Geothermal Gradient of Underground Structure Subjected to Fire and Blast Loads

The lack of space above the ground has forced human civilization to go underground for sustainability. It is not only space but also comfort that plays a key role in the development of such an underground structure. Nevertheless, it is crucial to acknowledge that underground structures, specifically tunnels, are vulnerable to fire and blast loads in the event of terrorist attacks. This rise in temperature due to fire, when accompanied by external threats like bomb blasts or missile attacks, may adversely increase the geothermal gradient at the point of action. Hence, necessary numerical studies need to be carried out to assess the effect of these kinds of external hazards on underground structures. In the present study, a combined fire and blast analysis of a tunnel built in sandstone rock is performed to predict the geothermal gradient generated across the tunnel cross-section. A three-dimensional non-linear finite element numerical model was developed with a circular tunnel of diameter of 5.6 m and in a rock domain of 200 m × 200 m × 200 m using ABAQUS GUI Interface. Initially, the thermal analysis was carried out with application of heat flux time history at the center of tunnel. Thereafter, the blast analysis was carried out for 50 kg PETN explosive at the center of the tunnel. The study conducted herein shows the strength degradation due to the thermal effect in the tunnel periphery. The findings of the study indicate that the critical zones susceptible to damage are primarily influenced by the magnitude of the pressure. This information can create an insight into the design of resilient underground structures with improved safety.