Armand van Wijck, TunnelTalk Europe Correspondent

In the past six years, the design of the planned bi-national Mont d'Ambin baseline rail tunnel on the proposed new rail connection between Lyon and Turin has seen several drastic changes due to heavy political debate and the fallout from the Euro Zone economic crisis. Through all of this, the safety concepts developed for the project by engineering consultancy Arcadis have managed to remain intact. A mix of European, Italian and French safety standards and assessment methods made for a strong starting point, however differing national attitudes towards risk have created design safety challenges. Armand van Wijck explores the situation.

As many as 15 trains are expected to be travelling through each tube of the 57km-long twin tube Mont d'Ambin base tunnel at any one time when the new 270km-long rail connection linking Lyon and Turin is completed in 2030. With both freight and passenger trains using the baseline link, operational safety within the long tunnel has been a particular concern for the both the project owner and government authorities.

Once operational, scheduled for 2030, the new line is expected to transport more than 40 million tonne of freight per year; a marked increase on the annual 13 million tonne that currently passes through the now-congested and higher-elevation Fréjus tunnel route. Double-deck high-speed TGV passenger trains, with a capacity of 1,200 passengers, will also use the link. As one of the largest infrastructure projects currently in development in Europe, the owner, Lyon Turin Ferroviaire (LTF), a subsidiary of Réseau Ferré de France (RFF) and Rete Ferroviaria Italiana (RFI), is preparing for a 2015 start on excavation of the main baseline railway tunnels. At the same time, Dutch consultancy Arcadis has been busy completing all aspects of the project's operational safety design.

Fig 1. Schematic of the tunnel's design and safety station locations

"Our safety concept has been ready since 2006," said Stefan Lezwijn, Advisor Tunnel Safety at Arcadis. "This includes some particular safety measures which are driven partly by the exceptional length of the tunnel at 57km, and partly by the fact that both freight and passenger trains will use the tunnel at 100km/hr and 220km/hr respectively.

With so many trains using each tube at the same time, the calculation of a minimum safe driving distance between them was critical. Based on possible risk scenarios, and involving the type of hazardous materials that will be permitted to use the tunnel, this minimum distance has been set at 4,200m. "LPG (liquid petroleum gas) for example is allowed to be transported but only in purpose-specific tanker wagons," said Lezwijn. "If, in an accident, these were to explode, they produce a deadly shockwave that radiates along the tunnel for up to 4,200m. This was a principle criterion that set the minimum headroom between trains."

In this eventuality, passenger trains can proceed to the nearest safety station, of which there are three along the length of the baseline tunnel. One of these, the Modane station, will also serve as a manned traffic centre. Each station will house a pressure sealed area in which up to 1,200 passengers can seek refuge. "That is the maximum capacity of a double-deck TGV, the largest train that will traverse the tunnel," explained Arcadis colleague Jorrit Nieuwenhuis. "From the surface, rescue workers will reach the safety stations via the four access tunnels that were excavated earlier as geotechnical research adits and access and muck-haulage during excavation and construction of the main tunnel tubes."

"In an emergency, rescue workers will need about an hour to reach the base tunnel," said Nieuwenhuis. "In a worst case scenario, for an emergency stop between safe stations, passengers can evacuate to the other tube, in which the installed ventilation system will prevent smoke from penetrating. We designed the distance between emergency exits at 333m to coincide with the length of a double-deck TGV high-speed passenger train to allow for as quick an evacuation as possible," said Nieuwenhuis.

Bringing travelling trains to a stop in the parallel tube for the safety of escaping passengers poses another challenge: "You need to deal with the air pressure effects generated when a train comes to a stop from full speed. Trains therefore need to stop and accelerate gradually while inside the tunnel," said Nieuwenhuis.

Fig 2. An fN-curve showing the possible risk scenarios, with the consequences in terms of fatalities on the horizontal axis, and the probability frequency on the vertical axis

Fig 3. The fN-curve with the two norm lines. The numbered circles refer to the computed scenarios

Safety model
With the base tunnel passing through the underground border between Italy and France, the project is shared between both countries. Owner Lyon Turin Ferriovaire (LTF) reflects the bi-national nature of the venture; it is a subsidiary of both Résseau Ferré de France (RFF) and Rete Ferroviaria Italina (RFI).

Due to the international alignment of the project, and the cultural differences between both countries, Arcadis has had to combine new European safety regulations with concurrent French and Italian national rules within its safety design. "The Italian approach to safety is based on a probabilistic point of view whereby as long as you are below a certain norm line, you are in the clear," said Nieuwenhuis. "In contrast, French regulations use the ALARP (as low as reasonably possible) principle, which is deterministic. It requires doing all you can to guarantee the highest safety level, as long as it is affordable and feasible. On the European Union level, certain minimum safety thresholds are demanded for tunnels inside the EU."

Mixing these principles created a new method, which, according to Arcadis, works surprisingly well. The possible risk scenarios are made visible in a risk acceptance graph, or fN-curve. The probability of an outcome is marked on the vertical axis, with consequences, in terms of fatalities, marked on the horizontal axis. Additionally, two norm lines are present in the fN-curve. "The upper line counts as an absolute norm. All safety scenarios should meet this norm," explained Nieuwenhuis. "Below this, lies the second norm line. For risk scenarios between these two lines, cost-effective measures should be taken to reduce these risks further."

The Mont d'Ambin base tunnel was one of the first projects for which Arcadis used this principle, but in the meantime it has been implemented successfully for the design of several other international projects. "We noticed that many tunnelling projects prefer to work with safety check-lists. However, as soon as certain measures involve large sums of money, people start weighing the costs," said Nieuwenhuis. "In our process, the cost deliberation is already incorporated into our tool. It gives a fundamental explanation of what the consequences of additional measures are and hence eliminates a lot of discussion."

Aftermath of the 1999 Mont Blanc tunnel fire

The tool might prove useful as a starting point to solve any cultural differences, a point that cannot be overlooked. Take for instance the 1999 Mont Blanc tunnel disaster: a burning truck led to a deadly catastrophe partly because of bad communication between France and Italy and partly because of incompatible fire-fighting equipment. "This historical tragedy probably made safety a predominant issue on the Mont d'Ambin agenda. Even during the very first design phase LTF had already embraced tunnel safety as a main theme," said Lezwijn.

All trucks using the Mont d'Ambin base tunnel on roll-on roll-off freight trains, will have to first pass through a heat detection gate, as is the case nowadays at the Mont Blac tunnel. When heat anomalies are found, such as overheated breaks or super-heated engines, these trucks are taken out of the traffic line immediately.

Cultural combinations
Despite the efforts of LTF to incorporate tunnel safety from the start, and Arcadis mixing the various safety philosophies, Lezwijn thinks cultural differences on any bi-national project will always remain one of the greatest challenges. "There are still safety measures, mostly in the technical domain, that need to be synchronised. One of these is selection of the automatic fixed fire-fighting systems will be used, and where, and when," he said, "and what does it need to protect; the structure, the trains or the people. These things will undoubtedly resurface the moment rescue workers get involved and have their say on the operational design of the project."

As a standard, the safety stations will be equipped with fixed fire-fighting systems, affording burning freight trains the opportunity of stopping there as well. For safety reasons related to steam development, the systems will not turn on as long as people are around. "If a fire breaks out elsewhere we are relying on the fire brigade to get there in time," said Lezwijn.

1999 tragedy has influenced Lyon-Turin safety demands

With regard to the ventilation system, air supply shafts will be linked to each access tunnel and the base tunnel tubes will be equipped with longitudinal ventilation. "In this way the escape areas, including the parallel tube, can be kept smoke free," said Nieuwenhuis. "Furthermore, ventilation is made up in large part by the trains themselves. All those trains passing through the tunnel one after another make for a continuous air flow in the driving direction."

The passenger trains, travelling at speeds of up to 220km/hr, did pose an additional challenge for ventilation design. Niewenhuis believes it will lead to large pressure differences. "It is like a mega-storm passing through, very different to what we are used to in conventional tunnels. We had to make very specific calculations and adjust the ventilation design accordingly."

All cultural differences set aside, LTF seems to have done the best it can to prevent a second Mont Blanc disaster. "While the tunnel design has been modified several times, the project directors have not altered our safety principles," said Lezwijn. "When we visited LTF five years after we completed the safety study, it was really good to hear them still talk about that minimum train distance of 4,200m."

This is the second article of a three-part series on the Mont d'Ambin rail tunnel. The next article will discuss the intense protests and debate that the project initiated in its early development phase, which directly affected construction workers.

References

Progressing the Lyon-Turin base rail link - TunnelTalk, August 2013
Extreme tunnel planned for Andes rail link - TunnelTalk, November 2012
Final finish for Gotthard Baseline excavation - TunnelTalk, March 2011
Sprinkler limitations for tunnel fire-fighting - TunnelTalk, September 2011
Fire-fighting system installation for Eurotunnel - TunnelTalk, February 2011
Aftermath of Mont Blanc tragedy - TunnelTalk, June 2001

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