Road tunnel fire risk analysis: Is 1-D analysis inferior to 3-D?

Road tunnels are one of the most critical infrastructure elements of modern road networks. Fire, probably the most dangerous accident in a tunnel, is rare but if they occur, they can have disastrous consequences. As far as the management of fire events is concerned, the common practice is based on risk analysis studies being conducted following the specific requirements each country has adopted. The country-based normative standards dictate whether a one-dimensional or a three-dimensional analysis shall be performed. Both kinds of analysis aim to provide a forecast of the evolution of the fire and, as such, to give a reliable indication of the threat that a fire event would pose to the successful evacuation of the users being trapped inside the tunnel. However, the 1-D and 3-D analyses do not always provide identical results and cautiousness should be shown not to introduce unnecessary, additional, resource-consuming safety measures or the opposite. This study provides an illustrative comparison of the one-dimensional and three-dimensional analysis to showcase the fallacies that may occur. The analysis is applied on a typical Greek tunnel fulfilling the minimum requirements of Directive 54/2004/EC. The results illustrate that differences in the obtained results may indeed occur. For instance, the gas temperature estimated from an 1-D analysis is higher than the 3-D analysis at low heights, close to the fire source (up to 50 m), while the radiant heat flux levels are much lower than the corresponding 3-D predictions. However, these differences are alleviated further upstream of fire location. By uncovering potential fallacies and challenges included in risk analysis studies towards fire events, this study aims to support tunnel operators and risk analysts in proposing realistic safety measures, when and if necessary.