Analysis Of Surface Subsidence And Influence Caused By Dewatering Of Confined Water

With the development of urban construction,underground space is constantly developed,high-rise buildings,open-cut tunnels,rail transit stations and other structures are constantly emerging in the city.With the increase in the number of foundation pit projects,they are also developing in the direction of large and deep,which increases the technical difficulty and safety risks for the foundation pit projects with complex construction mechanics behavior,such as weak underlying layer and confined aquifer encountered in the excavation process.How to overcome this problem from the perspective of technology and safety is the key to be solved in the process of urban construction.The research object of this paper is the foundation pit construction which is rich in confined water.The effective treatment method for confined aquifer is pumping water and reducing pressure.However,reducing confined water head will inevitably cause the settlement of surrounding buildings,which will endanger the safety of underground engineering structure and surrounding buildings or structures if improperly treated.Therefore,it is necessary to study the surrounding land surface settlement caused by the precipitation of confined aquifer.In this paper,the formula of surface subsidence induced by pumping of confined aquifer is derived by combining theoretical derivation with numerical simulation.Combined with Jian An tunnel in Bozhou,the law of surface subsidence under different precipitation modes is discussed,and the influence of different factors on the precipitation of confined aquifer is analyzed.The main contents and results are as follows:(1)Based on the whole well seepage theory and Mindlin displacement solution,the subsidence formula of the ground surface caused by pumping of confined aquifer is deduced.A three dimensional finite difference model of single well pumping pressure reduction in confined aquifer is established,and the law of surface subsidence after rainfall in confined aquifer is analyzed.The theoretical calculation is in good agreement with the model calculation results.The results show that: Depressurized precipitation will cause the compression deformation of aquifer soil.Surface subsidence shows a funnelshaped curve,and the farther it is from the dewatering well,the smaller the subsidence value is.(2)Based on Jian An tunnel in Bozhou,the FLAC3D numerical model is established to calculate the groundwater seepage and surface subsidence using different dewatering modes.The results show that dewatering outside the pit can reduce the head pressure outside the retaining wall and reduce the lateral movement of the retaining wall,but it has a great influence on the surrounding environment.When the pit dewatering mode with water stop curtain is adopted,the water stop curtain reduces the hydraulic connection between inside and outside the foundation pit and can effectively reduce the influence on the surrounding environment.However,the large water head difference between inside and outside the foundation pit will cause the retaining wall to move toward the inside of the foundation pit.(3)Based on the pit precipitation model with cantilevered water stop curtain,the influence of the insertion depth of water stop curtain,aquifer parameters and the arrangement of dewatering well on the surface settlement after dewatering of confined aquifer is analyzed.The results show that when the insertion depth of water stop curtain reaches 80%,the effect of water stop curtain is very obvious,the settlement value decreases by 24%,and the influence range decreases by 19%.The orthogonal test shows that the elastic modulus and Poisson's ratio are significant factors in the aquifer coefficient,and with the increase of the elastic modulus,the maximum value of the surface subsidence and the influence range of the subsidence decrease,but the decrease is not significant.When the number of dewatering Wells meets the requirement of dewatering,with the increase of the distance between the dewatering Wells and the water stop curtain within30 m from the center of the foundation pit,the surface settlement decreases continuously and the settlement curve decreases from steep to slow.In the range of 50 m from the center of the foundation pit,with the increase of the number of dewatering Wells in the foundation pit,the surface settlement becomes larger.