Study On Stability Of Tunnel With Small Clear Spacing Caused By Excavation Of Undercover Opening Slope At The Exit Section Of Tunnel

Due to geological conditions and natural factors,the deformation of the initial support of the tunnel is too large,which leads to the phenomenon of intrusion.The general treatment schemes include changing the arch and changing the dark hole,but for the shallow depth and poor rock quality.For the hole section,the latter option is obviously safer and more economical.At the same time,this will also cause the original design's side slope position to move backwards along the tunnel axis.As the area of the slope slope excavation increases,the scope and extent of the impact increase,and the excavation process is inevitable.It will cause the redistribution of surrounding rock stress,causing additional displacement of the tunnel,which seriously threatens the safety of the existing tunnel structure.Therefore,how to predict the influence of excavation on the stability of the tunnel during the dark-changing period and the reasonable selection of the process of controlling the additional displacement of the tunnel are of great significance.Based on the specific engineering background,this paper uses the combination of numerical simulation and measured data analysis to study the influence of the excavation of the curved section on the small clear distance tunnel in the hole section.The main contents are as follows:(1)Analyze the influence mechanism of different forms of excavation works on the existing tunnels below,and summarize the relevant factors affecting the additional displacement of the tunnels;(2)Combining the specific engineering background,the finite element software MIDAS/GTS is used to establish a three-dimensional model for numerical simulation analysis.From the point of view of additional displacement,the influence of excavation above the portal section on the tunnel with small clearance is studied,and the horizontal displacement,vertical displacement and longitudinal displacement of the tunnel below are obtained.The results show that the excavation of the slope causes the longitudinal displacement of the tunnel to be limited and presents uneven bulging.The additional displacement of the tunnel located directly below the excavation area is mainly vertical displacement,while the additional displacement of the tunnel on the side of the displacement excavation area Mainly horizontal displacement.(3)Using the software to establish a two-dimensional model to simulate the multi-case of the modified parameters of the typical section,and analyze the influence of the residual soil thickness of the tunnel vault,the integrity of the tunnel lining structure and different excavation methods on the additional displacement of the tunnel.degree.The results show that the displacement of the arch bottom of the tunnel during the shallow excavation is greatly affected by the unloading amount,and after reaching a certain excavation depth,the displacement of the arch bottom of the tunnel is more related to the thickness of the residual soil of the vault;When the load is constant,as the thickness of the residual soil of the tunnel vault decreases,the additional displacement of the dome position increases,and the additional displacement of the arch bottom position does not change.The stratified block excavation method can effectively control the lower tunnel.Additional displacement rate.(4)Summarize the control measures used to reduce the additional displacement of the tunnel,such as the support inside the tunnel,introduce the control measures of the specific project and conduct a detailed and in-depth analysis of the measured data of the tunnel and the slope.The results show that the overall variation of the measured values of the dome displacement of the tunnel is basically consistent with the numerical simulation results.The use of back pressure backfill and "T" shaped steel frame support can effectively control the additional deformation of the tunnel to ensure safety during construction.