Research on the mechanical response and damage mechanism of freeze-thaw fractured red sandstone in Pingshanhu mining area, Gansu Province

To investigate the mechanism of strength degradation and rupture mode of fractured rock mass under freeze-thaw, the red sandstone in Pingshanhu mining area was selected as the research object. A comprehensive feedback methods such as experimental research, theoretical analysis, and numerical calculation, as well as multiple index testing methods were used to conduct the freeze-thaw cycles, triaxial compression tests and numerical simulations of rock mass in the sliding zone. The evolution laws of mechanical properties and behavior of fractured rock mass under different freeze-thaw cycles and confining pressures were explored from the perspective of macro and micro damage evolution. Combined with PFC^3D numerical simulation, the entire deformation and failure process of freeze-thaw fractured rock mass was reproduced, revealing the crack evolution process and load damage mechanism. The results indicate that the water content present in the prefabricated fractures increases the contact area between water and rock, resulting in a higher rate of formation of new large and medium-sized pores than the rate of decomposition of small pores; Frost heave force leads to the expansion of micropores and fractures in rock samples, weakening of interparticle bonding, and continuous accumulation of damage; With the increase of confining pressure, the failure mode of intact rock samples after freeze-thaw changes from tension to shear, the rock samples with different fracture lengths all exhibit shear failure mode, and the confining pressure causes rock samples change from brittleness to ductility; The process of rock failure is accompanied by the transformation of strain energy into damping energy and particle sliding energy, the asymptotic effect of damage is quantitatively characterized based on the simulation of rupture energy characteristics. The AE ringing count shows a trend of increasing first and then decreasing with the change of stress, which is divided into four stages: slow increase during the steady period (no obvious deformation in the compression/elastic section), rapid increase during the slow increase period (a small number of microcracks in the plastic section), high-speed increase during the sharp increase period (a large number of cracks in the softening section), and sudden drop during the sharp decrease period (no new cracks are generated in the residual section), reflecting the severity of particle bonding failure in the rock in real time; By changing the volume of water particles to simulate the freeze-thaw process of water, the volume constitutive equation is introduced into the freeze-thaw simulation experiment. The total number of cracks in the rock sample is negatively correlated with the length of pre-existing fractures, with tensile cracks as the dominant factor and shear cracks as the secondary factor. The number of cracks undergoes three stages of small, large, and multiple increases, corresponding to the three processes of smooth-steep-smooth crack evolution curve; The cracks in the intact rock sample gradually develop from the periphery to the interior, while the cracks in the fractured rock sample first appear at the prefabricated fractures. The freeze-thaw damage exacerbates the macroscopic rupture degree of the fractured rock, but the shape does not change with the increase of fracture length. The greater the initial damage caused by fractures, the weaker the rock mass ability to resist deformation under the combined action of the occurrence environment and external loads.