Publication
A new concept of road runoff management, based on diffuse infiltration over the shoulder, was tested in a real-scale experimentation site located near Grandson, Switzerland. This new concept consists in diffusely infiltrating the road runoff in the infiltration slope adjoining the road shoulder; where road contaminants are retained. This presupposed that the road shoulder was as tightened as possible to effectively drive the road runoff to the slope, and that the infiltration slope effectively retains all road pollutants. Those two main postulates needed strong scientific verification. For this purpose, five different shoulder designs were tested in Grandson to assess which one presented the best hydraulic performance, i.e. which shoulder was the most impervious. Those designs were made of gravel mixed with humus (SGL), gravel mixed with clay (SGC), gravel seeded with lawn (SGL), a prolonged HMF road base (SH), and a bentonitic geotextile tightening (SB). Also, to verify the retention of pollutant, two infiltration slopes adjoining SGL (LW) and SH (LH) were set as lysimeter (basal geomembrane collecting road effluents); the third infiltration slope (LB) had a direct connection with the aquifer which was closely monitored (6 piezometers located up- and downstream from the road; influence of the road runoff was thus emphasized). Finally, to ensure that this new concept do not lower the road bearing capacity, deflectometers were placed in the road structure; 3 campaigns of leveling assessed the road settlement. Hydraulic assessment of the shoulders, based on 112 natural precipitation as well as 3 artificial watering tests, showed strong discrepancies between all shoulders behaviours. They had to deal with potentially high amount of water. Lag times ranged from a few minutes to several hours. Results showed that lag times were only function of the precipitation mean intensity. The infiltration process could be assessed by a modified Green & Ampt equation. The moisturizing front was frank and linearly progressed downward. Shoulders vertical hydraulic conductivities ranged from 4.6·10-6 (SGC) to 2.2·10-5 m·s-1 (SGL). The exfiltration volume is function of the water volume available at the shoulder surface. The drought preceding the rain event has no influence on the stock variation or the exfiltration volume. Road runoff used two different infiltration paths: the macroporosity drives the water efficiently and rapidly, while the microporosity is less efficient and retains the moisture longer. Tracer tests (Brine) proved that the contamination first flush passed over the shoulders. This is also confirmed by the flux calculation. Runoff coefficients CR ranged from 0.3 (SGL) to 0.9 (SB). This coefficient decreased rapidly once the shoulder is wet. Only SB really fulfilled its task, letting water through only in harsh hydraulic conditions (100 mm; 40mm/h). It is highly recommended for further road development: for new roads as well as old roads being refurb
Charlotte Grossiord, Christoph Bachofen