Evolution of Broken Coal’s Permeability Characteristics under Cyclic Loading–Unloading Conditions

This study conducted a cyclic loading–unloading (CLU) test on broken coal samples with three particle sizes (0–5 mm, 5–10 mm, and 10–15 mm) under four different stress path conditions. The evolution permeability characteristics of samples during repeated compaction were investigated. The dimensionless permeability and the porosity variation law were obtained under CLU conditions. The permeability loss difference (PID) index was defined, and the permeability damage was analyzed. The permeability evolution model under mining influence conditions was constructed. Results indicate that an increase in maximum loading stress (MLS) exacerbates the seepage network channel destruction of broken coal. As the MLS increases, there is a decrease in permeability recovery rate during the unloading stage and an increase in permeability loss rate during the loading stage. The first stress loading results in a rapid reduction in the porosity, while the subsequent CLU has a minor impact on porosity variation. Results of the PID analysis show positive correlation between the permeability attenuation degree and the MLS. Furthermore, both the permeability recovery rate and the permeability loss rate increase with increase in particle size, indicating that the effects of pressure relief and stress recovery have a greater influence on larger particles. Theoretical permeability values of model were verified with test results, and their high consistency proves the permeability evolution model’s feasibility. The results will help provide theoretical guidance for gas extraction in goaf.