Study On Deformation Characteristics Of Surrounding Rock During Excavation Of A Metro Tunnel With Variable Section In Qingdao

With the rapid development of urbanization in our country,it is a great tendency to develop urban rail transit engineering in a wide range.The large-section variable-section of metro tunnel has its particularity.At present,most experts and scholars seldom consider the weakening of mechanical parameters of surrounding rock in the excavation process when conducting numerical simulation calculations,so the conclusions drawn are bound to be different from the actual situation.Based on a large cross-section variable section of Qingdao Metro,the stressstrain curve,mechanical parameter weakening,crack propagation,macro failure and other characteristics of granite samples were analyzed through theoretical analysis and laboratory triaxial test,and the weakening characteristics of key parameters required by MIDAS/GTS/NX simulation were obtained.The weakening values required by the simulation were obtained by combining these features with the field geological survey report and assigned to each corresponding construction stage.Then MIDAS/GTS/NX finite element simulation software was used to dynamically simulate the field construction scheme,and the characteristics of the arch settlement displacement,arch bottom uplift displacement,left and right arch waist convergence displacement,surrounding rock stress and plastic zone were analyzed during the excavation of surrounding rock in the variable section.Optimize the site construction scheme to provide reference for subsequent similar projects.The main research contents of this paper are as follows:(1)The research progress of surrounding rock stress and deformation in indoor triaxial test and large-section tunnel construction was summarized by reading and analyzing relevant domestic and foreign literatures.(2)For granite samples with different weathering degrees collected and processed on site,laboratory uniaxial tests,conventional triaxial tests and constant axial pressure unloading confining tests were designed to analyze and obtain the stress-strain curve,mechanical parameter weakening,crack propagation,macroscopic failure and other characteristics of granite samples under different confining pressures and different unloading rates.(3)A numerical analysis model was established by MIDAS/GTS/NX finite element simulation software based on the field monitoring data of a Qingdao metro tunnel with variable section.According to the process of the on-site construction scheme,the 3D dynamic simulation is carried out considering the granite parameter weakening characteristics,and the comparison is made with the monitoring data of the corresponding points on the site,so as to ensure the reliability of the model.On the basis of the above finite element simulation,laboratory test and field monitoring data,the characteristics of vault,arch bottom,left and right waist displacement,surrounding rock stress and elastoplastic zone after completion of excavation are analyzed and their mechanisms are expounded,which can provide reference for future projects.(4)On the basis of dynamic simulation analysis of the site construction scheme,further analysis of the vault,arch bottom,left and right arch waist and other indicators to carry out the stress and strain characteristics of surrounding rock,put forward the construction optimization scheme,and compared the site construction scheme with the construction optimization scheme,to obtain the stress and deformation characteristics of the large cross-section of variable section of surrounding rock.