Analysis On The Influence Of Shield Tunnel Passing Through Existing Tunnel In Short Distance

With the development of underground transportation in China,more and more subway lines and tunnels are built.More and more tunnels and pipelines will be traversed during the tunneling of new shield tunnels.Under the condition that the clear distance between the new and old tunnels is less than one time of the tunnel diameter,a subway tunnel passes through existing underground tunnel in initial support state at a short distance.In this paper,a model is established based on the engineering background,and the main research work includes safety analysis,impact analysis and reinforcement measures analysis:(1)This paper expounds the initial in-situ stress state before shield tunnel excavation and the stress redistribution state after tunnel excavation.It makes a detailed summary and analysis of the action mechanism of shield tunnel passing through the existing underground tunnel in parallel and the interaction mechanism between the tunnel support system and surrounding rock during the process of shield tunnel excavation.Based on the Mindlin solution,it deduces the pressure and side of shield earth bin The calculation formula of soil displacement around the shield under the action of wall friction and the calculation formula of total additional load generated on the existing tunnel during the shield crossing process.(2)The maximum values of displacement and surface settlement of the existing tunnel in the field monitoring data and numerical simulation results are compared with the control values,and the safety analysis is carried out.The results show that no matter the surface settlement,the settlement of the existing tunnel vault or the convergence of the clearance,the results of the numerical simulation of the side crossing section have little difference with the field measured data,which proves the reliability of the numerical simulation.However,the maximum value of the deformation is less than the control value,whether it is the numerical simulation result or the field measurement value.It can be seen that the impact of new shield tunnel excavation on the existing tunnel is within a reasonable range when the clear distance between the new and old tunnel is less than one time of the tunnel diameter.(3)On the basis of the side tunnel model,the new and old tunnel position relations are changed to establish the shield tunnel model of double line underpass and single line underpass.According to the results of numerical simulation,the displacement,internal force and surface settlement of the new and old tunnels are analyzed.The results showthat with the change of the location relationship between the new and old tunnels,the displacement,ground settlement,internal force and bending moment of the existing tunnels will change under the conditions of double track side crossing,double track underpass and single track underpass.(4)Based on this project,the reinforcement measures of setting isolation piles in the soil between shield tunnel and existing tunnel are studied,the partition effect of isolation piles is analyzed,and the influence of pile length,pile diameter,pile tunneling distance and pile spacing on the partition effect of isolation piles is studied.The results show that the vertical and surface settlement of existing tunnels can be effectively reduced in the course of shield tunneling.However,the effect of decreasing horizontal displacement is lower than that of reducing sedimentation.However,no matter the pile length,pile spacing,pile tunnel spacing or pile diameter,the variation of the parameters of the isolation pile will have a certain impact on the isolation effect of the isolation pile.In terms of the sensitivity of the isolation effect of the isolation pile,the pile length >pile spacing > pile tunnel spacing > pile diameter.(5)Change the reinforcement method of the existing tunnel,and analyze the reinforcement effect of three common reinforcement schemes of the existing tunnel combining with the economy and stability.The results show that the reinforcement effect of three common reinforcement methods in the tunnel is generally steel arch >spandrel support plus transverse tie > vault vertical support + lengthened and densified anchor bolt.As for the limitation of horizontal displacement,there is little difference in the effect between the steel arch frame and the arch shoulder support plus horizontal tie.The steel arch frame is slightly better than the arch shoulder support plus horizontal tie,while the lengthened and densified foot locking bolt has certain limitation on the horizontal displacement of the existing tunnel,but the effect is not as good as the former two.However,for the reinforcement scheme of spandrel support + transverse tie in the tunnel of this project,although the effect is not as good as that of steel arch frame,it still meets the safety requirements.Compared with steel arch frame,it has low cost,simple construction and economic benefits.The selection of reinforcement scheme is reasonable.