Study On Failure Law And Meso-mechanism Of Sandstone Under Triaxial Loading Coupled With Different Water Pressures

In recent years,the long-term operation safety of high dam reservoirs has become the focus of both engineering and academia.After the high dam reservoir project has been impounded,the water-rich state and water pressure environment of dam foundation and reservoir bank rock mass have changed,resulting in stress redistribution and deformation of bank slope rock mass,thus inducing the engineering problems such as bank slope instability and dam collapse,and poses a huge threat to navigation safety,life and property safety of residents on both banks.Rock mass stability problem caused by reservoir impounding of the high dam is a progressive failure problem of fractured rock mass under changing water pressure,which is a hot and challenging problem in current research.In this thesis,the action regularity and failure mechanism of the pre-pressure damaged sandstone under different high constant pore water pressure is intensely studied by triaxial compression tests.The failure regularity and mesoscopic mechanism under different water pressure coupling triaxial loading conditions combined with acoustic emission monitoring,CT scanning technology and PFC numerical simulation.The mesoscopic mechanism of complex sandstone failure under water-rock coupling is revealed from the mesoscopic level.The main research work and conclusions in this thesis are as follows:(1)Research work.Triaxial compression test schemes under different constant high pore water pressures was designed,and sandstone specimens was prepared according to ISRM.The MTS815 test machine was adopted to carry out triaxial compression tests under different constant pore water pressures at 10 MPa and 80 MPa,the acoustic emission signals were simultaneously collected and analyzed by PCI-2acoustic emission instrument,CT scanning technology was used to study the internal deterioration law and failure pattern of the specimens,and the PFC software is used for numerical simulation.The methods can verify the test results and explore the waterrock coupling mechanism.(2)Macro-mechanical regularities.Based on triaxial compression tests under different constant high pore water pressures,the effects of different confining pressures and different water pressures on the mechanical properties,fracture modes and failure mechanisms of sandstone samples were studied.It is found that with the increase of pore water pressure,the deterioration damage effect of sandstone specimens is enhanced and the macroscopic failure mode changes from plastic failure to brittle failure.Confining pressure inhibits pore water pressure and delays destabilization and destruction.Under 10 MPa confining pressure and different water pressures,sandstone specimens have a single fracture surface,i.e.,S-shaped single fracture surface for low water pressure conditions and Y-shaped fracture surface at high water pressure condition,which generally belongs to inclined section failure.Under 80 MPa confining pressure and different water pressures,the fracture surfaces of sandstone specimens are complicated,such as horizontal section failure,Z-shaped failure and multi-direction failure in middle area.(3)Mesoscopic fracture mechanism.Acoustic emission characteristic parameters are obtained utilizing the acoustic emission monitoring system and analyzed.It is found that water pressure can change the failure mode of tensile shear of specimens,and the degree of bond damage under different pore water pressures is quantified.Under 10 MPa confining pressure,the acoustic emission signals are mainly tensile failure signals,and the specimen is tensile failure.Under 80 MPa confining pressure,with the increase of water pressure,the proportion of tensile failure signal increases,and the proportion of shear signal decreases.The failure mode changes from shear failure to tensile failure.The distribution law of dominant frequency is different in each loading stage.Pore compaction and bond failure in stage A lead to the development of micro-cracks.Low-frequency signal is dominant and a few highfrequency signals are characterized.The water pressure in stage B weakens the bond structure of sandstone specimens,reduces the bond strength of particles,and produces a small amount of dominant frequency signals.When the water pressure-confining pressure ratio increases to 50%,instable failure occurs.The main control fracture surface has been formed in stage C.The dominant frequency is mainly low-frequency signal.The high-frequency signal increases as water pressure increases,and the highfrequency signals are dense.The specimen fails suddenly.From the water pressureconfining pressure ratio,the increasing rate of bond damage at high confining pressure is higher than that at low confining pressure.Under certain confining pressure,the increasing rate of bond damage degree under low water pressure is greater than that under high water pressure,and the degree of bond damage tends to converge.If the water pressure is continuously increased,the specimen will destabilize and failed during the stabilization stage.(4)Water-rock coupling mechanism.Using Particle Flow Code(PFC)software,a model was established and the micro parameters of particles and bonding were calibrated.The calibrated model was used to verify the results of different waterpressure coupled triaxial loading tests,and the mechanism of water-rock coupling was revealed at the mesoscopic level.The tensile shear failure mode of the model is consistent with the experiments,and the development trend of typical fracture surfaces is consistent.It is found that water pressure can promote crack concentration.Under 0MPa water pressure,cracks are distributed throughout the entire specimen.The entry of water pressure promotes the concentration of cracks that are initially dispersed inside the specimen to form a fracture surface,while cracks in other areas decreases.As the water pressure and confining pressure increase,the occurrence time of cracks advances,the number of cracks increases,and the contact force increases.The contact force reaches its peak at the peak stress,and then decreases significantly.After the peak,the contact force blank area appears at 80% post-peak stress.