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Safety conduct

Further horrific tunnel fires have lent particular significance to the refurbishment work for the Mont Blanc tunnel. Adrian Greeman reports

Three months into rehabilitation of the fire-devastated Mont Blanc road tunnel, the work is proving more difficult than originally thought. Opening date for traffic to run in the restructured tunnel is likely to be September this year rather than the hoped for July.

Perhaps difficulties were to be expected. When the joint owners of the tunnel, the French and Italian side concessionaires announced a programme of rebuilding and upgrading works in October last year it was clear that an extremely tight schedule was being set out. Excavation and concreting for multi-point new fire shelters and ventilation had to take place more or less simultaneously along the length of the 11.6km long tunnel (see box).

French-side contractor - a Bouygues/GTM/Impregilo consortium - had just 23 weeks, including a six week planning period, from the September contract start date, before the electrical and mechanical specialists moved in. There was also a complex programme for the whole tunnel, with specific time allowances for various tasks.

For example, two days is allowed for excavating a safety niche in the tunnel wall.

'This work overall is unlike normal tunnelling where you have one or two active work faces and a route in and out, ' says Laurent Samama, project manager for Maitrise D'Oeuvre, a joint venture of specialist consultant Scetauroute/Spea. 'Here there are worksites at intervals all along for 12km. It is a difficult logistical challenge.'

On top of that the client and its design and supervision team has imposed tight safety standards on both the construction work and its organisation. In October last year they were at pains to describe these measures in almost as much detail as the permanent safety measures being built into the tunnel itself. After witnessing one of Europe's most horrific fire disasters, in which 39 people lost their lives (NCEI May 1999), the joint tunnel operators clearly did not want to see accidents or further fatalities.

Even less did they want yet more adverse press and government attention.

But having met complex supervision and management requirements for the tunnels and a 1ha, two-level worksite platform at the portal, the contractors are now being confronted with new safety demands. On the French side in January this year the local Inspection de Travail, a safety and working conditions body, demanded that the tunnel be cleared for 300m each time there is any explosive work, which has meant trucks and workers further into the tunnel must remain in place so as not to pass the affected area, even if the drill and blast is taking place in a side chamber.

On the Italian side, where the contract reflects a different legal environment, the authorities have prohibited work at road floor level if other workers are busy in the crown overhead, such as in areas where the new ventilation ducts are being made. Both restrictions meant rethinking the logistics, says Samama.

The operations were already a jigsaw. Following clean up work (under separate contracts which began in the spring) and restoration of tunnel services, the contractors on each side now have to build a sequence of different excavated structures in the hard granite at the side of the tunnel.

Under a Euro 25M ($27M) contract, the French JV has to prepare rock chambers, vent ducts and tunnel niches for its own 5.8km half tunnel length, as well as a central excavation for a new underground fire station at the tunnel mid-point. Some 55,000t of rock is to be removed on this side, with another 5,000m 3ofconcrete taken out. Around 22,000m 3of concrete goes in, along with 5,000m 3of shotcrete.

The Italian work is similar but also includes cleaning up the damage to a 1km length of the spalled and crumbled 700mm thick concrete lining where the truck fire burned most fiercely, reportedly at nearly 1,000degreesC.

Concrete has been high pressure water-jet blasted away to reveal sound material and the tunnel relined. Three of the original uncompleted lane widening garages have also been excavated.

To ensure completion on time, some 60 to 70 workers are busy on any single shift making a total of 300 working on the 24 hour, six day a week programme. Main excavations are carried out by three Atlas Copco jumbo rigs with rock either blasted or split using an expansive gel in the smaller niches and ducts. Spray concrete uses Aliva robot rigs and a fleet of smaller hydraulic excavators and dumptrucks is operated from sites close to the portal.

Once they are done, the E&M contractors will move in to fit the complex of cables and communications which will, it is said, make the Mont Blanc tunnel one of the safest in the world.

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