the couple effect between dewatering and excavation,and to provide some useful guidance for protecting the environment.In this paper,based on the engineering characteristics in Shanghai,the characteristic and control measures of dewatering on groundwater variation and soil deformations in a free-field and during the excavation were studied by field measurement,statistical analysis and numerical simulation.The influence features of dewatering confined water on groundwater variation and soil deformations development in a free-field were investigated.An analysis method for investigating the effect of dewatering confined water on ground deformations during the excavation were proposed.The control effect and mechanism of waterproof curtain and recharge on dewatering-induced environmental influence were investigated.The previous research results and the application effect of environmental protection measures were verified and analyzed by two deep excavation cases.The main contents and conclusions of this dissertation are as follows:(1)On the basis of revealing the responses of groundwater head and soil deformation in multiple aquifers to the dewatering in a confined aquifer,the distribution pattern of water head variations and soil deformation in each soil layer was obtained,and the prediction method of dewatering-induced ground surface settlement was proposed.Results show that:dewatering in the confined aquifer I will firstly lead to a significant reduction of the water head and soil layer compression in aquifer,and the aquitard I and aquitard II will be extended slightly because of the deformation compatibility.Then,the decrease of water head and compression deformation occurs in the aquitard,and the phreatic aquifer is extended slightly.After that,the water level of the phreatic aquifer decreases and all soil layers are compressed.The numerical results and measured data show that the distribution characteristic of ground surface settlement is different from that of water head reduction.Therefore,the prediction method of land subsidence based on stratified summation method is modified and the empirical prediction method of land subsidence distribution is proposed.(2)An environmental impact analysis method for dewatering in a deep foundation pit is proposed,and the influence and mechanism of dewatering on soil deformations during the excavation was revealed.Results show that:Dewatering in the confined aquifer during the excavation will aggravate the ground surface settlements and influence range will be greater than 4H_e behind the wall,where He is the excavation depth.The maximum dewatering-induced ground surface settlement appears at near 2H_e behind the wall.The dewatering-induced ground surface settlement profile beyond 2H_e behind the wall is inconsistent with drawdown profile in the confined aquifer,while the settlement profile within 2H_e behind the wall is quite different from the drawdown profile.The ground surface settlement caused by dewatering in the confined aquifer mainly results from the compression of the confined aquifer.It is found that the expansion occurs at the soil layers above the confined aquifer within 2H_e behind the wall because of the interaction of wall and soil and the unloading effect.Based on the above research,the distribution pattern of dewatering-induced ground surface settlement in deep excavation is proposed.(3)Three indexes,settlement control coefficient,recharge influence coefficient and recharge efficiency,were proposed to evaluate the control effect of waterproof curtain and recharge on dewatering-induced drawdown and ground surface settlement.Results show that:Increasing the wall penetration depth can effectively reduce the ground surface settlement beyond 2H_e behind the wall,but it is not conducive to controlling the ground surface settlement caused by the dewatering and excavation within 2H_e behind the wall.When the wall depth increases,the settlement control coefficient decreases,and the control effect on the dewatering-induced settlement will be better.Recharge has obvious effect on controlling the drawdown and soil deformation of recharge aquifer,basically has no effect on the overlying and underlying aquifers,and has little effect on adjacent aquitards.The response of groundwater level and soil deformation in adjacent aquitards lags and is slow.Under the same recharge flow,the further the recharge well is from the excavation,the smaller the recharge influence coefficient is,which means the recharge has a weaker effect on the pumping inside the pit,and the greater the recharge efficiency is,which means the recharge has a better control effect on the dewatering-induced settlement.Under the same recharge position,the larger the recharge flow is,the greater the recharge influence coefficient and the recharge efficiency are.(4)Through field testing and analysis of two foundation pit in Shanghai,this paper studied the response of pore pressure and soil deformation in each soil layer to the dewatering and recharge in the confined aquifer I,and discussed the actual control effect of recharging in the confined aquifer II.In addition,the design scheme of recharge in construction was optimized based on the evaluation indexes of recharge mentioned above.Results show that:the method mentioned above can well predict the distribution of dewatering-induced surface settlement.Recharge can reduce the soil compression in the recharge aquifer.Consequently,the ground surface settlement can be reduced.However,the compression in the above aquitard and aquifer induced by the preliminary dewatering can not be effectively reduced.In the presented case of Hanzhong Road station,recharge was conducted simultaneously with the pumping,so the dewatering-induced drawdown and ground surface settlement around the protected building had been well controlled,and final settlements of the building were controlled within 2.0 mm.