Study On Unloading Effect Of Subway Tunnel Caused By Building Basement Excavation

Developing rail transit is an inevitable choice for urban development.Nowadays,urban subway tunnels crisscross and extend in all directions.All along the subway tunnel are the hot spots for the development of large commercial and living areas,but the commercial development often lags behind the development of rail transit,which makes more and more large-scale foundation pit projects close to the subway tunnel in operation.The relationship between the position of foundation pit and tunnel is more and more complicated.Therefore,it is necessary to further study and analyze the new characteristics between the foundation pit engineering and the tunnel structure,and the results have certain reference value to the actual engineering.Based on Chongqing Central basement foundation pit project,the finite element software is used to numerically simulate the influence of foundation pit excavation on the stress and deformation of metro Line 1 in operation.The main research contents and relevant conclusions are as follows:(1)The mechanism of uplift deformation and horizontal convergence displacement under unloading of surrounding rock is analyzed.The additional stress calculation method based on Mindlin solution and Winkler elastic foundation model can be used to obtain the uplift deformation location value of the existing tunnel structure,which verifies the feasibility of the numerical simulation results.(2)ABAQUS was used to numerically simulate the unloading of surrounding rock around the tunnel under different engineering factors,and explore the influence of tunnel burial depth,length of foundation pit,width of foundation pit,depth of foundation pit and thickness of surrounding rock retained on both sides of the tunnel on the existing tunnel.The results show that the stress and deformation of the tunnel are different under the influence of different engineering parameters.Generally speaking,the influence degree of different engineering factors on the tunnel is ranked as the depth of the tunnel,the excavation length of the foundation pit,the thickness of the retained surrounding rock on both sides of the tunnel,the excavation depth of the foundation pit,and the excavation width of the foundation pit.The analysis shows that the difference between the maximum displacement and the minimum displacement in the vertical direction of the tunnel along the axis direction of the tunnel is used as the basis to judge the unloading effect,and this value can be used as a reference value to judge whether the tunnel is safe.(3)The numerical simulation method is used to study the minimum thickness of surrounding rock above and on both sides of the tunnel when the basement of Chongqing Center is excavated,in order to ensure the safe operation of metro Line 1 under the condition of surrounding rock unloading.The sudden change of stress in the tunnel and the difference of vertical displacement along the vertical axis of the tunnel are proposed as the basis to judge whether the tunnel is safe.The calculation results show that: when the surrounding rock of subway Line 1 tunnel is unloaded,the surrounding rock with a thickness of 4m is retained above the tunnel,while the surrounding rock with a thickness of 6m on both sides of the tunnel,there is a sudden change of stress and the difference of vertical displacement is relatively significant.Therefore,when the basement foundation pit of Chongqing Central is excavated,at least 4m surrounding rock thickness should be retained above the tunnel structure of Metro Line 1 and at least 6m surrounding rock thickness should be retained on both sides.The numerical simulation results are co MPared with theoretical analytical calculations,and the results are in agreement with each other,which shows that the standard proposed in this paper is feasible.(4)For the surrounding rock unloading project of the subway,reasonable reinforcement measures can ensure the stability and safe operation of the tunnel structure.In this thesis,two tunnel reinforcement methods are selected for co MParative analysis:non-backing platform reinforcement(reinforcement 1)and backing platform reinforcement(reinforcement 2).It is found by numerical simulation that both reinforcement methods can effectively control the deformation of the existing tunnel structure,but the control effect achieved by reinforcement 2 is more significant.In the actual project,the two sides of the tunnel adopt the backout reinforcement measures and achieve the purpose of controlling the deformation of the subway tunnel,which shows that the research results of this thesis are correct.