Study On The Variation Of Water Level And The Deformation Due To Dewatering Of Deep Foundation Pit Of Subway Station

A significant number of literatures have reported on research achievements of dewatering mechanisms outside deep foundation pits and the resulted deformation of underground diaphragm walls and ground surface.But if the aquifers show a large thickness and there is dewatering in foundation pits,it is impossible for underground diaphragm walls to completely isolate the aquifer insides and outside the foundation pits due to the limitation of the technical and economic conditions.Besides,dewatering wells in foundation pits are mainly partially penetrating wells and dewatering is expected to exert an influence on the underground water to some depth below the wells,causing by-pass seepage at the bottom of the underground diaphragm walls to further impact the deformation of the surrounding ground surface and underground diaphragm walls.There has not been any deep and systematic research on the variation of water level outside of foundation pits and deformation of surrounding ground surface and underground diaphragm walls caused by dewatering of deep foundation pits in the event of by-pass seepage.In respect of key concerns about the impacts of dewatering of deep foundation pits of metro stations in the event of by-pass seepage,a deep and systematic research on the mechanisms,laws and control measures for the variation of water level and deformation outside foundation pits caused by dewatering of deep foundation pits of metro stations in the event of by-pass seepage has been developed by means of laboratory model experiments,theoretical analysis,numerical modeling and field monitoring and so on.Specific work involves:firstly,the mechanisms and laws of the variation of water level outside foundation pits caused by dewatering of foundation pits in the phreatic aquifer and artesian aquifer are studied based on laboratory model experiments and theoretical analysis and then the mechanisms of ground surface settlement and the laws of deformation outside foundation pits caused by dewatering of foundation pits in the phreatic aquifer and artesian aquifer are researched;secondly,based on the actual deep foundation pits engineering of metro stations,a 3D fluid-structure interaction numerical model is built,which has been verified by field monitoring data,and then a theoretic calculation method is verified by the computations of the model and field monitoring data;finally,the laws of the variation of water level outside foundation pits,ground surface settlement outside foundation pits and deformation of underground diaphragm walls caused by dewatering of foundation pits in different dewatering conditions of the phreatic aquifer and phreatic-artesian aquifer are studied based on the numerical model,influence factors exerted on ground surface settlement outside foundation pits by dewatering of foundation pits in the phreatic aquifer and phreatic-artesian aquifer are analyzed,and control measures are proposed for the settlement of adjacent buildings and ground surface outside foundation pits due to dewatering of foundation pits in the typical aquifers like phreatic aquifer and phreatic-artesian aquifer.The main work is summarized as follows:(1)Dewatering model tests of partially penetrating wells of foundation pits are designed,based on which the laws of the effective influence depth variation of dewatering of partially penetrating wells in the phreatic aquifer and artesian aquifer in the event of by-pass seepage of underground water and laws of the pore water pressure variation in the soil layers outside foundation pits are analyzed.And then a computing method for the effective influence depth of dewatering of partially penetrating wells in the phreatic aquifer and artesian aquifer,computational formula for water level variation curves outside foundation pits,and computational formula for partition of by-pass seepage zones outside foundation pits are proposed.The laws of the pore water pressure variation in soil layers outside foundation pits caused by large-depth and layered dewatering of partially penetrating wells in foundation pits in the typical aquifers of phreatic aquifer and phreatic-artesian aquifer are revealed.Water level variation laws outside foundation pits caused by injecting separately at different depths and inclination angles outside foundation pits after dewatering of partially penetrating wells in foundation pits are put forward.(2)Based on the dewatering curve formula outside foundation pits determined by model tests and theoretical analysis,an accurately corrected computational formula for ground surface settlement outside foundation pits caused by dewatering of partially penetrating wells in the foundation pits in the phreatic aquifer and artesian aquifer is raised.The verification results show that the corrected computational formula is more consistent with the test results.The laws of the ground surface settlement and variation of underground diaphragm walls outside foundation pits caused by large-depth dewatering of partially penetrating wells in foundation pits in the typical aquifers of phreatic aquifer and phreatic-artesian aquifer are revealed.The laws of the ground surface settlement and variation of underground diaphragm walls outside foundation pits caused by injecting separately at different depths and inclination angles outside foundation pits after dewatering of partially penetrating wells in foundation pits are put forward.(3)Based on actual deep foundation pit engineering of metro stations,a 3D numerical model of full size is built,of which the computations are verified by the acquired field monitoring data of water level variation outside foundation pits,ground surface settlement outside foundation pits and deformation of underground diaphragm walls.And then based on the theoretical methods obtained in this paper,the water level variation and ground surface settlement outside foundation pits caused by actual dewatering of foundation pits of metro stations are calculated.The theoretical calculation results are compared to the numerical modeling and field monitoring results to verify the accuracy and reasonability of the theoretical methods in practical engineering applications.(4)Relying on actual deep foundation pit engineering of metro stations,laws of water level variation outside foundation pits,deformation of underground diaphragm walls and ground surface settlement outside foundation pits caused by dewatering of foundation pits in different dewatering conditions are proposed based on typical aquifers of phreatic aquifer and phreatic-artesian aquifer,and the relationships between different dewatering plans and foundation pit size are discussed.(5)The ground surface settlement laws outside foundation pits caused by dewatering of foundation pits are revealed in different influence factor conditions in the typical aquifers of phreatic aquifer and phreatic-artesian aquifer based on numerical modeling.The variation sensitive range of ground surface settlement outside foundation pits corresponding to the osmotic coefficient,anisotropy coefficient and gravel cohesive soil layer thickness of different aquifers are proposed.(6)Measures to control settlement of adjacent buildings and ground surface caused by dewatering of foundation pits in terms of typical aquifers of phreatic aquifer and phreatic-artesian aquifer are studied in combination with laboratory test results with the limited urban space conditions considered.The researches show that:all 20° inclined injection wells for dewatering of foundation pits show the optimal control effect on settlement of adjacent buildings for 20° inclined injection wells can significantly improve the ground surface settlement outside foundation pits,greatly increase the scope and extent of the control influence exerted by injection wells on the ground surface settlement on the side away from underground diaphragm walls,and at the same time decrease the extent of the control influence on that on the side close to underground diaphragm walls.