Study On The Failure Mechanism Of Fractured Rock Masses Under The Coupled Hydro-mechanical Loading And Unloading Conditions

With the rapid development of infrastructure construction,there exist large number of large transportation and water conservancy and hydropower projects in China,where effect of hydro-mechanical loading on the stability in rock slopes and underground constructions plays an important role.The coupling hydro-mechanical loading and unloading conditions becomes important factors inducing such problems,and the fractured rock mass is a kind of engineering medium widely encountered in practical engineering problems,which has complex mechanical properties and permeability characteristics.In this paper,theoretical analysis,laboratory tests and numerical simulations were combined to systematically investigate the failure mechanism of fractured rock masses under both hydro-mechanical loading and unloading conditions.The whole paper is organized in the following parts.(1)Based on the theory of fracture and damage mechanics,the crack initiation and propagation law of rock under stress-seepage was studied.The compression-shear airfoil crack model considering confining pressure unloading was established,and the model was compared and analyzed by means of finite element software.The main influencing factors of the crack initiation strength of the compression-shear crack are the closure degree of crack,confining pressure,osmotic water pressure,crack inclination and rock properties,etc.The existence of osmotic water pressure increases the stress intensity factor at the crack tip,which causes the crack initiation strength of the compression-shear rock to decrease and the branch crack propagation length to increase remarkably.The existence of confining pressure constrains the crack propagation,which causes the crack initiation strength to decrease and the branch crack propagation length to decrease significantly.The extension length of the airfoil branch crack considering the confining pressure unloading action is mainly related to the confining pressure unloading level.During the confining pressure unloading process,the stress intensity factor at the crack tip and the airfoil branch crack propagation length increase significantly.When the confining pressure is unloaded to be close to the osmotic water pressure,the branch crack expands into an unstable state.(2)A conventional triaxial compression test and triaxial compression pre-peak unloading test with osmotic water pressure and non-permeability hydraulic pressure were carried out,which used a rock multi-field coupling triaxial apparatus for a standard specimen containing a single through-type crack made of a rock-like material.The internal cracks of the damaged specimen were reconstructed in three dimensions by CT scanning system and AVIZO software.The test results show that the deviatoric stress increment,confining pressure unloading increment and uniform confining pressure drop parameter of the specimen decrease with the increase of initial axial pressure level.After the specimen is unloaded,as the initial axial pressure level increases,the final confining pressure and ultimate yield strength increase.It indicates that the higher the initial axial pressure level,the smaller the confining pressure unloading amount required for the specimen to be destroyed,the shorter the confining pressure unloading duration,and the higher the damage degree,that is,the more likely the specimen is damaged.In addition,there is no linear relationship between the deviatoric stress increment and the fracture angle,but the overall rule is that the smaller the fracture angle is,the larger the stress increment is.The dilatancy angle decreases with the increase of the fracture angle and the initial axial pressure level,and increases with the decreases of the initial confining pressure.The dilatancy angle of the specimen in the unloading test is greater than that of the conventional triaxial compression test.The cohesive force in the unloading test is lower than that of the conventional triaxial compression test,and the cohesive force increases exponentially with the increases of the initial axial pressure level.However,the internal friction angle is higher than that of the conventional triaxial compression test,and it decreases with a polynomial relationship as the initial axial pressure level increases.In the two stages of confining pressure unloading,the circumferential strain increment is larger than the axial strain increment,showing obvious lateral expansion.(3)In the triaxial compression pre-peak unloading test with osmotic water pressure,the peak strength of the specimen decreases as the osmotic water pressure increases,and increases as the initial axial pressure level increases.In addition,the uniform confining pressure drop parameter decreases with the increases of the initial axial pressure level,and decreases with the increases of water pressure.It indicates that the higher the initial axial pressure level,the greater the water pressure,the smaller the confining pressure unloading required for the failure of the specimen,that is,the more easily the specimen is destroyed.The peak cohesive force and residual cohesion increase exponentially as the initial axial pressure level increases.The cohesive force and internal friction angle of the specimen decrease with the increase of water pressure,and the peak friction angle and residual friction angle decrease with the increases of the initial axial pressure level.The maximum dilatancy angle of the specimen decreases with the increases of the initial axial pressure level and increases with the increases of water pressure.The damage variable decreases with the increases of the initial confining pressure,which reflects the restrictive effect of the confining pressure on the hoop strain of the specimen.The damage variable slightly increases with the increases of water pressure,indicating that the existence of water pressure will promote the development of the circumferential strain of the specimen.(4)In order to observe the crack initiation,propagation and failure process more intuitively,and better explain the failure mechanism of fractured rock mass under different stress path,therefore,the experimental process was reproduced by FLAC~3DD numerical simulation software.The elastic-brittle damage model was used to simulate the triaxial loading and unloading test of single-fracture specimen,and the crack propagation process and failure of the specimen under different conditions were compared and analyzed.The results show that the smaller the confining pressure under loading condition,the closer the crack propagation angle is to the vertical direction.And under the unloading condition,the confining pressure unloading increment is larger as the confining pressure increases,the crack growth becomes more concentrated,and the brittle characteristics are more obvious.(5)Based on the calculation model under waterless,the fluid-solid coupling method was used to study the evolution of crack under different water pressures,different confining pressures and different initial axial pressure levels,as well as the loading and unloading failure characteristics of specimen at various fracture angles.The crack propagation process and failure mode of the specimen will vary with the fracture angle.Among them,the specimen with the fracture angle is 0°and 90°are quite different from the other specimens with the fracture angle.