Analysis Of Impact Of Foundation Pit Excavation On Adjacent Existing Tunnels

Foundation pit engineering is the most common underground engineering activity in urban construction.With the construction of urban subways,more and more foundation pits are adjacent to existing tunnels.Foundation pit engineering construction will have an adverse impact on existing adjacent tunnels,especially when excavating foundation pits in water-rich areas.Both precipitation and excavation unloading will change the stress field of the surrounding soil,which will cause existing tunnels to generate internal forces.And deformation.Light cases cause water seepage in the tunnel and severely affect the safe operation of the tunnel structure.Therefore,it is of great engineering significance to reasonably analyze and predict the impact of foundation pit excavation on adjacent existing tunnels.This article mainly uses theoretical calculation methods to carry out research on this subject.The main contents and conclusions are as follows:(1)Starting from the theoretical analysis,the horizontal and vertical additional stresses of the surrounding soil caused by excavation and unloading of the foundation pit are calculated based on the Mindlin basic solution.The Dupuit assumption is introduced to determine the precipitation infiltration curve of the foundation pit.The calculation formula of the vertical effective stress in the influence range of pit precipitation is to obtain the vertical additional stress in the precipitation range caused by precipitation and excavation unloading.Through hypothetical analysis,the effects of precipitation and the stress release of the side wall of the foundation pit on the additional stress of the tunnel in the affected area were verified.The results show that the existing tunnel additional stress obtained by considering the effect of precipitation and the side wall unloading of the foundation pit is more accurate.(2)The shield tunnel is equivalent to a homogeneous tube according to the principle of continuous equivalence.Based on the Winkler foundation model,the equivalent existing tunnel is assumed to be a long elastic beam embedded in the ground.Internal forces and deformations under stress.Based on two examples of foundation pit excavation projects adjacent to existing tunnels,the theoretical analysis method is used to calculate the displacement of the adjacent tunnel caused by the excavation of the foundation pit,and the theoretical calculation results are compared with the engineering monitoring data.The distribution curve agrees with the field measured data.(3)Based on the engineering research background of Yungu Road Station Tunnel of Line 1 of Hefei Metro,based on the finite element numerical software MIDAS GTS NX,a one-dimensional beam element is used to simulate an existing tunnel and a three-dimensional model of foundation pit excavation is analyzed to analyze the excavation of the adjacent tunnel caused by excavation Internal forces and deformation.The theoretical calculation of the displacement of the tunnel and the curve of the internal force are basically the same,and the maximum position of the tunnel displacement and the internal force are basically the same.The maximum errors of the horizontal and vertical displacements of the tunnel calculated by the theoretical and numerical calculations are 15.94% and 11.63%,respectively.The maximum bending moment errors are 6.7% and 8.5%,respectively 38.07% and 4.8%.(4)By analyzing the parameters of the tunnel stress and deformation caused by the excavation of the foundation pit,the depth of the foundation pit,the distance between the side wall of the foundation pit and the tunnel,the buried depth of the tunnel,the length of the foundation pit,the depth of the foundation pit,and the water supply are studied.The influence of degree on the internal force and deformation of the existing tunnel.The results show that the length of the excavation,the depth of the excavation,the depth of the tunnel burial,and the distance between the side wall and the tunnel of the foundation pit have more significant effects on the tunnel,while the width of the foundation pit and the water supply have a smaller effect on the tunnel.