pollutant retentionAucune traduction française n'est disponible pour cette page.
Research program in progress involving A. Parriaux and J. Zigliani
Objectives
The Federal Office for the Environment (FOEN) recommends the infiltration of road runoff water directly on the slope of the structure. This new concept, that modifies a lot the classical management of road runoff, had to be verified and optimized in order to implement it in the practice of road construction. The present research has the following main objectives:
- to determine how this system can be built in the frame of common construction works, taking into account their specificities
- to test in situ different kinds of shoulders which ensure that the main part of the runoff arrives to the infiltration slope and also preventing a too important infiltration of water in the structure (risk of instability)
- to verify the hydraulic and environmental efficiency of the infiltration slope as it is designed in the FOEN recommendation.
Methodology
To achieve these objectives, an experimental station has been built at the scale 1/1 on a new cantonal road between Grandson and Champagne. This road was divided in several segments of 10 or 20 meters long, in order to test several kinds of shoulders. Two lysimeters have allowed to measure and sample the water after the infiltration through the soils of the slope. Another compartment was devoted to the quantification of immissions in the aquifer below the road. Due to the very complete monitoring, it was possible to measure the losses through the shoulders as well as the water and pollutant fluxes at all the strategic points of the station. An observation on a longer term allowed to check between 2003 and 2009 the evolution of the shoulders, the infiltration slope and the aquifer.
Results
The construction of this pilot road showed that this infiltration system can be built as a normal road building operation. The finest part was the construction of the infiltration slope with its two soil layers. The building of the shoulders did not lead to difficulties.
Among the various types that were tested, the bentonitic geotextile shoulder obtained the best performance. Its almost perfect impermeability makes an impressive difference with the other shoulders. It does not show any performance reduction during the period 2003 and 2009. Its self-healing property is certainly responsible for this stability. A shoulder made of a gravel - clay mixture gave a much lower efficiency but still better than the other shoulder types. A progressive erosion of the clay fraction was observed. A cover of the bentonitic geotextile by a layer of a gravel-clay mixture would be certainly a very performing design for long term efficiency of the shoulders.
The water that was collected at the base of the slope soil (lysimeter) is much cleaner than the runoff. The natural improvement of the quality is due to the retention and biodegradation of the two layers of the slope. The need of the lower layer (B horizon) is also reinforced by the progressive colonization of rodents of the upper layer (A horizon). The dense galleries in this layer transform the granular porosity into a conduit porosity which is less performing for pollutant retention. Fortunately the cleaning potential of the infiltration slope is not significantly reduced, partially thanks to the B horizon. A slight creeping of the slope has been also measured, however without practical influence up to now.
By comparison between piezometers upstream and downstream of the road, no immission in the aquifer was noticeable. This fact is in agreement with the analyses in the lysimeters.
Conclusion
The system of road runoff infiltration on the slope of the structure can be constructed without major difficulty. It is hydraulically and environmentally efficient if the hydrogeological conditions are favorable (permeable underground, groundwater protection). The impermeability of the shoulder that injures the transition between the pavement and the infiltration slope is a key point for the performance of the system and for the mechanical stability of the road structure.
Related paper
Parriaux A., Piguet P., Zigliani J., 2009, Rétention des polluants des eaux de chaussées selon le système „infiltrations sur les talus“, Vérification in situ et optimisation. / Road Runoff Pollutant Retention by Infiltration through the Roadside slopes. In Situ Verification and Optimization. Bulletin n°1253 de la VSS et Office fédéral des routes, Berne, août 2009.
