Insight into the effective-stress deviation and thermal-damage weakening of coal under real-time THM coupling

Coal-bearing strata in deep underground engineering are commonly subjected to coupled thermal, hydraulic, and mechanical actions, which may alter the applicability of classical effective-stress concepts. In this study, real-time THM triaxial compression tests were conducted on coal specimens under different temperatures, confining pressures, and pore water pressures. The effects of thermal, hydraulic, and mechanical conditions on deformation behavior, peak strength, energy evolution, and failure characteristics were systematically investigated. The results show that increasing temperature reduces the peak strength and pre-peak deformation capacity while modifying the energy-storage behavior and failure mode of coal. At 25 °C and 50 °C, the peak-strength response remains broadly consistent with the classical effective-stress relationship, whereas a clear deviation occurs at 75 °C, indicating enhanced temperature-related structural deterioration. Based on these observations, a revised peak-strength framework was proposed by extending the classical effective-confinement formulation to consider hydraulic weakening and temperature-related stiffness degradation. The framework provides a better description of the high-temperature strength variation and shows satisfactory prediction capability for the 75 °C dataset without additional calibration. These findings provide new insights into the evolution of coal strength under THM coupling and highlight the importance of distinguishing hydraulic weakening from temperature-induced structural degradation.