Shani Wallis, TunnelTalk

More refuge niches, twice as many exhaust air ducts, and regular escape connections from the tunnel to the fresh air ducts beneath the road deck are the principal elements of reconstruction required in the Mont Blanc road tunnel to improve its safety and allow it to resume operation in September, 2001.

Aftermath of the fierce fire that claimed 39 lives

Investigations following the horrific tunnel fire that claimed the lives of 39 travellers and closed the vital France-Italy Alpine route to all traffic in March 1999, revealed that safety aspects of the Mont Blanc highway tunnel fell far short of adequate standards for the busy 11.4km single-tube, two-lane, bi-directional road tunnel. Prior surveys had highlighted fire/life safety issues and the need for improvement. Rehabilitation was in the programme. The fire illustrated just how critical was that need.

Mont Blanc is the privately operated facility owned by the ATMB (Autoroutes et Tunnel du Mont Blanc), a consortium of French companies, and in collaboration with SITMB (Societea Italiano di Tunnel di Mont Blanc), the local government of Aosta Valley on the Italian side.

Known together as the Societe d'Exploitation du Tunnel de Mont Blanc, each entity operates and controls its own half of the tunnel. Tunnel excavation started in 1959, and the route was opened in 1965. As a major tourist and trading route between France, Italy and Switzerland, the private road and its toll tunnel has seen traffic volumes – particularly heavy lorry traffic – increase significantly over recent years. Prior to the accident, some 2,000 vehicles/h were passing through the tunnel, at the 80km/h in-tunnel speed limit, and toll revenues were up to about US$150,000/day.

Damage and reconstruction
The intense 1,300°C heat of the fire at Mont Blanc caused major structural damage to the tunnel's in-situ concrete final lining and to its services over a 2km stretch in the Italian controlled section. Some 15 days were needed for temperatures to cool down before survey teams could enter the area.

Wreckage of one of the burned out vehicles

But repairing this damage was clearly insufficient in preparing the tunnel to reopen. Major re-engineering of its structure and upgrading of its services and systems was required to bring the tunnel's safety measures up to standards befitting its increased single-tube, two-way traffic flow.

Structural measures recommended by a scientific and technical committee of six experts and approved by the Franco-Italian Intergovernmental Control Commission, are based on new excavations into the hard granite rock walls of the tunnel. Although clearance for the large freight trucks that use the tunnel is very tight, these excavations are not enlargements. Instead, they are local excavations into the rock walls of the tunnel to create three particular types of safety features.

New refuge niches at regular intervals along the tunnel will be about 1.5m wide and 1.2m deep, and will be fitted with isolating protective doors. Inside they will be equipped with emergency telephones, fire extinguishers, and fire hydrant connections.

New rock shelters at 300m intervals along one side of the tunnel will provide large rock chambers with stairway connections to the four fresh air ventilation ducts beneath the tunnel's road deck. These lead individually from the portals to the four quarters of each controlled half of the tunnel. A separate duct beneath the road deck is dedicated to extraction of exhaust air.

Reconstruction of the bi-directional highway tunnel

An existing series of ventilation ducts embedded in channels in the tunnel's rock walls extract exhaust air through vents in the tunnel crown and direct it into the exhaust duct for discharge at the portals. These existing ducts are to be enlarged and new ducts in between will double the number of exhaust vents along the tunnel. The positive pressure of the forced fresh air ventilation, in conjunction with forced extraction of exhaust air from the tunnel, will keep the safety chambers, the escape stairways, and the ventilation ducts beneath the road deck free of smoke or toxic fumes in the event of an in-tunnel fire or accident.

In addition, there will be a large chamber at the tunnel mid-point, designed to house a monitoring and fire emergency station.

A light fire emergency vehicle, a larger fire tender, and three on-duty fire-fighters will man the station round-the-clock, and work in association with the fire and emergency stations at the two tunnel portals.

Construction works
In the first 12 months following the accident, little was undertaken in the closed tunnel while various design, contractual, and management issues were discussed, studied and clarified. Eventually, in October 2000, two contracts for the estimated $22 million civil construction works were let, one for each half of the co-operated tunnel. A visit to the French side of the tunnel in February 2001 was an opportunity to witness how the engaged Bouygues/GTM-Dumes/Impregilo JV has managed the complex phases of excavation work within the contract’s tight construction period.

Reshaping the profile of the highway link

"The programme is to complete all civil works by mid-May to provide access to the restructured tunnel for the mechanical and electrical contractors to fit the new lighting, ventilation, signalling, and communication and monitoring systems, and have the tunnel ready for a scheduled reopening by September 2001," explained Ing Martinasso Bruno, Construction Production Manager for the JV. "With so many different excavations to be completed at different intervals along the tunnel, and within such a tight deadline, we decided to work from the mid-point and excavate all the new refuge niches first. Then, while concrete lining works progressed in these excavations, we went back and completed all the larger safety chambers and escape connections to the fresh air ducts beneath the road deck.

We are now finishing the last of the exhaust ventilation ducts, again starting at the mid-point and working toward the portal."

Excavation in the hard granite of the alpine rock tunnel is largely by drill+blast, although non-explosive methods are also used in areas where safety in close confines is an issue, and where potential damage to existing structures must be avoided. In all, a reported 55,000 tonne of rock and some 5,000m³ of demolished concrete lining is to be excavated to make way for the new facilities.

Immediate support of the new excavations comprises 3m long rockbolts and layers of steel-fibre reinforced shotcrete. Three different types of rockbolts are being installed according to specific support requirements. These are Swellex, mechanical shell rock dowels, and cement grouted rebar bolts. All drilling for blasting and rockbolting is managed by Atlas Copco Boomers and Montabert jumbos. Excavated material is removed using a fleet of articulated dump trucks.

Shotcreting
For shotcreting, the JV has a set of three Aliva Duplo rotor pumps and compact AL-504 nozzle booms mounted on small rubber-tire carriers. These small mobile nozzle boom carriers can manoeuvre easily into the tight spaces of the new excavations, while the hoppers of the free-standing Duplo pumps remain within the profile of the existing road tunnel to provide easy access for discharging ready-mix trucks.

Shotcrete unit for tight spaces

The wet-mix steel fibre-reinforced shotcrete has a content of 32kg/m³ of Dramix 35/5 steel fibre and is further dosed at the batching plant with superplasticiser. Liquid alkali-free accelerator is fed into the thin stream shotcrete at the nozzle and is metered by the accurate, PLC-controlled AL-403 dosing unit.

"A minimum thickness of 20cm of shotcrete in the new excavations is built up in layers of 5cm thick," explained Ing Bruno. "In the various localized areas of excavation, shotcrete cycles total about 15m³/session. Under such operations the Duplo rotor machines – two smaller AL-262 units and a lager capacity AL-285 machine – are efficient and economical. Rebound against the angular surfaces of the new excavations is up to about 20%, but there is no concrete wasted in clean-up or priming between cycles. The long clean-up and priming operations needed when using dense-stream wet-mix concrete piston pumps for shotcreting are eliminated with the Duplo compressed air operated rotor pumps."

"Duplo machines can work with either dry or wet-mix shotcrete, and were used to apply dry-mix shotcrete on the rehabilitation of the Channel Tunnel between France and England following the freight train fire in 1997," said Serge Costarella, owner of the French equipment supply company Le Materiel Technique SA, L.M.T. of Thionville and Aliva's agent in France. "But here at Mont Blanc, wet-mix is specified for safety."

Safety in the busy tunnel construction site is paramount. The speed of construction vehicles within the tunnel is limited to 40km/h and wet-mix shotcrete is specified to reduce dust and maximize visibility along the construction area. Some 3,000m³ of wet-mix shotcrete was applied by the three Aliva units at the various excavation sites along the French half of the Mont Blanc tunnel. "The three Aliva systems have held up to the tough schedule imposed to meet the contract's tight construction schedule," said Jean Gornik, the shotcrete technician assigned to the Mont Blanc project for L.M.T. The new chambers are given a final lining of 45cm of non-reinforced in-situ concrete, and Glasal panels supplied by Eternit are being installed.

Aftermath of Mont Blanc
Within five weeks of the devastating Mont Blanc accident, a second road tunnel fire in the Tauern single-tube, two-way road tunnel in Austria claimed the lives of four travellers. The two disasters have prompted a pan-European standard of minimum safety requirements within road tunnels.

All new road tunnels will be subjected to these minimum design requirements. In addition, some 48 existing road tunnels in the European Union with lengths of more than 1km are required to be re-engineered and modernized to meet these minimum safety standards by 2005. The basic design and operation of each tunnel is different, and therefore the safety re-design for each is also different. All, however, must undergo reconstruction and modernization in order to maintain public confidence in tunnels as safe transportation facilities.

Following Mont Blanc, the next road tunnel in France to receive attention is the Alpine Frejus Tunnel opened in the 1970s.

References

Historic review of Mont Blanc Tunnel after 46 years of operation - TunnelTalk, Sept 2011
Eurotunnel back to full capacity - TunnelTalk, Feb 2009
PP fibres to resist fire-induced concrete spalling - TunnelTalk, Nov 2010
Fire fighting system unveiled by Eurotunnel - TunnelTalk, Feb 2011
Sprinkler limitations for tunnel fire fighting - TunnelTalk, Sept 2011

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