Research On Effect Of Deep Excavation On Adjacent Tunnel And Corresponding Risk Transfer

With the demand of urban development,there are more and more foundation pit projects near existing tunnels.However,the excavation of foundation pit is bound to have certain effects on the structure of adjacent tunnels.At the same time,if some safety risk accidents occurs during excavation process,leading to excessive deformation or even collapse of the foundation pit,the degree of influence on the adjacent tunnel will be aggravated.Taking a deep foundation pit near an existing tunnel in Beijing as an example,this paper explores the deformation of foundation pit and adjacent tunnels which suffer from the excavation and support process of foundation pit by numerical simulation.The main research contents and results are as follows.(1)Introducing the deformation of retaining structure,basement uplift and surface settlement and the deformation mechanism of them.Then analyzing the reasons of tunnel structural deformation caused by foundation pit excavation,collecting some engineering examples of foundation pit construction near existing tunnels.Moreover,this paper introduces the related concepts of Bayesian network model and risk management,and the method of how to establish risk evaluation index system for foundation pit construction.(2)By using MIDAS-GTS finite element software,then analyzing the lateral displacement of retainng structure,uplift of foundation pit bottom,surface settlement and deformation law of tunnels caused by foundation pit excavation,and verifying the feasibility of supporting scheme.In addition,this paper analyzes the applicability of MC model and modified MC model(soil hardening model)in simulating foundation pit construction,and explores the influence of foundation pit construction on adjacent tunnel by changing the net distance between foundation pit and tunnel and by changing the excavation depth.(3)Based on Bayesian network model,the risk level of pit risk events which may cause great disturbance to adjacent tunnels is preliminarily assessed by using the method of fuzzy comprehensive evaluation,and simulate the main risk events of the foundation pit,such as support instability and retaining structure instability,which may affect the foundation pit and tunnel.(4)By numerical simulation,it is found that unloading of foundation pit excavation will cause lateral displacement of retaining structure towards foundation pit and uplift of foundation pit bottom,which will cause lateral displacement and settlement of surrounding soil,resulting in horizontal displacement and vertical settlement of adjacent tunnel,and each value meets the design requirements;MC model does not consider the non-linear relationship between soil modulus and stress level,nor can it distinguish load-unload relationship,then it can not give reasonable deformation prediction of soil behind walls,so the modified MC model is more suitable for foundation pit excavation analysis;The tunnel deformation decreases with the increase of the distance between the tunnel and foundation pit,and the relationship between maximum horizontal displacement,vertical displacement of tunnels and net distance can be fitted by power function;The maximum deformation of the tunnel is positively correlated with the excavation depth,and under a certain depth range,the relationship between them can be fitted by a straight line;If the instability of support or retaining structure which forces the retaining structure to undergo large deformation,it will pose a great threat to the adjacent tunnel.