Distributed fiber-optic monitoring of internal temperature and strain of rock support system subject to fire

• The temperature sensitivity coefficient of polyimide fibers was calibrated. • The capacity for temperature and strain sensing by polyimide fibers was validated. • The integrated rock support system specimen was subjected to fire heating and water-cooling. • Spatiotemporal temperature changes and strain were monitored by the OFDR technique. Current experimental findings about the spatiotemporal response of rock support systems under fire remain limited. This study innovatively employs advanced distributed fiber optic sensing (DFOS) technology to investigate the temperature and strain distribution of an integrated rock support system specimen, comprising shotcrete, concrete lining, grout, granite, and rock bolt, subjected to fire heating and subsequent water cooling. The temperature sensitivity coefficient of polyimide fibers was calibrated, and its capacity for temperature and strain sensing was validated. After 136 min of heating, the peak temperature of the rock bolt reached 117.9 °C, which can generate evaporation pressure, and the peak strain of + 2251 με was observed, potentially harming the concrete integrity. Considering the peak temperature and strain in granite attained at 78.3 °C and 0.265%, respectively, the associated rock thermal damage is non-negligible. The heat conduction effect of the rock bolt in the surrounding rock is considerable. The cumulative axial displacement of the rock bolt after heating is 15% larger than that of the granite, which may degrade the bonding between them. Water cooling cannot effectively reduce the temperature inside concrete. The cooling rate at 5 cm away from the fire-exposed surface is −1.1 °C/min. The strains on the granite remain stable throughout the cooling process. The contraction deformation mismatch during cooling may cause secondary damage to the bonding between the rock bolt and the rock. The experimental results could lead to improved understanding of the thermal damage caused by underground fire and subsequent cooling to components and interfaces of the rock support system.