Study On The Relaxation And Mechanical Properties Of Granite And Corresponding Engineering Application

The mechanical test of rock relaxation is an important method to study the rock relaxation phenomenon as well as analyzing and evaluating the long-term safty and stability for rock engineering.With the rapid development of China's transportation,large-scale hydropower projects,the problems for the safety and long-term stability of underground engineering and rock slope excavation engineering are becoming more and more serious.It is necessary to conduct researches about the relaxation and relaxation properties of harn rocks,especially for uniaxial and triaxial relaxation tests.On account of this and on the basis of previous researches,the present study combines laboratory test,theoretical analysis and numerical simulation to explore the uniaxial and triaxial relaxation characteristics,as well as strength and failure characteristics of deep hard rock under different stress conditions.Moreover,constitutive model is derived from the previous theoretical study.Furthermore,secondary development is conducted with the help of FLAC^3D software based on the constitutive model.Also,simulation studies are presented a certain engineering project.The main research contents are as follows:?1?The experiment of subcritical crack growth and fracture toughness of granite under the condition of constant displacement relaxation was carried out by the AG-I 250kN precise universal material testing machine.The mechanical characteristics of relaxed rheology of hard rock are studied,and the fracture toughness and subcritical crack growth threshold of granite were obtained.In the meanwhile,combining the fracture toughness and subcritical crack growth threshold of granite and Burgers creep model,this paper presents a calculation model of transient and relaxation crack growth in hard rock during loading and relaxation,and the calculation model is applied to the calculation of microcrack growth in hard rock under different stress relaxation conditions.?2?Relaxation experiments at different axial strain levels and different confining pressure unloading rates were carried out by using the MTS 815 multi-functional electro-hydraulic servo-controlled rigid testing machine,then the effects of relaxation conditions on the behavior of hard rock relaxation under triaxial test are studied.The results show that the rock can show a certain stress relaxation behavior under different strain levels.It is found that the stress relaxation of rock and the length and speed of relaxation crack have the same variation trend.The difference between the axial pressure and the confining pressure of the rock specimens,that is,the greater the deviation pressure,the more obvious the stress relaxation behavior of rock.At the same time,the unloading rate of confining pressure has a significant influence on the relaxation effect of the rock material.At the same time,the unloading rate of confining pressure has a significant influence on the relaxation effect of the rock materials studied.As the unloading rate of confining pressure increases,the relaxation behavior of rock becomes less and less obvious.?3?MTS5504 universal testing machine,3D digital image related technology and Olympus SZX16 stereoscopic microscope were used to conduct conventional uniaxial compression test and multistage relaxation relaxation test on granite specimens containing pre-existing cracks,and the characteristics of crack growth and macroscopic strain during stress relaxation are analyzed.The effect of stress relaxation on the strength and failure of hard rock is introduced from the perspective of macroscopic stress and strain,macroscopic crack growth form and microcosmic fracture characteristics.It was found that the maximum axial strain and axial compressive strength of specimens increased with the increase of relaxation test at different axial strain levels.At the same time,the lateral strain in the area near the surface crack was relatively obvious from the crack initiation phase to the final failure.The lateral strain values along the crack growth direction show a certain time sequence during the loading process,and the crack faces of specimens show two main fracture modes under the microscope,which are tensile fracture mode and shear fracture mode.The failure mode of specimens in conventional uniaxial compression test is mainly axial splitting failure,while the fracture surface of specimens in stress relaxation test can be found to have friction and shear characteristics.?4?Combined with the previous research results,based on the modified Mohr-Coulomb criterion and implicit constitutive integral algorithm,a stress and strain model of rock considering plastic-damage coupling effect is established.Combing the loading conditions of uniaxial compression and stress relaxation tests for granite specimens,the stress and strain relationship and damage evolution of rock under uniaxial compression and relaxation conditions are predicted by using the proposed numerical solution of rock stress and strain considering plastic-damage coupling effect.Meanwhile,the calculated results are compared with the experimental data to verify the reliability of the model.?5?Based on the established model,which considers the plastic-damage coupling effect,the stress and strain of rock during loading and relaxation are redeveloped and imported into the FLAC3D software.The long-term stability of the Tonghu tunnel exvation project is analyzed by using the loading and relaxation model,and the long-term stability of rock engineering under complex stress is predicted.