Model Test Study On Displacement Of Ground Surface And Existing Tunnel By Shield Construction

Shield construction will inevitably affect the displacement of the surrounding soil,and the displacement of the soil will extend in a certain range in all directions until it is stable.Therefore,deformation occurs on the surface above the tunnel,and when the shield is worn under construction,not only the surface deformation but also the displacement of the existing tunnel will be affected,and the displacement value is too large to cause safety hazards.Therefore,it is very important to study the law of shield construction on the existing tunnel and surface deformation,which is of great significance to ensure the safety of existing tunnels and surrounding environment.Taking the shield construction of a cross section of rail transit in Hefei as the research background,the influence of shield tunneling on the displacement of the ground surface and the existing tunnel when the buried depth ratio and the net spacing are different is studied through the shield model test.The whole test process is divided into two steps:excavation and post-grouting.Firstly,the law of the influence of parallel tunnel shield construction on surface deformation is studied.The results show that:(1)When the left-line tunnel is excavated,the surface settlement curve of each stage of the left-line excavation conforms to the normal distribution proposed by Peck,and the surface settlement value increases with the increase of the soil loss rate.There is a linear relationship between the maximum settlement value of surface settlement and the soil loss rate at each stage.When the ratio of buried depth is constant,the sedimentation tank width coefficient “i” decreases slightly with the increase of soil loss rate.When the soil loss rate is constant,the sedimentation tank width coefficient “i” becomes larger as the buried depth ratio increases,and the larger the settlement tank width coefficient “i”,the larger the influence range of the lateral surface settlement.(2)After the excavation of the right line is completed,the influence range of surface settlement will increase as the depth of the parallel tunnel increases,and the surface settlement of the tunnel will decrease and the shape of the settlement tank will be moderated.(3)After grouting in the tunnel wall,the surface lift value of grouting changes linearly with the increase of grouting rate.With the increase of buried depth,the effect of grouting on surface elevation is reduced.Secondly,the law of the tunnel under construction on the deformation of the existing tunnel and the above ground surface is studied.The research results show that:(1)The maximum value of surface settlement at each stage of soil loss after excavation of theleft-line tunnel changes linearly with the increase of soil loss rate.The settlement rate is the fastest and the final settlement is maximum when the penetration angle is 90°.(2)There is a linear relationship between the maximum value of the dome settlement and the soil loss rate in each stage of the left line excavation.The undercut construction of the shield tunnel will cause uneven settlement in the longitudinal direction of the existing tunnel.When the tunnel penetration angle is 15°,the net spacing between the upper and lower tunnels is different.When the undercut angle is 90°,the settlement value of the tunnel vault becomes larger as the clearance between the upper and lower sides of the tunnel increases,and the surface settlement is about 1.5 times that of the vault.(3)Post-wall grouting can control the displacement of the surface and existing tunnels.When the amount of grouting is appropriate,the position of the surface and existing tunnels can be restored to the initial state,and the displacement rise value increases with the increase of grouting rate.