The transition to free flow on the A 13 makes it possible to de-artificialize 28 hectares which will be sown and planted using the materials present on site.
On the Paris-Normandy motorway, it has been a year since the gantries replaced the 14 toll barriers. These are dismantled little by little, and the tracks put back on the axis. In some places, the road will be reduced from 20 to six lanes, freeing up vast spaces. “While artificialization is becoming a national problem, we are going to return 28 hectares to nature,” says Arnaud Quémard, general director of Sanef, concessionaire of the A 13 – A 14. But not just anyhow.
In this type of operation, the usual procedure consists of extracting the different layers of the infrastructure then replacing them with topsoil from other construction sites or agricultural areas, spread 40 to 60 cm thick. A solution that put off the dealer: “Finding topsoil is often difficult, especially in Ile-de-France. It is expensive, and taking soil to transfer it elsewhere seemed questionable to us,” explains François Cornier, Sanef construction director. Setec, project manager of the free flow transition project, then proposed an original approach: reconstruct the ground using the materials present on the site. “We are going to make our own land,” summarizes François Cornier.
First stage: the roadway is demolished to a thickness of 30 cm. The subgrade layer then appears, made up of inert materials such as gravel or cement gravel. These are analyzed to check the absence of pollution then decompacted to a depth of 1 m and mixed over 40 cm with 10 to 15% compost. The new soil is then left to rest for a year before being sown or planted. Two years of experimentation on the Buchelay (Yvelines) and Dozulé (Calvados) sites have demonstrated the effectiveness of the process. “The compost aggregated well, plant mortality was low, and we quickly identified earthworms, a sign of a functioning soil,” notes Antoine Faure, project engineer at Setec Lerm. The small species work the soil on the surface, then the long worms descend deeper, aerate the soil and allow water infiltration. The micro-organisms and bacteria provided by the compost also help to make this soil alive.
Reduction of earth movements
The teams tested around twenty plant species chosen for their suitability for the site – soil texture, physicochemical parameters, pH or even exposure: prunus, maple, elderberry, orchardgrass, trefoil, clover, etc. “This technique has already been used, but this is the first time that it has been applied in a motorway environment and on such a surface,” emphasizes François Cornier. Although it requires a long period of experimentation and a lot of monitoring work, it considerably reduces earth movements, in one direction or the other. “With the usual method, in Buchelay alone, we would have had to evacuate 800 trucks of materials and bring in almost as many containing topsoil,” specifies the construction director. With the method chosen, 90 trucks of compost will be enough.
