Study On Deformation And Failure Mechanism Of Surrounding Rock In Soft And Hard Interbed Tunnel

With the continuous development of tunnels and underground construction in China,various unfavorable geology,such as karst caves,fault fracture zones,frozen soils,muddy interlayers and soft rock sections,are often encountered during the construction process.The sedimentary rocks with layered structure in nature account for two-thirds of the land area.The layered structure contains a large number of soft and hard interbedded structures.Due to the particularity of the soft and hard interbed structure,the soft and hard interbedded rock masses are destroyed.The pattern is mainly manifested in three forms: one is the slip damage along the level,the other is the shear failure at the oblique level,and the third is the splitting damage along the level.When tunnels are excavated in such soft and hard interbedded rock masses,it is easy to cause instability of the surrounding rock of the tunnel and severely cause tunnel collapse.Based on the Sibianshan tunnel project,this paper focuses on the impact of tunnel excavation location,soft and hard interbedded rock dip angle and rock thickness on the stability of tunnel surrounding rock.Throughout the analysis process,indoor uniaxial and triaxial compression tests were first carried out through self-made soft and hard interbedded rock masses to analyze the mechanical properties and deformation and failure modes of soft and hard interbedded rock masses.Then the numerical simulation of the indoor compression test of the rock mass is carried out by the discrete element software PFC2 D.Finally,the tunnel excavation of the soft and hard interbedded rock mass in Simian Mountain is numerically simulated to study the excavation position of the tunnel,the inclination of the rock formation and the thickness of the rock formation.The influence of crack propagation law and deformation failure mechanism on surrounding rock of soft and hard interbed tunnels.The research contents are as follows:(1)Through the uniaxial and triaxial loading tests on the soft and hard interbedded rock mass,the effects of different confining pressures on the mechanical properties and deformation and failure modes of different dip angles and layer thicknesses are obtained.According to the experimental results,under the uniaxial compression,the peak strength of the rock mass with the inclination angle of 30° is smaller than the peak strength of the rock mass with the inclination angle of the rock layer of 0°,and the soft rock of the rock mass with the same inclination angle.The body thickness increases the peak strength of the rock mass.When the inclination angle and the layer thickness ratio are the same,the peak strength of the rock mass increases with the increase of the confining pressure;with the increase of the confining pressure,the peak intensity and the inclination angle when the inclination angle is 0 degrees is 30 degrees.The difference in peak intensity is getting smaller and smaller.(2)Using the discrete element software PFC2 D to simulate the uniaxial and triaxial numerical simulation of soft and hard interbedded rock specimens,and verifying the reliability of the mechanical properties and deformation and failure modes of soft and hard interbedded rock mass by PFC2 D software through comparison with laboratory tests.From the microscopic analysis,the distribution law of crack propagation in soft and hard interbedded rock mass is obtained: under uniaxial compression,the soft layer rock body first produces cracks.When the inclination angle is 0°,when the rock layer thickness is 2:0.5,the rock mass is generated.The cracks running through the soft and hard rock layers,when the thickness of the rock layer is 2:1,the rock mass has cracks,but it does not penetrate the soft rock mass;when the rock dip angle is 0°,the expansion of the rock mass crack is affected by the confining pressure.The final failure mode changes from crust failure to shear failure with the increase of confining pressure.When the inclination angle of the rock is 30°,45°,60°,the failure mode of the rock mass is shear failure,when the inclination of the rock mass is At 90°,the failure mode of rock mass is splitting failure;from the relationship between the number of fractures in soft and hard interbedded rock mass and strain,it can be seen that in the triaxial compression test of soft and hard interbedded rock mass,when the degree of damage of hard rock mass is small,the relationship between the number and strain of rock mass fractures is "S" type Hard rock layer when a large extent of the damage,the number of rock fracture strain relationship to form an "S" slow growth,and rapid growth.(3)The excavation of soft and hard interbedded rock mass in Simianshan tunnel is simulated by discrete element PFC2 D software.The deformation and failure law of surrounding rock and the distribution law of crack extension are studied by changing the excavation position,thickness and dip angle of the tunnel.According to the simulation results,when the tunnels in different locations are excavated in the soft and hard interbedded rock mass,the location of the weakest part of the tunnel is related to the location of the hard rock mass in the soft and hard interbedded rock mass.When the tunnel is under the hard layer,The weakest part of the surrounding rock of the tunnel is at the waist.When the tunnel passes through the hard layer,the weakest surrounding rock of the tunnel is at the arch;when the tunnel is excavated,the soft layer rock is damaged to a large extent,the rock mass is finely divided,and the hard rock mass The degree of damage is small,and the hard rock mass breaks at the arch waist and above,and a large extent of collapse may occur.Therefore,for safety reasons,the hard rock mass and the soft rock mass should be treated differently.This phenomenon The increase of the dip angle of the rock stratum and the increase of the confining pressure are more obvious;the softness of the soft rock mass in the soft and hard interbedded rock mass is better,because the integrity of the hard rock mass of the tunnel is better,because it is affected by the soft and hard interbedded rock mass.When the force is applied,the soft layer rock mass is easily compressed and deformed.With increasing layer thickness of the soft rock,the rock stress soft layer has greatly deformed,bear most of the stress.