TRAM: a New Quantitative Methodology for Tunnel Risk Analysis

The paper illustrates and describes the structure of a new quantitative model of risk analysis for road tunnels named TRAM (Tunnel Risk Analysis Model). The result of the model, in accordance with the European Directive and the Italian Legislative Decree, returns the F-N curves of societal risk, in other words functions that relate the frequency of occurrence of an accidental scenario (F) with the expected consequences in terms of potential victims (N). Starting from two types of initial events, a fire and a Dangerous Goods (DG) release, a total of 18 accidental scenarios was defined. The frequencies of occurrence of each accidental scenario is obtained using the Event Tree Analysis (ETA) technique. For each scenario, the number of fatalities, expressed in terms of deaths, is obtained by simulating the formation dynamics of the queue of vehicles, using a model able to calculate the queue length, depending on traffic, the vehicle type, as well as the closure time of the tunnel. Then, a distribution model of the potentially exposed users has been defined and coupled with an egress model. The users’ tenability is estimated on the basis of the egress model and the evolution of each accidental scenario, which is evaluated using a zone model. The proposed model can simulate each of the 18 accidental scenarios in several different positions along the tunnel, considering the impact that different tunnel infrastructure measures, equipment and management procedures can have on the users egress and on the propagation of the effects of the accidental scenarios. The model is able to consider the interdependence between these measures and their reliability in terms of their availability in an emergency situation. Finally, to validate the model, comparisons are made with the QRAM software developed by PIARC for some representative case studies. Through this model, it is possible to perform the risk analysis of a tunnel in an actual configuration and compare the expected value of damage with the corresponding one of the tunnel in a virtual configuration, as prescribed by the Italian decree compliant with the European Directive 2004/54/EC.