Structural fire behavior of tunnel sections: assessing the effects of full burnout and spalling effects

This paper outlines a study to evaluate the structural response of a reinforced concrete (RC) tunnel lining subjected to a full-burnout RABT ZTV (train) fire using a coupled thermo-mechanical finite element model that simultaneously accounts for soil-structure interaction, cooling phase of fire, and concrete spalling, aspects often neglected in performance evaluations. A simplified, rate-based spalling model to bound outcomes (no-spalling maps to lower bound; spalling maps to upper bound) is implemented, and damage is classified using four indicators: reinforcement temperature, depth of concrete above 300°C, residual displacements, and cracking potential on the soil-facing side. Results show that spalling increases heat penetration and shifts damage class: circumferential rebar peaks at about 650°C with spalling versus about 400°C without; the heated-concrete depth above 300°C increases from 75 mm to 105 mm; and peak steel temperature occurs during cooling, underscoring the need to model the cooling phase. Soil stiffness mainly affects residual crown displacements (dense: 5 mm; loose: 9 mm) but does not change damage class for the considered case study, and no cracking was found on the unexposed side. The framework supports post-fire assessment and performance-based design of tunnel linings where spalling risk is non-negligible, acknowledging the use of a uniform spalling representation with a 2D plane-strain model.