Evolution Characteristics of Coal Fractures under High-Temperature Coupling with Triaxial Stress

Underground coal fires pose severe hazards to safe mining and environmental sustainability, with fracture development in coal serving as critical pathways for oxygen supply and heat transfer. However, the dynamic evolution of fractures under coupled high-temperature and triaxial stress conditions remains poorly understood. In this study, bituminous coal samples from a fire-affected mining area in Xinjiang, China, were subjected to high-temperature treatments (200 and 400 °C) and subsequently loaded under triaxial compression (confining pressure 5 MPa) while being monitored by an industrial CT scanning system. Full stress-strain curves were obtained, and VG Studio MAX software was used to quantitatively analyze fracture morphology, volume, density, porosity, and 3D fractal dimension across five successive loading stages. The results reveal that temperature governs the mechanical behavior of thermally treated coal. With increasing temperature, the compaction stage shortens, elastic modulus decreases, and peak stress first increases then decreases, while the durations of plastic deformation and postpeak failure stages first shorten then lengthen. The porosities of RC, C200, and C400 coal samples at the first scan were 0.07%, 0.28%, and 0.70%, respectively, indicating that high-temperature treatment increased the initial porosity of the coal samples. Based on fracture density, porosity, and fractal dimension, fracture evolution is divided into three stages: smooth development, slow growth, and rapid growth. The 3D fractal dimension further confirms this staged evolution. These findings provide a theoretical basis for predicting fracture network evolution in coal fire zones and offer practical guidance for fire prevention strategies.