Modeling and Experimentation of Fragmented Coal Porosity Evolution Under Stress, Temperature, and Moisture

The spontaneous combustion of residual coal in the goaf poses serious threats to mine safety, particularly in accumulation zones. Porosity plays a pivotal role in governing oxygen transport and thermal accumulation, and is markedly affected by stress, temperature, and moisture conditions. To quantify these effects, this study develops a porosity evolution model incorporating multi-field coupling based on fractal and Weibull distribution theories. Validation experiments were conducted using a low-temperature oxidation apparatus on pressure-fractured coal. Model predictions closely matched experimental results, with strain prediction errors remaining below 3% across a temperature range of 23°C–120°C under 15 MPa axial stress, and a maximum deviation of 9.2% at 60°C. The results reveal that (1) increased stress promotes uniform particle size distribution; (2) coal compaction follows a three-stage process of fragmentation, rearrangement, and interparticle extrusion; and (3) rising temperature slows porosity reduction due to thermal expansion, whereas high moisture reduces elastic modulus and promotes volumetric expansion. This work offers theoretical and experimental insights into porosity evolution under coupled fields, contributing to improved understanding of spontaneous combustion risks in goaf environments.