Structural Fire Safety of Circular Concrete Railroad Tunnel Linings

In many tunnel designs, lining cross section is selected based on construction requirements rather than design loads. A constant cross section is typically used along a tunnel especially for tunnels constructed by tunnel boring machines (TBMs). Factor of safety against failure is typically high at shallow depth regions of alignment. Minor repairable damage is targeted at rare events such as earthquakes, train derailments, explosions, and long-duration fires, and a reduction to service load factor of safety is applied in these types of events. The focus of this paper is to analytically investigate structural fire safety of circular tunnel linings in terms of reduction in service load safety due to time- and temperature-dependent material degradation and increase in load demand in a tunnel fire, and to develop recommendations for preliminary assessment of structural fire endurance of circular tunnel linings. Analytical methods accounting for thermal nonlinearity, material degradation, tunnel lining-ground interaction, and fire time stages are available to assess the structural fire safety of the concrete tunnel linings. Results of hydrocarbon fire tests of tunnel segments can be estimated by available analytical methods. TBM tunnels at soft soil can have a better fire performance compared to the ones located at stiff conditions with similar initial loading