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Emma and the Parisians

Paris A86 - Chronic congestion in the French capital is being tackled through the construction of an usual double-deck tunnel. Ed Owen reports from Paris.

Burrowing furiously under Paris' most expensive and exclusive suburbs is a vast machine, as hot and noisy as a Victorian foundry but using modern technology to create a very special tunnel.

The single 11.56m diameter Herrenknecht tunnel boring machine (TBM) 'Emma' is creating a huge 10.4m diameter tunnel. Such a large tunnel - 3m wider than the Channel Tunnel's - will be partitioned to create two road decks on top of each other.

The tunnel will complete the 'super-Peripherique' A86, which orbits Paris in its outer suburbs, and which has been built in two distinct phases.

The 4.5km phase one is having the finishing touches added, to open in October. The 5.5km phase two is being bored, and should open in 2009. Coroute won the design, build, finance and operate (DBFO) contract, subcontracting the work to Scoatop, a consortium of Vinci, Eiffage and Colas (see box).

Entering the belly of the mechanical beast, somewhere under the Versailles World Heritage Site, the TBM creeps forward at a snail's pace and is cluttered with giant reels. 'As the machine moves forward, we add new pipe - power, air, water, bentonite, ventilation, ' shouts François Renault, methods engineer for Socatop, over the deafening clatter of machinery.

In the TBM's control room, noise is reduced to a loud squeal. 'We are travelling at 43mm per minute, ' he announces. 'It is possible to go off-course if there are unexpected changes in the geology, but this is never more than a few centimetres.' Local geology does vary, and the TBM has been adapted to deal with both hard and soft ground. It is an earth pressure balance machine with bentonite injection used to lubricate the and ease spoil removal. Rock spoil is removed from the cutting face using a screw conveyor.

In wet soils, such as the local Fontainebleau sand, the TBM is changed to hydraulic mode, where the screw is replaced with pumps and pipes to extract the extremely runny spoil. 'Changing modes takes about one week, but it can take up to a month to reach full working speed again, ' says Renault.

Safety refuges are dug every 200m along the tunnel, each large enough for 100 people to shelter in an emergency.

Refuges vary in size and shape, depending on how hard the ground is. 'At the moment we are cutting into limestone, which is quite stable. However, when we move to sand we have to freeze the area, which makes it quite hard, like brick, and we use a jackhammer to get it all out, ' he says. In very sandy ground two separate refuges are built to ensure the structure is safe from collapse.

In areas where the water table is high, pumps are being used to lower it around the tunnel.

As we advance through the TBM towards the cutting head, space becomes increasingly cramped. The final room before the face is tiny, resembling the interior of a submarine, complete with port hole.

'Occasionally there needs to be work done on the face. In this instance it might be done under pressure and the workmen must come into this chamber to get up to pressure, and then again on the way back to depressurise, ' says Renault.

Just behind the cutting face, a train delivers 11t, 2m wide lining segments, made in Lyon, and huge quantities of grout, used to fill the annulus between the lining rings and the excavated bore.

Unusually, the train runs on tyres, as trains with rails would have difficulty with the tunnel gradient.

Rings are assembled within the TBM shield. Connections are bolted.

At the rear of the TBM, an extra car, unique to this tunnel, prepares the walls for the three separate decks. A machine grinds the tunnel walls to provide a rough surface and drills holes for reinforcement rods. These are used to cast concrete shelves along the length of the tunnel on which will sit prefabricated slabs to form the double deck. 'We make prefabricated slabs on site, up to 24m per day, ' says Renault.

The road decks are installed only once the tunnelling work is complete.

The finished tunnels have curved walls, the bottom deck needing a solid kerb to prevent cars driving up around the curved surface if they lose control. Tunnels will have the Finnish 'Hi-Fog' system installed, which can contain and put out fires extremely quickly using small quantities of water, delivered through 17,500 nozzles.

Work is racing ahead with 3.5km of the second tunnel complete. Once it meets the first tunnel, the French will once again have completed a mustsee piece of engineering to rank alongside the Eiffel Tower and the Millau Viaduct.

The A86 in brief Construction Phase one links Malmaison, where the A86 currently ends, with the A13 - one of the major arteries linking Paris to the west of France. This tunnel is complete and is undergoing fit-out ready for opening in October 2007.

Phase two, due to complete in 2009, will complete the A86, linking the two ends from the A13 junction down to Jouy-en-Josas, and is being bored. This will open in 2009.

A third phase, for a separate tunnel to take freight around Paris, is still under negotiation.

Once the A86 sections are complete, city traffic is expected to reduce by 15%.

Finance The project is being delivered under a 21.5bn (£1.1bn) design, build, finance and operate contract, rising to £1.6bn if the third stage goes ahead. Cofiroute won the bid, subcontracting the work to Scoatop, a consortium of Vinci, Eiffage and Colas.

Cofiroute will run the concession for the next 70 years.

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