Damage mechanism of coal samples under the coupling effect of sulfate erosion and wet-dry cycles: Experiments, constitutive models, and numerical simulations

The coal pillar is a critical structural component of underground reservoir dams in coal mines. Prolonged cyclic storage and drainage of sulfate mine water can cause significant damage and degradation to these pillars. Understanding the damage evolution of coal under the combined effects of sulfate solutions and dry-wet cycles is vital for assessing the stability of coal mine reservoirs. This study investigates the degradation patterns of coal's physical and mechanical properties by conducting uniaxial compression tests on coal samples subjected to varying dry-wet cycles in de-ionized water and sulfate solutions. The results reveal how sulfate erosion and dry-wet cycles influence the coal's characteristics. A numerical model was developed to examine the microscopic particle-level damage mechanisms of the coal, and a refined damage constitutive model was established. The findings indicate that sulfate solution and dry-wet cycles increase the coal's water absorption rate and decrease P-wave velocity. Additionally, both the peak strength and elastic modulus of the coal decrease. The failure mode transitions from tensile failure to shear failure as the number of dry-wet cycles increases. In the numerical simulations, the contact network loses its load-bearing capacity, even before extensive damage occurs, with force chains concentrating at the ends. While the total number of cracks decreases, shear cracks become more prominent. The degradation effect under sulfate solution is more pronounced. The modified damage model closely aligns with the experimental results, providing theoretical support for the durability of coal pillars in sulfate mine water environments.