Strenth And Creep Properties Of Peak Front Unloading Damaged White Sandstone Materials

Deep underground projects in a deeply buried or high ground stress environment will inevitably encounter the stability of the subsurface rock mass.Compared with shallowly buried engineering rock masses,deep rock masses are the carriers of the direct action of deep resources and underground space development.During the extraction of deep underground resources,the surrounding rock undergoes a complex process of loading and unloading,so that the extraction of deep engineering rock masses is accompanied by strong unloading disturbances.For hard rock underground engineering,loading and unloading of stresses often leads to two forms of instability and damage:(1)gravity-driven block instability controlled by structural surfaces,such as ground collapse and rock collapse;and(2)stress-induced damage or yielding,such as spalling,slabbing,swelling and rock bursting.It poses a great threat to the stability of the rock surrounding the underground project and the personal safety of the site personnel.Therefore,the study of deformation damage mechanism of rock body under unloading conditions not only has important theoretical value,but also provides theoretical basis for designing reasonable support means before destabilization of deep roadway,and has the purpose of controlling large deformation damage of roadway surrounding rock.This paper is based on the"Study on the Equivalent Law of Creep Instability of Sandstone after High Temperature"(ZR2021QE202)of the Natural Science Foundation of Shandong Province and the"Study on the Equivalent Law of Creep in Porous Media by Stress-Damage Field of Underground Engineering"(JYBSYS2018204)of the Key Laboratory of the Ministry of Education for Safe and Efficient Mining in Coal Mines.(JYBSYS2018204),the uniaxial compressive strength test of peak-unloading damaged white sandstone,the triaxial compressive strength test of peak-unloading damaged white sandstone with the combined effect of peak-unloading damage and surrounding pressure,and the uniaxial creep test of peak-unloading damaged white sandstone were carried out to investigate the effects of peak-unloading damage and surrounding pressure on the strength and creep properties of white sandstone.The effects of pre-peak unloading damage and surrounding pressure on the strength and creep properties of the white sandstone were investigated.The microstructure of the fracture surface of the peak-unloading damaged white sandstone was observed by scanning electron microscopy,and the fine-scale damage form was analysed in conjunction with fractography,and the evolution mechanism of the peak-unloading damaged white sandstone from macroscopic to fine-scale damage was established.Based on the energy dissipation principle,a damage intrinsic model of white sandstone was established,and creep damage variables were proposed based on the time-dependent damage effect,and finally a creep model of white sandstone Xiyuan was established considering the pre-peak unloading damage effect.Based on the experimental results,a comparative numerical simulation of the uniaxial re-load test of the peak front unloading damaged white sandstone was carried out using ABAQUS software,and the experimental results were in good agreement.The main research elements and conclusions are as follows:(1)Triaxial compressive tests on the effect of peak preload relief damage and underground engineering envelope pressure on the strength of white sandstone in the fracture zone of deep shafts were carried out to determine the peak preload relief points of white sandstone as peak preload relief strength(₇)=0?50%?60%?70%?80%?90%)and peak preload relief damage envelope pressure as 1 MPa.The basic physical and chemical properties of the white sandstone raw material were tested and the preparation of the peak preload damage white sandstone specimens was described.(2)Uniaxial compression tests were carried out on white sandstone specimens with pre-peak unloading points₇)=0?50%?60%?70%?80%?90%and an unloading damage perimeter pressure of 1 MPa to investigate the strength characteristics and deformation patterns of pre-peak unloading damaged white sandstone,and to observe and analyse the microscopic fracture structure of the specimens with the aid of scanning electron microscopy.The uniaxial compressive strength of the Lemaitre strain-equivalence principle is used to describe the uniaxial compressive strength of the Lemaitre strain-equivalence principle.Based on the principles of energy dissipation and energy release,the damage intrinsic model of peak preload damage white sandstone was established from the perspectives of concrete and rock mechanics.The damage intrinsic model of peak preload damage white sandstone can better describe the stress-strain law of white sandstone under different damage degrees,and the relationship between the critical damage value D_cr and the damage degree was analysed.The model was then improved by considering the energy loss of acoustic energy and optical energy,and the improved model could better describe the damage variation of the peak-unloading damaged white sandstone.A triaxial compression test was then carried out to investigate the strength characteristics and deformation pattern of the white sandstone under the action of peak front unloading damage(₇)=0?50%?60%?70%?80%?90%)and surrounding pressure(1 MPa,2 MPa,3 MPa).Stress Mohr circles were plotted based on the experimental data and the M-C yield criterion to obtain parameters such as cohesion c and friction angle?.Numerical simulation compression tests of white sandstone under the weakening influence of peak front unloading damage were carried out using ABAQUS numerical simulation software,and the effects of peak front unloading damage action on the uniaxial strength properties and deformation law of white sandstone were analysed,and the results of the study proved the rationality of the numerical simulation tests carried out.(3)Creep tests were carried out on white sandstone specimens at different levels of damage pre-peak compressive strength(₇)=0?50%?60%?70%?80%?90%).The white sandstone specimens are elasto-plastic materials and exhibit significant creep properties.A particle swarm optimisation BP neural network(PSO-BPNN)prediction model was developed based on the creep test of peak preload damaged white sandstone,which takes into account the physical and mechanical properties of peak preload damaged white sandstone.The PSO-BPNN prediction model was able to predict the long-term strength of the pre-peak unloading damaged sandstone better than the BPNN prediction model.The Nishihara creep model can better describe the decelerating creep phase and the stable and accelerating creep phases of the pre-peak unloading damaged white sandstone.The non-linear creep model is based on the creep damage variables of the time effect,and the theoretical curve fits the test curve very well.