Study On Macroscopic And Microscopic Failure Mechanism Of Marble Under Constant Axial Stress Unloading Confining Pressure

The excavation of underground engineering is an unloading process,and the rock underground engineering excavation is an unloading process,and there is a clear difference between the failure mechanism of the rock mass under the loading path and the unloading path.There have been many studies on the mechanism of rock unloading failure,and scholars have carried out a large number of macro-failure experiments,but the study of the mesomechanism of rock unloading failure has not been perfect.In this paper,a combination of test and numerical simulation was used to carry out the conventional triaxial and constant axial pressure failure tests and granular flow mesoscopic simulation of the marble,and analyze the mechanical characteristics and failure modes of the marble under the unloading path.As well as the law of energy conversion,the effect of unloading rate,initial unloading position and confining pressure on rock failure is discussed.The main achievements of the paper are as follows:(1)Carry out conventional triaxial and pre-peak and post-peak failure tests of constant-pressure unloading confining pressure on marble,and analyze the variation law of elastic energy and dissipation energy in each stage of marble deformation under loading and unloading paths.The results show that the confining pressure suppresses the energy dissipation,thereby delaying the release of elastic strain energy.This phenomenon is particularly evident in conventional triaxial tests.Under the unloading path,the elastic energy release rate is accelerated,and the model damage is more severe.(2)Based on the test data of the marble failure process,the Fish language programming is used to establish the particle flow numerical model in the PFC2 D program,and the variable control method is used to systematically study each meso parameter in the particle flow program PFC2 D to the model macro parameters(such as elastic modulus,Peak stress,Poisson's ratio)and the influence law of the form of failure,finally a set of meso parameters that can correctly reflect the deformation characteristics of marble.The simulation results show that the meso model's curve shape,peak strength,peak strain,failure mode and other aspects are consistent with the laboratory test results under external load.(3)The particle flow program was used to simulate the failure of marble under constant axial pressure and unloading confining pressure,and the influence law of the confining pressure,unloading rate and initial unloading position on the meso-fault of marble was analyzed.After the unloading starts,under different unloading rates,the difference in the number of internal cracks in the rock sample mainly occurs during the plateau period of the stress-strain curve(ie,the curve section where the axial force remains constant);the unloading rate will change the internal cracks in the rock sample Distribution form: the higher the unloading rate,the faster the suppression effect of confining pressure decreases,which makes secondary cracks more likely to spread laterally,and the distribution trend along the main shear plane is weakened;after the model is broken,particles are distributed along both sides of the failure plane,In the opposite direction.The average particle displacement increment affects the formation of the failure surface to a certain extent;the confining pressure can change the distribution ratio of tensile shear cracks.As the confining pressure increases,the ratio of shear cracks to total cracks increases greatly.