Research On The Infulence Of Foundation Pit Excavation On The Deformation Of Adjacent Metro Tunnel

Foundation pit excavation is a widespread underground engineering activity in current urban construction.More and more foundation pit projects are adjacent to existing subway tunnels.These adjacent excavation will have adverse effects on the existing subway tunnels,especially for the shield tunnels in soft soil areas,which may cause the tunnel seepage and mud leakage,even the damage of the tunnel structure and the distortion of the track bed,which seriously affect the stability of the tunnel structure and the operationalsecurity.How to accurately predict and control the influence of foundation pit excavation on adjacent soft soil metro tunnel is of great engineering significance.In this paper,mathematical statistics,theoretical analysis,orthogonal test,numerical simulation and other research methods are used to conduct in-depth research on the effect of the deformation of metro tunnel in soft soil due to adjacent excavation.The specific work is as follows:(1)32 typical excavation casesin soft soil areas in China which are adjacent to existingmetro tunnels were collected and sorted.By means of mathematical statistical analysis,effects of the horizontal distance betweenfoundation pit and tunnel,the relative height difference between foundation pit the tunnel,the space-time effect,soil reinforcementand dewatering were studied,and the basic rules of the adjacent tunnel caused by the excavation were summarized.(2)Starting from the theoretical analysis,a simplified calculation method for the deformation oftunnels caused by adjcent excavation was proposed.Forthe first step the Mindlin solution was used to derive formulas of the tunnel longitudinaladditional stresscaused by foundation pit excavation;the second stepthe deformation equation of metro tunnels was established based on model of double elastic beam and was solved by the finite difference method.The results from theoretical calculationwas compared with the finite element numerical simulation and the in-situ measurement data to obtain good consistency.The results show that model of double elastic beam is more advanced to analyze the deformation of the tunnel.The parameters were changed to study the effects of factors such as the horizontal distance between the foundation pit and the tunnel,the excavation depth of the foundation pit and the soil of different foundations on the deformation of the tunnel.(3)Aimed at Hangzhou typical soft clay,the relative height difference between the foundation pit and tunnel,the horizontal distance between the foundation pit tunnel,the stiffness of the retaining structure and the vertical spacing of support were taken as factors,and the maximum horizontal and vertical displacement of the tunnel were taken as the assessment index.The orthogonal array of L9(34)was selected for the orthogonal experiment,and the results of each test were calculated with the Plaxis 3D finite element software.With the method of range and variance analysis,the order of importance and the influence mechanism of the factors on tunnel deformation were revealed,and the optimal combination of factors to controll the deformation of the tunnel and the suggestion of parameter optimization were obtained,which can provide reference for similar projects.