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Shelter strategy

Political and economic factors rule out construction of a second parallel bore for the Month Blanc tunnel, the safety solution chosen for the Tauern tunnel in Austria where 12 people died in a fire in May 1999, just two months after Mont Blanc. The second Tauern tunnel will allow safer unidirectional traffic, which offers an escape route and rescue access.

Instead the Mont Blanc tunnel will use three methods for safety - new refuges, enhanced firefighting and fire prevention systems.

The major provision is a series of shelters hewn into the hard granite walls at 300m spacing along the north side of the tunnel, like one side of a 5.5m high arched chamber and covering a surface area of 37.5m 2. These will replace the alternating 'garages' at 300m intervals on the right and then the left, where the tunnel widened to a third lane hard shoulder for a short distance.

Every second garage, ie at 600m intervals, had a small brick-walled fire shelter with telephone.

The shape of the new shelters will vary each location, says Laurent Samama, each with a double door sealing the chamber. First entering an antechamber with the shelter on one side and equipment on the other, a second door opens the refuge where inside is a further door, operated by the fire control centre if necessary, leading downstairs to four under-road ventilation ducts, high enough at 2.5m to serve as an escape route. They are separated from a fifth larger hot smoke exhaust duct.

Ventilation through these ducts will be boosted from a current 4MW portal fan capacity to 10MW. Air will feed at 82.5m 3/sec into small road level grills at 10m intervals creating a semitransverse system for day to day use. Air will also pass into the escape refuges to maintain an over-pressure in emergencies.

Smoke exhaust is also to be boosted with the number of extraction points being trebled.

Currently there are smoke vents every 300m, drawing crown air down through a tunnel side duct. The 1.2m 2cross-section ducts connect to the under road exhaust culvert through which smoke is drawn to the portals.

New 1.5m 2ducts at 100m intervals will be formed. These will have remotely operated vents so that only those over the fire will be open, says Samama, concentrating the effect.

He says the tunnel air movement required complex study. Frequently the barometric pressure difference across the Alps means that there is a wind in the tunnel of perhaps 3m/sec speed. To counter this and prevent mixing of layers, 76 additional jet fans are being installed in pairs; these will reduce wind velocity and allow the stratification of the smoke so that it can be vented.

A battery of computerised and video detector systems will pick up overheated lorry loads, spot traffic jams and locate accidents. Embedded coils in the road will monitor flows and computer algorithms analyse TV images.

Automation will be supplemented by round the clock human observation at a main monitoring safety station at the French portal and a linked backup station on the Italian side, which will operate remote controls for traffic barriers and electronic signs to prevent snarl-ups.

They will also direct fire appliances in the event of a disaster. And a further two appliances, one light and one heavy will maintained at a new fire station at the tunnel midpoint, excavated from the rock.

Three people on duty here will also monitor videos and instruments.

Appliances will tap into new hydrants, with both French and Italian fittings, installed at 20 fire point niches, 1.2m deep in the tunnel walls. Another 58 similar niches, will house small extinguishers and telephones for the public to report incidents.

Bertrand Levy, director of the French side operator Autoroutes et Tunnel de Mont Blanc, says the chosen solution will be at least as effective as a separate emergency escape tunnel parallel to the main bore.

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