Research On The Influence Of Shield Tunneling Under Subway Tunnel On Existing Tunnel And Its Contro

With the influx of large numbers of people into the city,China’s urbanization process is advancing rapidly.As long as the increasing number of urban population base,ground traffic has been faced with huge pressure.In order to alleviate the current situation of ground traffic congestion in daily travel,metro traffic has been rapidly developed.The scale and number of urban subways have been expanded and increased,especially the number of shield crossing projects has also been increased.The disturbance to the existing tunnel lines has inevitably been induced by the construction of new shield tunnels through existing tunnels,and the settlement deformation of existing tunnels is the most important issue in the construction process of underpass tunnels.Thus,the prediction and control of settlement deformation of existing tunnels during underpass construction has become a difficult issue for similar projects.Therefore,it is of great practical significance to investigate the process of shield tunnel underpassing existing tunnels.Based on an underpass tunnel project in Nanning,,finite difference simulation and other methods have been used in this dissertation to study the impact of small angle tunnel underpassing on the existing tunnel deformation through theoretical numerical calculations,and the factors affecting the existing tunnel deformation have completely analyzed.Finally,targeted control measures for the influencing factors have been proposed.The main research content and results of this dissertation are as follows:(1)Through the Euler-Bernoulli elastic foundation beam model based on Winkler foundation,the equilibrium differential equations were firstly established by the two-stage analysis method,and then the numerical solution of the settlement deformation of the existing tunnel was solved byanalyzing the ground displacement caused by the construction of new tunnel.Finite-difference software was used to analyze the effects of the spacing between the two tunnels,the pinch angle,the ground loss and Poisson’s ratio on the deformation of the existing tunnel.The results show that the ground loss has a greater influence on the existing tunnel deformation.With the increase of the ground loss rate of the existing tunnel,the settlement deformation increases,but Poisson’s ratio changes have a smaller impact on the existing tunnel deformation.The spacing between the two tunnels also affects the existing tunnel settlement deformation,in other words,the larger the spacing,the smaller the impact on the existing tunnel,while the existing tunnel settlement deformation increases with the decrease of the angle between the two tunnels.(2)An engineering model is established by finite difference software to simulate the construction process of shield tunnel underpassing existing tunnel at a small angle in close proximity,and then to analyze the ground settlement deformation caused by the shield construction process and the settlement deformation law of the existing tunnel.The results show that the impact range of shield construction on the existing tunnel is greater within 4D(D is the diameter of the tunnel excavation)away from the intersection location,and the fastest rate of settlement deformation of the existing tunnel and the ground are both within 1D from the existing tunnel,while the differential settlement of the existing tunnel is the largest,and the torsional deformation is also the maximum in this range.And the maximum differential settlement between the top and bottom of the existing tunnel is also within this range.So this is the time when the structural safety of the existing tunnel is at its highest risk.(3)The factors affecting the loss of strata during the construction of the shield tunnel underpass were analyzed,and the formation causes and control methods of different effects were explored.Compared with the effects of different grouting pressures on the settlement of existing tunnels,the settlement deformation of existing tunnels could be controlled by adjusting the grouting pressure near the tunnel underpass location.It can be noticed that the grouting pressure is inversely proportional to the settlement of existing tunnels,namely the grouting pressure increases while the settlement of the tunnel decreases.