Study On Triaxial Unloading Creep Test And Microscopic Mechanism Of Mudstone With Different Water Content

After the excavation and unloading of underground caverns,the stress release of surrounding rock causes the side wall,vault and arch bottom of the cavern to be subjected to horizontal lateral pressure,top pressure and eccentric pressure,which is more prone to engineering problems such as floor heave,roof settlement and even caving.Especially for weak rock mass,the deformation of surrounding rock lasts for a long time and converges slowly after excavation,and the plastic deformation is more significant under the influence of groundwater.Based on this,this paper selects a tunnel excavation mudstone as the research object,and the evolution mechanism of unloading creep damage of mudstone with different water content are studied by laboratory tests,theoretical analysis and the establishment of PFC^3D particle flow model.The main conclusions are as follows:（1）The results of triaxial loading and unloading failure test of natural mudstone show that the rock sample is more damaged under unloading conditions.The initial confining pressure will limit the internal damage development of the sample,and the failure mode of the rock sample is mainly affected by the loading process,the unloading process is a further promotion under the loading process.After unloading failure,the surface micropores of rock samples developed obviously,which showed shear fracture morphology.The fractal dimensionof mudstone is between 1.390 and 1.468,and the higher the axial stress level,the more sufficient the development and expansion of micro pores in the rock sample during unloading.The numerical test results show that the unloading effect mainly improves the friction strength of internal particles inside the rock sample,which has little effect on weakening its own material strength.The greater the initial axial stress level and confining pressure,the closer the bond at the starting point of particle unloading,and the greater the contact force between particles.Compared with the starting point of unloading,the three-dimensional fabric rose diagram of the unloading failure point changes obviously,and the maximum contact force and the maximum number of contacts are significantly reduced.The unloading leads to the aggravation of the internal damage of the rock sample and the fracture of the bond between the particles.（2）With the increase of water content,the mineral particle structure and pore structure continue to evolve and develop,which become loose,the micro-pores between mineral particles increase,the particle connection effect is weakened,and the pore fractal dimensionincreases exponentially with the increase of water content.The results of triaxial compression creep test of mudstone show that the rheological properties of mudstone with different water contents are quite different.With the increase of water content,the long-term strength of the rock sample decreases,the total creep time decreases.The creep failure modes of rock samples with different water contents are complex,and there are many cracks along the fracture surface.The greater the water content,the more serious damage of mudstone.The creep test results of mudstone with different water content under axial pressure and unloading confining pressure show that under different unloading stress levels,the axial and lateral instantaneous strain and total creep of water-bearing mudstone are much larger than those of dry rock samples.Near the creep failure stage,the lateral strain increases rapidly,and the lateral creep deformation develops more rapidly than the axial direction,showing a significant lateral expansion.The unloading creep failure mode of the rock sample is characterized by tension-shear composite failure.（3）The results of triaxial compression creep simulation test of mudstone show that the maximum contact force value inside the rock sample decreases with the increase of water content.With the increase of axial stress level,the micro-cracks expand from both sides to the center,and the micro-cracks inclination deviates from the direction of the maximum principal stress and the force chain spreads from both ends of the sample to the inside,resulting in the failure of the rock sample.And the anisotropy coefficient of contact force of mudstone with different water content has the same form of expression.With the increase of stress level,it shows a trend of increasing first and then decreasing.The lower the water content of rock sample,the more gentle the change of anisotropic trend coefficient(6.The creep simulation test results of mudstone under axial compression and unloading confining pressure show that the random fracture characteristics of particle bonding are obvious under high water content,while the internal damage expands from both ends to the interior under low water content.The number of contact forces shows a’peanut’distribution state with large on both sides and small in the middle.The number of contact forces of higher water content is"fatter"than lower water content samples at the failure stage.With the decrease of confining pressure,the anisotropic trend coefficient(6 shows a trend of increasing first and then decreasing.However,the greater the drop range of anisotropy coefficient a under higher water content,the contact force inside the sample changes sharply and the damage degree is more severe.（4）Based on the traditional Kelvin element model,considering the influence of water damage and loading damage on rock degradation during creep.Based on the variation of elastic modulus with different water contents,the creep curves under various stress states can be accurately predicted.The viscoplastic body with aging characteristics is introduced to describe the characteristics of mudstone accelerated failure stage of mudstone.The creep constitutive model can reflect the creep characteristics of mudstone with different water contents.（5）The verification results of PFC^3D and FLAC^3D numerical coupling show that under the condition of unloading rheology and groundwater coupling,the force chain is transferred to the arch foot position to form a strong chain area,and the arch bottom is mainly squeezed by the normal stress in X direction,leading to the continuous increase of arch deformation.Compared with the unloading rheological condition,the groundwater effect will significantly affect the internal equilibrium state of the particles,the critical coordination number is slightly lower,and it takes longer to reach the internal stable state,which is more in line with the actual situation.