Shear mechanical properties and microscopic evolution characteristics of fractured sandstone under normal disturbance

To study the shear mechanical behavior and micro-evolution characteristics of fractured rock masses under normal disturbance, direct shear tests of fractured sandstone under constant normal stress were carried out, and using PFC2D, discrete element model considering different fracture angles was established to study the shear strength and failure characteristics of the specimen under constant normal stress and normal disturbance. Based on the microscopic evolution laws during the shearing process, the crack propagation pattern and energy dissipation mechanism of fractured sandstone under normal disturbance are analyzed. The results indicate that prefabricated cracks significantly reduce the shear strength of rocks by 9.79%-20.28% and 11.12%-16.78% for different crack angles under two loading modes. As the crack angle increases, the slope of the stress-strain curve decreases, and shear strength and peak strain exhibit a first increase and then decrease trend, showing a clear angle effect. Shear strength and crack angle have a cubic function relationship. Normal disturbance weakens the influence of angular effects, and accordingly the ability of the rock to resist shear deformation is weakened and damage is advanced. During the shearing process, the internal force chain of the rock exhibits an evolving trend consistent with the direction of the load, and microcracks mainly arise and develop from both ends of the specimen and the ends of the prefabricated cracks. Microcracks are mainly distributed at angles between 20° and 80°, with shear cracks accounting for 78% to 81% after failure. The disturbance causes an increase in the number of different types of cracks, the initiation displacement decreases and the proportion of shear cracks increases. The development of acoustic emission ringing count can be divided into four stages: calm, slow increase, outbreak and stable periods. Under the disturbance, the “calm period” is shorter, and a stepwise increasing trend appearred during the “slow increase period”. The total energy at the failure point of the specimen is lower in the shear test under normal perturbation. Most of the input energy is stored in the sample in the form of elastic strain energy until the sample is destroyed, and the elastic energy under the normal disturbance is significantly lower than constant normal stress effect, with a reduction of 9.87% to 13.94%. The disturbance reduces the energy storage capacity of the sample, increases dissipation energy, facilitating crack formation and ultimate failure.