Study On Reliability Of Surrounding Rock Tunnel Support In Badong Formation In Northeast Chongqing

As an important project in underground space,tunnel construction has received extensive attention for its safety.However,there are few researches on the reliability of supporting structures at present,that is,there are still deficiencies in the risk prediction and effective safety assessment of tunnel engineering.In this regard,this paper takes the Baicungou tunnel excavation project in Fengjie County,Chongqing City as the engineering research background,with the help of MIDAS/GTS and FLAC^3D software,MATLAB software and origin software to study the initial support of the tunnel under the geological surrounding rock of the Badong Formation.The main research contents are as follows:（1）First,the creep characteristics of the surrounding rock of the tunnel are introduced,and then the interaction process between the initial support and the surrounding rock is explained,and finally two basic creep processes of the rock are analyzed,namely stable creep（decayed creep）And unsteady creep（non-attenuation creep）.Combined with the settlement and convergence of the tunnel surrounding the tunnel,it can intuitively and objectively reflect the mechanical properties and stability of the support structure during the tunnel construction process,and focus on displacement to study the reliability of the tunnel’s support for the initial support structure.（2）By using MIDAS/GTS finite element software to simulate and calculate the length of the anchor rod under five working conditions of 2.0m,2.5m,3m,3.5m and 4m.In contrast,when the selected anchor rod length is 3m to 4m,the difference between the displacement of the vault and the side wall is very small.Considering economic issues,that is,the anchor rod length is 3m in this simulation.（3）Use the finite element software MIDAS/GTS to simulate and calculate the bolt density under the four working conditions of 0.8m,1m,1.2m,and 1.5m.In contrast,when the bolt spacing is 1m,the impact of support deformation is small,Considering economic issues,so 1m is taken as the best value of the bolt spacing in this project.（4）Use FLAC^3D to simulate excavation with CD method,namely DK718+775section（CD method）;use FLAC^3D to simulate excavation with core soil method for circular excavation,namely DK718+760 section（circular excavation method with core soil）.The analysis shows that the stability of the supporting structure of the tunnel section is also different when different construction methods are used.When the CD method is used for excavation,the displacement and deformation of the supporting structure are relatively small.Based on this,it is judged that the CD method is safer,that is,the DK718+775 section is a relatively safer section.（5）Use FLAC^3D software to perform numerical modeling analysis on DK718+775and DK718+760 sections.Program through FLAC^3D,arrange monitoring and measurement points,analyze the stress changes and deformations in different directions at each characteristic point of the support structure as the face of the face advances,and use the origin software to determine the displacement-time curve of each section of the support body Perform a cubic polynomial fit.According to the fitting equation,the time-dependent failure probability of the corresponding observation point is calculated,and the reliability of the tunnel section can be judged by the failure probability.The addition of the time-dependent failure probability can realize the dynamic assessment of tunnel engineering risks and comprehensively predict the risks at different time points in the whole process of tunnel construction.It is of great significance for the design optimization and construction risk avoidance of underground tunnels.