Fire-induced damage in tunnels: Thermo-mechanical modeling incorporating support system and geological conditions

Abstract

The thermo-mechanical response of underground tunnels to fire loads is governed by interactions between the tunnel structure and the surrounding geological formation. Accordingly, for accurate prediction of damages in the tunnel structure subsequent to a fire event, it is critical to incorporate the state of the support system as well as in-situ conditions in the surrounding geology at the time of the fire event. Current tunnel-fire studies are mainly developed adopting beam-spring elements, thus disregarding the intricacies of the surrounding geological formation, its true constitutive behavior, as well as realistic tunnel-rock interactions. Another limitation in current tunnel-fire studies is assuming uniform temperature loads within the tunnel interface using time–temperature curves obtained from design codes, therefore ignoring the spatial nature of fire-induced temperatures along the tunnel interior. Temperature gradients can generate additional damage to the tunnel structure. This paper presents a comprehensive integrated numerical framework aimed to predict fire-induced damages in underground tunnels, incorporating four novel components: (i) detailed design of the tunnel support system and in-situ conditions; (ii) degradation of the original tunnel support system over the years due to presence of chloride and sulfates in nearby environment; (iii) thermal gradients generated along tunnel cross section due to real fire events; (iv) and thermo-mechanical interactions between the tunnel structure and the surrounding geological medium, incorporating thermal-induced strength degradation in the concrete and in the steel support elements following design standards from Eurocode. The numerical framework consists of fire simulation using computational fluid dynamics, and thermo-mechanical modeling of the tunnel response to fire. The proposed numerical framework has been utilized to simulate the Caldecott tunnel fire as the case study in this paper, and to evaluate the impacts from the in-situ stress regime and rock strength parameters on the thermo-mechanical behavior response of tunnel to fire events. Results provide a novel insight into the geomechanical response of horseshoe tunnels to fire. Findings reveal the critical zones prone to damage during a fire event to be highly governed by the in-situ stress field and the strength parameters of the surrounding geological strata.