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Double trouble

Tunnels A86

A major tunnel fire was just one of the obstacles that had to be overcome by the builders of the new A86 tunnels around Paris. Words and pictures by Adrian Greeman.

Perhaps the ghosts of the Emperor Napoleon and Empress Josephine are responsible, or maybe the Seine river sprites; but someone has it in for the A86 tunnel in France.

Just getting the project off the ground proved almost impossible. Although most of the outer ring motorway for Paris was completed years ago, the 10km long western Paris section was different. This upmarket quarter includes the Palace of Versailles and Josephine's Malmaison home, as well as homes for financiers, politicians and film stars. 'Nimby' ruled OK!

A giant tunnel would keep the surface clear and environment clean argued contractor's consortium, Cofiroute. The estimated US$1.5bn costs could be raised with private money from banks and equity from the group's 'name' contractors such as Campenon Bernard and Dumez GTM, together with road operator Colas. Other members, Entreprise Jean Lefebvre, GTM Construction and Fougerolle, would contribute.

But even after Cofiroute had convinced the government and gained the 1996 concession, environmentalists and local towns invoked European competition rules to block work. Portal construction was suspended in February 1998.

Eventually, the European Court decided the award did not contravene regulations, but the French state council put the project out to open tender anyway.

Cofiroute won and tunnelling restarted in November 1999, under a design build contract with Socatop, a contractor consortium with the same shareholders as the concessionaire.

Then came the Mont Blanc tunnel fire, and its 39 deaths.

Already controversial for its unique two stacked decks (see box) for cars, the fire had a huge impact on what was to be France's largest tunnel project.

Even with the safety advantage of lorries travelling in a separate tunnel, removing major fire and accident hazard, and unidirectional levels cutting the risk of high speed head-on collisions plus significant electronic and physical safety measures, the French government wanted tighter rules. Cofiroute was instructed to double the emergency points, to every 200m.

These connections provide refuge shelters and escape stairs to the other, unaffected, level.

A lot more hand tunnelling work is now needed, all potentially very difficult, says Pierre Boutigny, technical director for tunnelling. 'The tunnel is really two projects, ' he says. 'First is the main bore, which is big and technically complex, passing through some difficult ground including fine running Fontainbleau sand.'

Then comes fitting out. Three interior slabs are needed for lower and upper car decks and top and bottom ventilation ducts. The refuges need side excavations, or 'adits', each investigated, dug and fitted out once the TBM has passed.

The main 11.2m diameter tunnel is achievable by using a well designed machine, says Boutigny. 'But for the adits we will have to use every method in the book, ' he adds, 'because they will be in waterlogged ground, or fine sand or both.'

'Another problem will be the logistics, ' he goes on. 'I expect to have 10 adits going simultaneously. In the Mont Blanc renovation, which also had multiple smaller works, a ready mixed concrete truck might take one hour to reach its tunnel work site.'

Currently the Herrenknecht multimodal TBM is the focus. It operates as an earth pressure balance machine in better ground of marls, chalk and limestones and will switch to bentonite slurry mode in the Fontainbleau sand, which has not yet been reached. A compressed air bubble system is fitted for fine tuning the balancing pressures.

'We also use it in open mode where there is no water table, ' says Boutigny. This keeps muck drier, and better suited to the conveyor carrying spoil to a riverside loading point for barges.

The machine erects a 2m wide eight section segment ring as it passes, which is sealed with a conventional grout, 'though with a high proportion of fly ash for later strength gain', says Boutigny.

Construction progressed well until March this year when a supply locomotive, bringing reinforcement bars into the tunnel, caught fire. The TBM had driven about half the 4.5km of the Phase one tunnel which joins Rueil Malmaison to an A13 autoroute junction.

Workers putting in the precast slabs some 600m back from the TBM battled with the fire, but burning hydraulic fluid ignited the conveyor belt above and the tunnel filled with fumes. Nineteen workers escaped forwards to the TBM, a rehearsed procedure to take advantage of its 12 hours of compressed air supplies. Fire brigades brought masks to rescue them.

Though no-one was hurt, nearly three months of production were lost. Lifting gear for precast floor slabs had to be replaced and more problematically, the tunnel needed cleaning. Soot deposit over a 1,000m length is not only unpleasant but is also electrically conductive.

This is not a good idea when a TBM uses 9,000kW of power, suggests Boutigny.

Worse still, complex electronic controls have had to be cleaned out and circuit boards tested on the otherwise undamaged TBM.

'There is also some concrete spalling along an 80m section, ' says Boutigny, though this can be repaired by jet cutting and reinstating it with mesh and steel fibre shotcrete.

Work has now resumed and the tunnel drive should complete in March or April next year. Adits and more fitting out will follow with the first section open to traffic in 2005.

By then the TBM will then be driving the second longer 5.5km tunnel section, starting from the south. Everyone is hoping that will be another, less fraught, story.

Stacked for safety Cofiroute's 10.4m internal diameter tunnel allows for two road decks for cars and light vehicles, each 2.55m high.

There is space for an adult to stand with room above for electronic surveillance cameras and lighting. Each deck has two full car lanes and an emergency vehicle lane/hard shoulder.

Above or below are ducts for transverse ventilation, independent for each deck.

Vertical ventilation shafts, which also act as escape routes, are spaced every 1,200m.

Traffic will be monitored by automatic TV and slow vehicle detectors every 100m and 'red cable' fire temperature sensors. Lane allocation lights will be sited every 200m and verbal warning display boards every 400m.

Passengers can escape at refuges connecting to the other deck every 200m.

Lorries and trucks will run in the separate, full height West Tunnel, routed further out of Paris. But this has been held up by the government to allow consideration of its safety provisions.

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