A86 West Tunnel, Paris, France

The A86 West, when finally completed in 2010, will form the final link of the 80km A86 ringroad around Greater Paris, allowing the journey from Malmaison to Versailles to take only ten minutes, rather than 45 minutes.

At a cost of €2.23bn the A86 West will be formed of two toll tunnels; one will be an innovative 10km double-deck tunnel (duplex motorway) for light vehicles, the other, at 7.5km, is along more traditional lines with just a single deck designed for all vehicles including HGVs.

Started in 1999, the first phase of the project between Rueil-Malmaison and the A13 interchange was due to be opened in October 2007. This has now been put back to spring 2008, with the second section between Pont Colbert and the A13 interchange opening in 2010.

A86 WEST UNDERGROUND TOLL ROAD

The A86, including the A86 West, is being built to relieve traffic congestion and improve traffic links between the suburbs of Paris. The completion of an outer ring road will greatly relieve pressure on the existing road infrastructure and guarantee moving traffic.

However, there have been some historical issues which have seen the debate as to how best reach a solution continue for thirty years.

The proposed route for the A86 includes a stretch through the West of Paris; this is a problem area where road building is concerned. A mainly residential area it features some important historical monuments in Versailles and protected woodland areas. After rejecting 22 proposals the A86 West underground solution was accepted as being a viable solution.

ENGINEERS AND CONTRACTORS

The French State granted Cofiroute the concession for the A86. Cofiroute have great experience in motorway construction and operation having done so with 890km of motorway in France up to 2001. Cofiroute appointed Societé de Construction de l'Autoroute de traversee de l'Ouest Parisien (Socatop) as consultant engineers and contractors, their remit to include construction work. Socatop is an association that brings together three of the biggest construction and road industry companies in France, VINCI, Eiffage Construction and Colas.

Reinforcing steel was provided by SENDIN and Formwork has been provided by several companies, including: Bernold-Ceresola AG, PERI S.A.S. France, RB-PIM and Simpra. Bonnard & Gardel provided consultant engineering services for the project (geotechnics and hydraulics) and multi-service vehicles were provided by TECHNIMETAL.

THE EAST AND WEST TUNNELS

The A86 West project involves two tunnels, the East and West tunnels. The East tunnel is exclusively for light vehicles with 2m-clearance height and will feature two decks each with three lanes. Each level will be independent of the other, watertight and with individual air ventilation systems. It will feature an intermediate intersection with the A13 motorway.

The West tunnel will accept all vehicles including HGVs, its smaller internal diameter allows for two carriageways, one in each direction with the same ventilation system being used.

TUNNEL BORING

The tunnels were constructed using an all-terrain, made-to-measure tunnel boring machine (TBM) fabricated and supplied by Herrenknecht AG of Germany. The TBM used had three operating functions being able to simultaneously drill underground, remove of excavated materials and perform progressive structural finishing (through the installation of arch segments).

The TBM was adaptable so could be changed in diameter to meet the different bore requirements of each tunnel. Using a Mixshield the TBM could be configured to operate through different geological layers ranging from hard layers such as limestone to soft layers comprising of water and clay. Utilising a laser guiding system the TBM followed a precise boring course that saw an average of 30m being cut every day.

Tunnel boring for the first section was completed in October 2003 and fitting out of the tunnel as well as remodelling the A13 toll plaza was started in April 2004.

The ventilation system for the first section was completed in 2006. Following satisfactory inspections the first section will be open in spring 2008.

In June 2005 digging was started for the second section of the tunnel; the TBM had been dismantled and transferred to begin the work. The digging was completed in August 2007 and fitting out of the dual deck tunnel was started with an opening scheduled for 2010.

TUNNEL CONSTRUCTION

The final part of the TBM operation concerned the installation of arch segments to provide the structural finish to each tunnel. Seven of these arch segments comprise to make a ring together with one keystone. These segments were fabricated from prefabricated reinforced concrete and weigh 11t each. Over these a joint is fitted over the whole circumference to guarantee water tightness.

This construction technique provides an extremely strong and rigid structure that can easily handle the stresses placed upon it. The tunnel tube consists of a series of vertical slabs fixed to corbels embedded in the tube lining walls; these will form the structure to support the road and are again made from reinforced concrete.

Above and below the traffic lanes ventilation systems have been installed (31 in this 4.5km section). These will ensure that there is a permanently renewed atmosphere in the tunnel with highly efficient smoke extraction in case of fire. In the East tunnel these will be split by a vertical partition to provide extraction on one side and fresh air ventilation on the other. With just one level of traffic the West Tunnel will have an extraction system in the top of the tunnel and a fresh air ventilation system at the bottom.

The west tunnel has 31 emergency refuge areas and 13 of these are smoke evacuation ERAs. There are also 18 interconnecting stairways for emergency use in the tunnel. The emergency systems also include four emergency shafts including: Hauts Bénards and Bois de l'Etat emergency shafts in Rueil Malmaison; and Place Berthet and Le Butard emergency shafts in La Celle Saint Cloud.

Both tunnels will also use HI-FOG fire protection technology, which is a high pressure sprinkler system that produces a fine mist of water (droplets 50–120µm) to damp down smoke or extinguish and cool fires.

The system only uses water which is non toxic. The system was developed and is marketed by the Marioff Corporation.

The East tunnel will have 13 ERAs with smoke evacuation vents and interconnecting stairways. There will also be five emergency shafts: Prieuré in Marnes la Coquette, Carrousel in Versailles, Viroflay railway station in Viroflay and Porchefontaine in Versailles (here there will also be an ERA).

TRAFFIC CONTROL

It is of extreme importance that the tunnel has a smooth traffic flow rate and the mechanisms to quickly respond to any emergency events. This is achieved partly through an information system that automatically collects traffic data such and speed and density of traffic. This information is relayed to a safety and control office to allow quick and effective control of vehicles entering, inside and exiting the tunnel.

The tunnels also feature 350 DIVA cameras for the instantaneous detection of non-moving vehicles to supplement the fire alarms, air quality detectors and other traffic management systems that usually determine incidents and intervention response.