Effects of fire on the mechanical response characteristics of tunnel surrounding rock: insights from scaled model tests with similar thermal response

To investigate the degradation of the load-bearing capacity of the surrounding rock in tunnels under fire conditions, this work employed the surrounding rock of the Baishuishan Tunnel as the geological background. A nano-alumina-modified cement-based similar material was developed to reproduce the thermal response of the prototype rock, ensuring that its thermal damage and degradation characteristics were consistent with those of the natural rock. Based on this material, a scaled physical model of the prototype tunnel was constructed and subjected to mechanical loading after simulated fire exposure of varying durations. The temperature diffusion and evolution within the surrounding rock during fire exposure were quantitatively analyzed, and the effects of fire duration on the post-fire mechanical response of the surrounding rock structure were evaluated using acoustic emission (AE) monitoring and digital image correlation (DIC) techniques. The main findings are as follows: when the fire duration reached 60 min, the temperature at the tunnel crown exceeded 300 °C. With the extension of fire duration to 240 min, the high-temperature region expanded, although the rate of temperature increase diminished. Both the ultimate bearing capacity and the generalized elastic modulus decreased linearly with increasing fire duration, with reductions of 36.7% and 40.0%, respectively, observed at 480 min. When the fire duration was ≤ 240 min, the predominant failure mode was shear failure, whereas at durations was ≥ 360 min, a mixed tensile-shear crack network developed. The sustained and active AE signals provided a quantitative measure of the progressive damage within the surrounding rock.