Stability assessment of shallow tunnels subjected to fire load

During recent fire accidents in European tunnels, significant spalling with spalling depths up to 50% of the thickness of the concrete lining was observed. In addition to spalling, the remaining part of the lining was subjected to thermal degradation, resulting in an increase of the compliance of the lining support. In tunnels with high overburden, this increase of the support compliance activates the load-carrying capacity of the surrounding soil. In shallow tunnels, on the other hand, the soil as such may not be able to maintain the stability of the tunnel. In order to assess the stability of shallow tunnels with continuously increasing support compliance in consequence to fire load, the extension of the “beam-spring” model towards consideration of effects associated with tunnel fires is proposed. At first, layered finite beam elements are used for the discretization of the lining, allowing consideration of spalling by deactivation of layers following a prespecified spalling scenario. Secondly, the change of material properties in consequence of high temperatures is accounted for within elastoplastic material models employed for the description of the mechanical behavior of concrete and steel. Based on different fire-load scenarios and spalling histories, an answer to the central question “whether or not spalling can cause collapse of shallow tunnels” is attempted by the application of the proposed analysis tool to the Lainzer tunnel (Austria), a tunnel recently constructed and characterized by low overburden.