Experimental investigation of failure characteristics and permeability evolution of thermally treated granite under hydro-mechanical couplings

In recent years, the increasing frequency of tunnel fires has drawn significant attention. The high temperatures generated during such fires can severely damage the surrounding rock mass, leading to a deterioration of rock strength and enhanced permeability. Ensuring the stability and safety of tunnel rock exposed to fire is therefore of urgent concern. This study performed triaxial compression experiments on granite samples that had been heated to 25, 300, 500, and 700 °C in order to examine the patterns of change in the rock's characteristic stress, permeability, and strain energy under varying heat treatment temperatures and seepage pressures. The results indicate that P-wave velocity and hardness of granite significantly decrease above 500 °C. As heat treatment temperature and seepage pressure increase, granite's characteristic stress, elastic modulus, and brittleness index all decrease. Furthermore, thermal treatment exacerbates the detrimental effect of seepage pressure on rock strength. Heat treatment significantly impacts granite permeability, with permeability increasing by a factor of 57 at 700 °C. The permeability evolution curve exhibits strong correlation with changes in elastic energy proportion. Granite permeability enters a sudden increase phase after its elastic energy fraction reaches a peak. Scanning electron microscopy (SEM) observations reveal significant increases in thermal crack width and length after 500 °C, providing microscopic support for explaining the cause of this permeability surge.