Theoretical And Experimental Study On The Seepage And Deformation Of Metro Foundation Pit With Suspended Waterproof Structure

With the fast urbanization in many countries,a large number of underground infrastructures are constructed in central areas of cities.During the construction of these underground infrastructures,foundation excavation is a complex process and requires special attention.Engineers should not only pay attention to the safety of adjacent buildings during excavation,but also need to appropriately pump out groundwater.Especially in the water-rich strata,the deep foundation pit dewatering is an important measure to keep the foundation pit dry and excavation,and the pit dewatering often adopts closed waterproof structure and suspended waterproof structure.When the foundation pit is located in a thicker phreatic aquifer,suspended waterproof structure is more economical than closed waterproof structure.In order to improve the dewatering efficiency,it is worthwhile to comprehensively investigate the effect of the suspended waterproof structure on the groundwater drawdown.This study takes the subway foundation pit under the hydrological geological conditions of Nanchang as a case study,and carries out the research on the groundwater drawdown and deformation outside the pit caused by the dewatering in the subway foundation pit with suspended waterproof structure in the water-rich strata of Nanchang through model test,analytical analysis and numerical analysis.The main work of this paper is as follows.(1)With a deep foundation pit in Nanchang as the background,the model scale experiment is carried out by using similar materials to reduce the size of the pit project,and the mechanism of groundwater drawdown inside and outside the pit caused by the pit dewatering with suspended waterproof structure is studied through the model test,and the groundwater drawdown and difference of groundwater drawdown caused by dewatering under different length of waterproof structure is analyzed.(2)Based on the above model test,the numerical model of the model test is established by using Finite Element Method,and the process of pit dewatering with different drawdown depths and lengths of waterproof structure is simulated respectively.The accuracy of the numerical model is verified by comparing with the model test and analyzing the relationship on pumping time-depth-flow.Further,the changes of surrounding seepage field and groundwater drawdown caused by pit dewatering under different lengths of waterproof structure are obtained,and the effects of pit dewatering on the pit deformation and surface settlement are summarized.(3)The results of both model tests and numerical models show that the dewatering in the foundation pit with the suspended waterproof structure causes groundwater difference between the inside and outside of the pit,and the pit deformation and surface settlement can be reduced by controlling the groundwater drawdown difference.Based on the direction of the seepage paths calculated by the finite element software,the suspended waterproof structure of the foundation pit can be regarded as a partially penetrating well with a large diameter and the unsteady flow governing equation for large diameter partially penetrating well can be established.Boltzmann transformation is used to solve the unsteady flow equation of groundwater,and the equations of groundwater drawdown and flow outside the pit under the unsteady flow condition are derived,and a calculation method linking the water drawdown inside the pit and the water drawdown outside the pit is proposed to obtain the equation of water drawdown difference inside and outside the pit.The equation of water drawdown difference between inside and outside the pit is obtained.The sensitivity parameter analysis is carried out by normalization,and the factors affecting the difference of water drawdown inside and outside the pit are summarized.(4)Based on the model test of pit dewatering excavation,the mechanism of pit deformation and surface settlement under the action of dewatering excavation is studied by numerical analysis,which makes up for the shortcomings of the model test in the excavation,and the accuracy of the finite element model is verified by comparing the measured value of the model test with the calculated value of the numerical model.The numerical model of dry-sand excavation and dewatering excavation of foundation pit under different working conditions is further established,the horizontal displacement and bending moment of retaining wall caused by dry-sand or dewatering excavation of foundation pit are studied and analyzed,and the response of foundation pit under the coupling effect of water-soil is obtained.(5)Based on the case of subway deep foundation pit in Nanchang,a three-dimensional refined numerical model is established with Finite Element Method to study the three-dimensional seepage field caused by dewatering excavation of foundation pit with suspended waterproof structure,and analyze the deformation of foundation pit and surface settlement caused by different locations of foundation pit under the action of dewatering excavation,and obtain the law of foundation pit deformation and surface settlement caused by dewatering excavation of foundation pit under the coupling effect of water-soil,which provides reference for the design and construction of deep foundation pit dewatering.