A risk analysis in the event of a fire in a twin-tube road tunnel with considerations for resilience.

The paper is an extension of our previous study in which it was investigated the loss of resilience of a twin-tube motorway tunnel in the event of a traffic accident in the north tube with the resulting blockage of either the right lane or both lanes. The functionality of the tunnel system was assumed to be recovered by using the remaining undisrupted lane of the tube interested by the disruptive event (i.e., only one lane is closed) or reorganizing the traffic flow by using the parallel tube for bi- directional traffic (i.e., both lanes of the north tube are closed). In this study, the average travel speed of traffic flow was used as a resilience metric and coupled this with the risk level of tunnel users in the event of a simultaneous occurrence of a fire in the left lane of the north tube or in the adjacent tube, respectively. A Quantitative Risk Analysis, which is based on the results of Computational Fluid Dynamics (CFD) modeling, was developed to assess the risk of users under different combinations of fire scenarios in the aforementioned undisrupted lane or in the parallel tube when the north tube characterized by a traffic accident is partially or completely closed. The results of the QRA were found to be coherent with those of the tunnel resilience based on the average travel speed. A lower risk level was found with the partial closure of the tube in contrast to the complete one, and by activating the Variable Message Signs (VMSs) to suggest an alternative route to Heavy Good Vehicles (HGVs) only. This study can increase our knowledge on the operative conditions of road tunnels, and can help to make a more suitable choice in the recovery process of functionality accounting for the exposure to risk and resilience at the same time.