Duplex A86 - VINCI Construction Grands Projets

BACKGROUND

Completing the A86 was part of a development project for the Île-de-France region. This objective was set in the 1970s; therefore, 30 years were needed to implement this project. This tunnel infrastructure links the business hubs west of Paris, namely, Nanterre and La Défense, on the one hand, and Saint-Quentin-en-Yvelines and Vélizy-Villacoublay, on the other. The tunnel consists of two superimposed traffic levels (one direction per level to prevent head-on collisions) – this is a key innovation in response to the Mont-Blanc tunnel disaster that occurred on March 24, 1999. Thanks to control systems at the entrances to the tunnel, traffic conditions in the tunnel are fluid at all times. At the first sign of congestion, access is regulated by the closing of toll barriers, and traffic can be directed, if required, to other routes above-ground. Speed is limited to 70 kilometres an hour, which is optimal for ensuring both road safety and fluid traffic conditions.

TECHNICAL OVERVIEW

At 10 kilometres in length, this monotube tunnel with an interior diameter of 10.4 metres was one of Europe’s largest underground works. It features seven rescue shafts (ranging from 19 to 25 metres in depth), three ventilation units in the various interchanges, two intermediary ventilation units concealed in specific shafts, 40 niches with transfer staircases, and 28 smoke-extraction niches. Given the great diversity of soil types, excavation operations were carried out with a specially designed two-mode TBM: earth-pressure balance and hydraulic (or slurry shield). The A86 tunnel’s design is innovative: two superimposed, one-way roadways with a height of 2.55 metres. Each roadway has two traffic lanes and one emergency lane. Consequently, there is no cross-traffic and no risk of frontal collision.
Safety was built into this project right from the design stage. In the event of fire or severe accident, motorists can seek protection in one of the airtight and pressurised shelters located at 200-metre intervals. These shelters, which can accommodate about a hundred people, are connected by phone and videomonitoring to the control and monitoring centre at Rueil-Malmaison (where an operator is present 24 hours a day). In each shelter, a staircase allows users to reach the other tunnel level, which is then used as an evacuation and rescue channel. Every 1,000 metres on average, the tunnel is connected to the surface by rescue shafts. These shafts are equipped with a staircase and lift and allow emergency service crews to reach the tunnel near the fire or accident site at all times.
The tunnel’s interior features are also innovative: lighting is close to natural daylight; and the roadway and walls are grey for optimal visibility (thereby offsetting the tunnel’s smaller size as compared with traditional tunnels open to all types of vehicles).

In efforts to enhance driver peace of mind inside the tunnel, Cofiroute called on six social science researchers to carry out a study on local residents’ and future users’ acceptance of the A86 Duplex project. As a result, light (therefore, more reassuring) colours were used for the roadway and walls and more eye-catching colours (orange and green) were used for safety signs.

IMPACT

To optimise the infrastructure’s integration within its surroundings, Cofiroute asked Michel Regembal, the architect who designed the Stade de France, to create the structure’s architecture in response to the expectations of local residents and future users. If 90% of the structure is by definition invisible, since it is underground, for the remaining 10% efforts were made to optimise integration (compact and semi-underground interchanges, operational buildings with neutral architectural design that blend in well with the vegetation-rich landscape). This approach helps preserve the valuable heritage features – and green spaces – of the communities through which the infrastructure runs.
As a result of fluid traffic conditions, the improving environmental performance of vehicles, and the systems implemented by Cofiroute, the pollution generated in the tunnel remains limited. Studies and simulations have shown that the levels of pollutants in the tunnel are well below current French and European targets. Moreover, a permanent and independent air-quality-monitoring agency has been put in place.
Finally, in accordance with government commitments, the three interchanges were built below existing motorways and national routes. As a result, they are isolated from residential zones by treed mounds that absorb noise not only from the interchanges but also from the adjoining motorways.