Effect Analysis Of Foundation Pit Excavation On Proximate Station Deformation

With the rapid development of the national economy and the continuous acceleration of the urbanization,the number of permanent residents in China’s cities has been rising.The expansion of modern cities has created an intensified traffic pressure,the subways came into being due to it.The buildings along the subway lines have risen vigorously,now the amount of tall buildings as well as subway lines is still climbing.Under such circumstances,the situation is becoming more and more commonplace which the high-rise foundation pit and the subway line network are adjacent to each other,but the mutual influence between the both cannot be ignored.The unloading of excavation breaks the original equilibrium state of the surrounding,and the induced stress redistribution changes the initial stress field and displacement field of the soil,further causing additional deformation and internal forces in the subway station.How to carry out the construction of deep foundation pit safely and efficiently under the premise of not affecting the normal operation of existing subways has become an urgent problem to be solved.This paper studies on the foundation pit of the outer ring project of Longhu Financial Island in Zhengdong New Area adjacent to Zhengzhou Rail Transit Line 4,and adopts to the finite element analysis software Midas/GTS to construct a three-dimensional model containing soil,enclosure structure,supporting system and subway station.The model discovers the influence of factors like excavation process,thickness and length of enclosure and groundwater on the deformation of adjacent subway station.The research has achieved the following main results:Theoretically,a method of calculating the uplift(i.e.,the upheaval)amount of the pit bottom caused by soil unloading during the excavation has been put forward.Then,a deformation response calculation model for the underground structure surrounding the subway line is constructed.The Middelin formula is used to derive the additional internal force that is generated inside the structure of the under-operation subway station when the pit is excavated and unloaded.The appropriate foundation model and its parameters are further chosen,and the structural deformation formula of the existing underground subway station under the excavation unloading effect is obtained.The simulation analyses show that the foundation pit excavation makes the floor of the adjacent subway station overall float,and the most disadvantageous positionappears at the bottom of the station near the end of the foundation pit.However,the influence of excavation on the bending moment of the station floor is small,but the floor close to the wall appears negative bending moment,which may cause cracking of the walls.The simulation results show that the walls of the subway station deviate to the side of the foundation pit as a whole due to the influence of the adjacent excavation.Specifically,the excavation of foundation pit exerts a greater impact on the displacement of the station walls within its width,and a smaller impact on the bending moment of the walls.The maximum displacement occurs at 3-4m from the top and bottom of the wall.The simulation results also show that the excavation of the adjacent foundation pit causes the vertical deformation of the columns in subway station.The maximum displacement occurs at the end of the station near the foundation pit.At the same time,due to the uneven vertical displacement,the differential settlement occurs and the maximum value locates in the width side direction of the foundation pit.The excavation has little effect on the axial force of the column,but large effect on the pressure at the bottom of the column.Real-time monitoring should be implemented in order for preventive measures.