Study On Deformation Failure Mechanism And Support Technology Of Surrounding Rock Of Diversion Tunnel In Horizontal Layered Rock Mass

With the improvement of China’s comprehensive national strength,domestic water conservancy projects have achieved unprecedented development.Large-scale hydropower stations,diversion projects and various water conservancy projects have also emerged in the context of development.Diversion tunnels pass through various climatic environments and complex geological structures,connecting large and small water conservancy projects.It plays an irreplaceable role in water conservancy projects.In this paper,based on some diversion tunnels in the diversion project of Dajihuang,with the help of domestic and foreign data collection and field exploration data,the deformation and failure mechanism of surrounding rock of horizontal layered rock tunnel under different structural plane parameters and different rock thickness is studied.Firstly,the finite element software Midas GTS NX is used to simulate the construction stage of the diversion tunnel.The changes of rock thickness and structural plane parameters are considered in the model,and the interface contact element is used to model the weak structural plane.The displacement,plastic zone and stress change of surrounding rock of horizontal layered rock tunnel with different rock thickness and structural plane parameters are analyzed and summarized.Through orthogonal experimental design,nine working conditions are established,and the order of primary and secondary relations of influencing factors is obtained by grey correlation analysis.The concept of membership degree is introduced to explore the order of primary and secondary relations of various factors under comprehensive displacement and stress,and then the optimal scheme is given by combining the combination weighting method.Finally,in order to explore the feasibility of the optimization scheme,the optimization scheme is substituted into a specific tunnel of the Dajihuang project to establish a numerical model for safety analysis.The main conclusions are:（1）By summarizing and analyzing the existing literature and engineering data at home and abroad,it is concluded that the layered rock mass is usually divided into thick layered structure,thick layered structure,medium thick layered structure,interbedded structure and thin layer structure.When the dip angle of the bedding planeαsatisfiesα₁＜α＜α₂,the layered rock mass will undergo plastic slip failure along the bedding plane;Whenα＜α₁ orα＞α₂,the layered rock mass will not be destroyed along the layer;the vault and floor of the tunnel in the horizontal layered rock mass are prone to tensile failure,while the left and right side walls of the tunnel in the vertical layered rock mass are more vulnerable to bending failure.The main failure modes of surrounding rock of layered rock tunnel are bedding slip and bending fracture.（2）Through the comparative analysis of the finite element model results of 15 working conditions of different rock thickness and structural plane parameters of horizontal layered rock mass,it is found that the deformation law of surrounding rock is basically the same.Comparing the numerical models of three groups of different strength structural plane parameters,it can be seen that the vault settlement,floor heave and left and right side wall convergence decrease with the increase of rock thickness.With the decrease of rock thickness,the thinner the rock layer,the more the number of weak structural planes,and the worse the stability of the tunnel.The plastic deformation position and the maximum shear strain position in the surrounding rock of the tunnel are distributed along the structural plane.The large and small principal stress values are symmetrically distributed along the central axis,and the large and small principal stress values are larger at the left and right side walls.（3）Comparing the simulation results of different structural plane parameters of layered rock mass,it is found that the deformation law of surrounding rock is basically the same with the excavation of the tunnel.The displacement and deformation values of different positions decrease gradually with the increase of the strength of the structural plane.However,when the strength of the structural plane is large,even if the thickness of the rock layer is changed,the displacement of the surrounding rock of the tunnel is small.With the increase of structural plane parameters,stress and strain are gradually decreasing.（4）The order of influencing factors of displacement comprehensive score and stress comprehensive score is bolt length,steel arch spacing and excavation footage.Bolt length,steel arch spacing,excavation footage.The priority order of comprehensive influencing factors is bolt length,steel arch spacing and excavation footage.（5）The maximum correlation degree in the test condition is condition 9,and the correlation degree is 0.700.It can be concluded that condition 9 is the optimal scheme,and the tunnel stability is better and safer.The parameters of working condition 9 are modeled according to the actual geological parameters of the project.Through the comparison between the optimization scheme and the original scheme,it can be seen that the optimized support parameters have a good inhibitory effect on the tunnel displacement,maintain the stability of the surrounding rock,and reduce the distribution of the plastic zone and the maximum shear strain distribution range.