Work has restarted on the A86 Paris ring motorway tunnel following the devastating tunnel fire earlier this year. Over two months’ work was needed for cleaning and repair.

A delivery train caught fire in a large conduit beneath the tunnel’s floor slab in March, causing an inferno. Burning hydraulic oil destroyed the nearby precast concrete floor slabs, fit-out gantries and equipment above. It also caused spalling over 100m of concrete segmental lining. The cause of the fire is still is unknown.

The 11.24m diameter Herrenknecht TBM, boring the first 4.5km drive of the 10km tunnel, was not directly affected by the fire, but it had to be cleaned and partially renovated because of heavy smoke particle deposits across its surfaces. The control systems and circuit boards also needed cleaning.

“The fire affected the plastic material of the conveyor belt muck out system and this caused a very thick smoke. Some 1000m of the tunnel was covered in soot,” technical manager Pierre Boutigny explained. Apart from being dirty, the soot is electrically conductive. “The TBM demands some 9000kW of power and a short circuit is unthinkable,” he said.

New lifting equipment was needed for the slab installation work, which is carried out further back along the tunnel. The slab, eventually to be the lower of two car decks in the unique double-deck road tunnel, is also fitted as a construction platform for complex side adit excavations required for safety routes in the finished tunnel. An upper road deck and a ceiling slab are not fitted until much later.

The concrete tunnel lining will be pressure cut back to sound material and new concrete and mesh added to rebuild the segmental ring lining.

This first section of tunnel connects Rueil-Malmaison to a new interchange on the A13 motorway into Paris. A second 5.5km drive will complete the A86 Paris ring motorway forming an overall 10km long toll tunnel. Concessionaire Cofiroute is formed from contractors Vinci and Eiffage, which also forms JV Socatop. Road operator Colas is the client.