Coupling Effect Of Dewatering And Excavation On Surrounding Environment And Water Level Control In Deep Confined Aquifer

Environmental impact and safety control of deep foundation pit engineering are important problems faced by urban infrastructure and underground space development.The coupling effect of deep excavation and dewatering will inevitably cause the change of stress state and additional displacement of surrounding soil,which will threaten the service safety of adjacent underground structures.The Quaternary overburden in the Yangtze River Delta is thick,and there are multiple confined aquifers.Suspended waterproof curtains are often used for deep foundation pit dewatering scheme design in this area.The geological environment risk caused by groundwater extraction is extremely serious,leading to frequent engineering accidents and even causing serious economic losses and social problems.Therefore,it is of great theoretical significance and application value to reasonably predict the surrounding environment response law under the coupling influence of deep foundation pit excavation and dewatering,and then propose effective groundwater disaster control technology for the development and construction of urban underground space in complex environment.Based on the National Natural Science Foundation Project（51878157）,aiming at the typical deep foundation pit projects in the Yangtze River Delta（underground projects in Nanjing Yangtze River floodplain area and Changzhou Metro Line）,this paper lucubrates the coupling effect of dewatering and excavation on surrounding environment and groundwater control method in deep confined aquifer through the comprehensive technical means of in-situ test,numerical simulation,theoretical analysis and field test verification.The main research contents and achievements are as follows:（1）This paper comprehensively summarizes the research status of the impact of foundation pit dewatering and excavation on the surrounding environment at home and abroad,summarizes the research results of the effects of dewatering in confined aquifers,the coupling effects of dewatering and excavation,and the evaluation of dewatering control technology,and points out the problems that exist and need to be solved.（2）The distribution features of seepage field and displacement field caused by dewatering in confined aquifer under different soil stress history were discussed,and the influence mechanism of soil stress history on dewatering effect was analyzed and summarized.The reaults showes that:due to the consideration of the soil stress history,the deformation water release of the soil decreases,and the drawdown range and value of the confined water head under the same dewatering condition are larger than those under the condition without considering the stress history.The study also points out that the soil deformation under the working condition considering the stress history is significantly reduced in each dewatering stage,and the final maximum settlement range is more concentrated near the surface.（3）Based on the in-situ test,the determination method of the key geotechnical design parameter-state parameter OCR of typical soil layers in Changzhou area is given.Firstly,based on the modified Cambridge model,a semi-theoretical and semi-empirical formula for calculating OCR of clayey soil layers in Changzhou is proposed through CPT and laboratory tests.Secondly,the OCR empirical formula based on normalized cone tip resistance Q_t is proposed for silty clay sand layer and silty sand layer.Finally,through the regression analysis of DMT test and laboratory test data,the OCR empirical formulas of silty clay with silty sand layer and silty sand layer are proposed respectively.（4）The whole process of the dewatering and excavation of deep foundation pit with suspended curtain was simulated through the three-dimensional fluid-solid coupling calculation model considering transient dewatering.The deformation law of the soil around the foundation pit is revealed,and the division method of soil deformation influence zone around the foundation pit is proposed.The reaults showes that:due to the groundwater seepage outside the pit,the vertical and horizontal deformation range of the surrounding soil can reach 7 H_e（H_emeans the excavation depth）.Taking the vertical displacement of the soil up to 0.25%H_e and0.15%H_e,and the horizontal displacement up to 0.15%H_e and 0.10%H_e as the division standard,the soil deformation influence zone was divided into the main influence zone（I）,the secondary influence zone（II）and the general influence zone（III）.By defining the width coefficient M and depth coefficient N of the influence zone,and studying their variation law under the conditions of different maximum lateral displacement(δ_hm)of curtain and different permeability coefficient（k）of confined aquifer,the prediction method of the boundary of each influence zone are given.（5）According to the location relationship between the foundation pit and the adjacent underground structures,the method of dividing the unloading calculation area of excavation is proposed.As for the excavation of the suspended curtain foundation pit,the unloading equivalent method that can consider the combined effect of the unloading of the pit bottom and the curtain is proposed.Furthermore,the calculation method of the groundwater drop outside the pit and the calculation formula of the additional stress due to dewatering are proposed.Finally,based on the two-stage stress analysis method,combined with Mindlin’s solution and Pasternak’s two parameter elastic foundation beam model,the stiffness matrix of the tunnel and pile foundation deflection differential equation is given.The analysis of influencing factors indicates that during the excavation and dewatering of suspended curtain foundation pit,the changes of groundwater seepage field and confined water head outside the pit have significant effects on the tunnel.Compared with the distance between the excavation and the tunnel and the excavation depth,the size of the excavation has the least impact on the dewatering and excavation effect.（6）In view of the complex situations such as the application of multiple waterproof curtains in engineering practice,the existence of new and old diaphragm walls,and multi-wall interference of group pit dewatering,the control technology of multi-grade dewatering was studied in this paper,and the generalised method to calculate the head difference at two sides of each suspended curtain during multi-grade dewatering is proposed.It is pointed out that the multi-grade dewatering control technology can reduce the risk of engineering disasters and environmental impact by reducing the water head difference near the first curtain and extending the groundwater seepage path outside the pit.According to the thickness and permeability coefficient of the confined aquifer on the site,and the requirements of foundation pit excavation and the surrounding water level control,combined with the calculation method proposed in this paper,the pumping rate of the well,the relative depth of the curtain inserted into the confined aquifer,and the distance between the two curtains can be designed,thus forming the safety control and design method for multi-grade dewatering.