Study On Mechanical Characteristics Of Fractured Rock Mass Under The Coupled Hydro-mechanical Loading And Unloading Conditions And Numerical Simulation

Hydro-mechanical coupling of rock mass is a hot issue in the field of rock mechanics.Stress and water pressure are important factors affecting the stability of geotechnical engineering.The coupling effect of unloading and seepage is an important cause of rock mass instability.As an engineering medium widely encountered in practical engineering,fractured rock mass is rich in various macro fracture surfaces,which affects the distribution of stress field in rock mass,and its mechanical properties are more complex.Therefore,this paper focuses on the mechanical properties of fractured rock mass under the coupling of loading and unloading and osmotic pressure.Taking the metamorphic sandstone of zhedoshan tunnel of Sichuan Tibet Railway as the research object,combined with previous research results,the unloading mechanical properties,failure characteristics and crack propagation of rock mass with single fracture are studied by means of indoor test,similar material model test and numerical simulation.The research contents and achievements are as follows:（1）Based on the similarity theory,the similar materials of metamorphic sandstone are prepared and the cracks with different dip angles are prefabricated.The uniaxial test and conventional rock loading and unloading test are carried out.Under uniaxial and triaxial compression test conditions,the peak strength of the intact specimen is the largest.The larger the inclination angle of the specimen,the greater the peak strength.At the same stress level,the peak strength of rock decreases after unloading.The failure mode of specimens with different crack angles in uniaxial and conventional triaxial tests is mainly tensile splitting,and the macro fracture surface penetrates with the crack propagation at the crack tip.The shear failure is the main failure mode of the specimens under unloading.（2）The discrete element particle flow code PFC^2D is used to calibrate the meso parameters and determine the parameters of the numerical model on the basis of laboratory tests.Sensitivity analysis of hydro-mechanical coupling fluid parameters was carried out to determine the appropriate range.The results show that the fitting error of conglomerate loading seepage is less than 5%,and the coupling trend of sandstone unloading seepage is basically the same.（3）The mechanical properties of fractured rock mass under conventional loading are obtained by particle flow numerical simulation.The angle of fracture dip is positively correlated with the strength of fractured rock mass under uniaxial and conventional triaxial loading and unloading tests,that is,the larger the angle of fracture dip is,the higher the peak strength is.Under the hydro-mechanical coupling effect,the peak strength of intact and fractured rock mass specimens under various confining pressures decreases,and the slope of stress-strain curve also changes,which has little effect on the peak strength and peak strain（4）The model of fissured tunnel is built by PFC^2D,and the development process and energy release process of tunnel damage area under different fissure inclination surrounding rocks,different fissure distance chamber length,different fissure length,different in-situ stress,different side pressure coefficient and water pressure are analyzed.It is concluded that with the unloading of tunnel excavation,surrounding rock damage of cracked tunnel is different under different conditions.Under the same stress level,the damage degree is more significantly affected by the crack inclination and the length of the crack distance from the tunnel chamber.From an energy point of view,30°,45°And 90°In the process of stress redistribution,the energy release of fractured surrounding rocks is more intense and there is a short-term surge,which may lead to rock burst.According to the analysis of energy dissipation,the tunnel model of fractured rock mass under the action of hydro-mechanical coupling and the prediction criterion of rock burst are verified.