RISK ANALYSES OF TUNNELS USING THE SWISS GUIDELINE AND METHODOLOGY FOR RISK ASSESSMENT AND RISK EVALUATION

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1 RISK ANALYSES OF TUNNELS USING THE SWISS GUIDELINE AND METHODOLOGY FOR RISK ASSESSMENT AND RISK EVALUATION Niels Peter Høj **, Samuel Rigert *, and Marco Bettelini * * Amberg Engineering Ltd., Regensdorf-Watt, Switzerland ** HOJ Consulting, Brunnen, Switzerland ABSTRACT Significant experiences with the use of the new Swiss guideline for risk analyses of road tunnels have been gained during This paper presents the background of the methodology and illustrates its practical application. The methodology is applicable for new projects, in order to demonstrate the most efficient design options, and for existing tunnels, which may not be fully compliant with present day regulations, in order to decide on the best upgrade options. Based on this experience, the paper illustrates how the fire risk was assessed in two tunnels. The risk was estimated in the initial condition and subsequently with different safety measures including emergency exits of different types and distances. In the first example, it was proven that the construction of emergency exits was cost efficient and recommendable. In a second case, a comparison was made between a tunnel project with a high gradient and an alternative with a longer tunnel with a more moderate gradient. Using the new methodology it could be shown that the longer tunnel did not constitute a cost efficient alternative to the steep tunnel. Keywords: quantitative risk analysis, risk assessment, risk evaluation 1. INTRODUCTION At the end of 2014, the Swiss Federal Roads Administration (FEDRO) issued a guideline (ASTRA 19004, 2014) on Risk Analyses for Tunnels on the National Roads network. The guideline specifies in which cases risk analyses have to be carried out, the level of detail to be applied for the analyses and risk evaluation principles. In an additional document (ASTRA 89005, 2014) FEDRO specifies in detail the methodology, which shall be used for risk assessment and risk evaluation. The methodology includes also a parametric model for ventilation, smoke propagation and emergency egress. The guideline s goal is a high reproducibility of the results of risk analyses. A third document (ASTRA 89007, 2014) specifies the format of the results and gives a template for the reporting of the risk analyses. In the following, the methodology is briefly outlined and its application is illustrated based on two examples. 2. METHODOLOGY The core of the risk evaluation methodology is the ALARP principle and the application of marginal costs of substitution. This means that the fatality risk shall respect predefined limits and the safety shall be improved until further improvements can be shown cost-inefficient. The guideline specifies the weight factors on the consequences, the limits and the procedure for proving the tolerability of the risk.

2 The methodology and the background for the methodology is well documented in the technical reports associated to the guideline (ASTRA 89005, 2014) and has been presented in a number of papers. In the following, some key aspects are highlighted. Homogeneous segments Bayesian probabilistic nets o Models for influence on risk of various tunnel design parameters / safety measures o A parametric model for ventilation, smoke propagation and emergency egress Evaluation procedures for the risk and for safety measures A tunnel system is described by a number of parameters influencing the risk in the tunnel. These parameters (for example traffic, geometry etc.) may vary along the tunnel axis. In addition, the basic risk of events is dependent on the zones of the tunnel: entrance, interior and exit. For the quantitative analyses, each tunnel tube of the tunnel system is divided into segments, in which the parameters do not vary (the so-called homogeneous segments). For each of the homogeneous segments, the risk is assessed by use of so-called Bayesian probabilistic nets (BPN). These nets are able to model the dependence between various risk indicators (for example the describing parameters) and the outcome. BPN can be understood as further development of logical trees (event trees, fault trees) and offers in addition to these the possibility to integrate the correlation between the risk indicators. In addition, BPN can be presented in a more compact and transparent way than logical trees with many parameters. Figure 1 presents a principle illustration of the nets applied for each homogeneous segment. The actual BPNs used in the methodology consist of a large number of nodes representing all influencing factors and intermediate results. Reference is made to ASTRA for more details. The outcome of all nets is aggregated to represent the risk assessment for each traffic direction and the entire tunnel system. Figure 1: Bayesian probabilistic net risk model for tunnel segments (principle) The risk assessment integrates models for the influence of a large number of relevant influencing parameters: Traffic volume, variation of traffic over time, congestion frequency, percentage of heavy vehicles, traffic speed, accident frequency, exits and entrances, horizontal radius, gradient, number and width of lanes, one-way or two-way traffic, distance between emergency exits, lighting (normal/emergency), monitoring system and ventilation. In particular, the influence of the ventilation system and the ventilations strategy on smoke propagation has been integrated in the risk assessment model based on more than two million simulations of tunnel systems with various features. The simulations were carried out with the programs SPRINT and ODEM. (Riess et al, 2000 and 2010). Based on the simulations, response surfaces were fitted in order to achieve suitable parametric models (Riklin et al,

3 , Brandt et al, 2015). The parametric models can substitute adequately and with a high degree of accuracy individual smoke propagation and egress simulations. Finally, the Swiss guideline and the technical documentation specify the evaluation procedures for risk acceptance and for safety measures. The procedure is based on the ALARP principle with limits of tolerability and evaluation of cost efficiency based on marginal costs of substitution (MRS). 3. APPLICATION During 2015, significant experiences with the use of the new Swiss methodology for risk analyses of tunnels have been gained and in the following two cases are presented Case 1: Existing tunnel A number of relatively short tunnels in Switzerland with bi-directional traffic did not have emergency exits and an upgrade program was started to build emergency exits and/or escape tunnels. These upgrade projects are rather costly and the question is, whether the upgrade is cost efficient or if the tunnels could be improved through other, less costly measures. In the following it is illustrated how the fire risk is estimated in a (hypothetic) existing tunnel without emergency exits. The tunnel is located in a mountainous area with very steep gradients of 6%, has bidirectional traffic in one tube and its length is approximately 2 km. The traffic is neither extremely high nor extreme low, with vehicles per day in 2035 and a percentage of heavy vehicles of 8%. Transport of dangerous goods is not allowed. The ventilation system is rather weak and is categorized as natural ventilation. Figure 2: Case 1: Accident rates along the tunnel axis for the two directions First, the risk is estimated for the initial condition without emergency exits. Figure 2 shows the accident rate over the length of the tunnel in direction 1 (upwards) and direction 2 (downwards). The accident rate is increased at the portals. The accident rate is rather high due to the gradient and other parameters (horizontal radius and others) result in some variation of the accident rate along tunnel length.

4 The fire rate is shown in Figure 3 for direction 1 (upwards) and direction 2 (downwards). It is clearly visible that the fire rate is very high, in particular in direction 1, as a result of the large upwards gradient. Figure 3: Case 1: Fire rates along the tunnel axis for the two directions The fatality rate is shown in terms of fatalities per billion vehicle-km in Figure 4. The rate is very high in both directions and significantly over the defined upper tolerability limit (13 fatalities per billion veh-km). Vehicle fires are the main contributor to the fatality risk. Figure 4: Case 1: Fatality rates along the tunnel axis for the two directions For this reason, it is necessary to implement safety measures. The risk must be brought under the upper tolerability limit. Emergency exits with a distance of 300 m, 200 m and 100 m were identified as suitable safety measures to reduce the fire risk. In addition, improved lighting (in the following denoted as 5 cd. m -2 ) was investigated. The investment and running costs of the measures were calculated and, according to the guideline, transformed into annual costs (based on the annuity factor and inflation factor given in the guideline). The costs of improving the lighting appeared to be very low, whereas the investments for emergency exits were in the magnitude of 25 MCHF, resulting in annual costs in the magnitude of 1 MCHF. The annual costs of establishing emergency exists every

5 m, 200 m and 300 m did only increase by about 10% because a major part of the investment was a parallel safety tunnel to be used for the egress. The benefit resulting from the emergency exits every 300 m was a significantly reduced fatality risk in case of fire. With distances of 200 m and 100 m, this risk could be further reduced to approximately half and one quarter of the risk with exits at 300 m. The benefits in terms of reduced annual risk of fatalities and injuries were quantified in monetary units according to the specified weighting factors used for road traffic in Switzerland, which specifies a MRS of 5 MCHF and an equivalent of 31 injuries with 1 fatality. Based on this, the efficiency of the measures were determined as the ratio between the quantified annual benefit in monetary units and the quantified annual costs. The efficiency of emergency exits every 300 m was 1.425, for exits every 200 m it was and for exits every 100 m it was The efficiency of improved lighting was Even though the benefits arising from this measure were relatively modest, the very low annual cost made this measure the most efficient one. Following the principles of ASTRA 89005, the measures shall be introduced incrementally, starting with the most efficient measure, until further measures are no longer efficient. This is illustrated in Figure 5. In this incremental method, the efficiency of the remaining measures has to be re-evaluated after each step, because the efficiency of remaining measures is decreased by the risk reduction of already applied safety measures. This is why in the present example, each safety measure is cost efficient, but the situation changes if they are combined. Emergency exits every 200 m are efficient but if the distance was reduced to 100 m, the measure was not efficient. Finally, it was verified that the resulting fatality rate was under the upper limit, see Figure 6. Under the assumption that all relevant measures had been identified and assessed, the risk in the tunnel would thereby be acceptable. The package with the combination of measures could be recommended for construction. Figure 5: Case 1: Cost efficiency of combination of measures Figure 6: Fatality rate after combination of measures

6 Case 2: New tunnel project In a second example, the comparison was made for a project with a very high gradient and an alternative of a longer tunnel with a more moderate gradient. The tunnel was planned with one tube and two-way traffic. The traffic was modest (8000 veh./day), the design level of the tunnel was good with a cross sectional width of 12 m, a ventilation system with smoke extraction and a modern LED lighting system providing more than 3 cd/m 2 in the tunnel interior at day time. Furthermore, the tunnel had a service- and escape tunnel with emergency exits every 275 m. These design features are in line with the Swiss regulation for road-tunnel design, SIA 197 (2004) and other national norms. The key issue was the very high longitudinal slope. For topographical reasons and for connection to the existing road network the location of the portals was fixed. The initial project was an approximately 2000 m long tunnel with a gradient of 6.3%. This gradient is exceeding the limit of 5% for which the SIA 197/2 states: Due to the increased risk (accidents, smoke spread) the maximum longitudinal slope shall not exceed 5%. For this reason, an alternative was proposed, with a gradient just under 5%. Since the locations of the portals were fixed, the tunnel necessarily had to be longer, as it appears from the sketch shown in Figure 7. Figure 7: Alignment of the tunnel (initial project) with high gradient and alternative, which is longer with lower gradient Figure 8: Case 2: Accident rates along the tunnel axis for the two directions of the initial project with gradient >5% The risk assessment of the initial project demonstrated, as it was expected, the influence of the gradient. The accident and fire rates were increased compared to a tunnel with lower gradient. The fire rate was in particular increased in the upwards direction. The accident rate and fire rate are illustrated in Figure 8 and Figure 9.

7 Figure 9: Case 2: Fire rates along the tunnel axis for the two directions of the initial project with gradient >5% Because of the increased accident and fire rates, it would be expected that the fatality rate be increased. This is the case if one would compare the fatality rate of the actual project with a similar tunnel with 2% or 3%. However, the fatality risk is not increased to a level shown in Case 1, because the high design level with emergency exits, ventilation, light etc., which contribute to reduce the risk. Figure 10: Case 2: Fatality rates along the tunnel axis for the two directions of the initial project with gradient >5% It was decided to investigate the original design and its alternative from the point of view of safety. The result of the risk assessment for the alternative was a reduced fatality rate. In the initial design, the overall fatality rate was approximately 1.0 fatalities per billion vehicle km. In the alternative, the overall fatality rate could be reduced to 0.9 fatalities per billion vehicle km. In both cases, the vast majority of the fatality risk originated from accidents. The risk of fatal consequences from fires was very effectively reduced by means of ventilation and short distances between emergency exits. However, for the evaluation of the benefits of the measure, the weighted annual consequences of fatalities and injuries are decisive. In spite of the reduced rate, the 20% longer tunnel length resulted in an increased annual risk, because of the increased tunnel length. Fewer injuries from fires, approximately 5% more injuries from accidents and 10% more fatalities from accidents resulted in a total dis-benefit of approximately 5000 CHF/year.

8 Taking into consideration the approximately 25 MCHF additional construction cost for 500 m additional tunnel, the annual cost of the measure was estimated to about 1 MCHF. The cost efficiency of the measure was hereby , which is of course far from the cost efficiency of 1.00, where the measure is recommendable. Using the new methodology it could be shown that the longer tunnel did not (in this case) constitute a cost efficient alternative to the original design. 4. CONCLUSIONS One year after the release of the new Swiss methodology for risk analysis, a number of significant experiences could be collected. The new approach proved to be very reliable and provided very important contributions to a number of projects. The methodology proved to be an extremely valuable tool for deciding about project variants, and for basing decisions on a solid base. Using the methodology, reproducible results are obtained, such that projects are stabilized and responsibility is drawn away from the decision makers. Additionally, there is a strong focus on cost efficiency, which is important for tunnel owners and operators, who are interested in both, high level of safety and efficient operation. 5. REFERENCES ASTRA (2014), Risikoanalyse für Tunnel der Nationalstrassen, 2014, Bern. ASTRA (2014), Risikokonzept für Tunnel der Nationalstrassen / Methodik zur Ermittlung und Bewertung der Risiken in Tunnel, 2014, Bern. ASTRA (2014), Risikoanalyse für Tunnel der Nationalstrassen: Anwendungsbeispiel, 2014, Bern. Brandt, R; Schubert, M; Høj, N.P. (2012), On Risk Analysis pf Complex Road-tunnel systems; 6th International Conference Tunnel Safety and Ventilation 2012, Graz. Brandt, R., Riklin, N., Butty, V., Frey, S., Schubert, M., Høj, N. P. (2015), Gammeter, C., Gogniat, B. (2015), Quantification of Consequences of Road-Tunnel Fires ISAVFT 2015, 16th International Symposium on Aerodynamics,Ventilation & Fire in Tunnels, Seattle, Brandt, R., Schubert, M., Høj, N.P., Gogniat, R., & Gammeter, C. (2016), Swiss guideline and methodology for risk analysis of road tunnels. 7th International symposium on Tunnel Safety and Security (ISTSS), Montréal, Riess, I,, Brandt, R., (2010): ODEM: A One-Dimensional Egress Model for Risk Analysis, 5 th Symposium Tunnel Safety and Ventilation, Graz, Mai 2010, Graz, Riess, I., Bettelini, M, Brandt, R. (2000): Sprint A Design Tool for Fire Ventilation, 10 th Int.Conference, Aerodynamics and Ventilation of Vehicle Tunnels, bhr, 2000, Boston. Riklin, N., Brandt, R., Butty, V., Frey, S., Schubert, M., Høj, N. P, (2014): Einfluss der Tunnellüftung auf das Risiko eines Strassentunnels, Fachbericht ASTRA N , 2014, Bern. Schubert, M., Høj, N. P., Köhler J., Faber M. H., (2011), Development of a best practice methodology for risk assessment in road tunnels, Research Project ASTRA 2009/001, Report Nr. 1351, 2011, Bern. SIA 197/2 (2004) Projektierung Tunnel, Strassentunnel Schweizer Norm, Schweizerischer Ingenieur- und Architektenverein (SIA), 2004, Zürich.