Effects of soil surface degradation and vehicle momentum on dust emissions and visibility reduction from unpaved roads

Unpaved roads are built on the basis of soil compaction. Therefore, the operation of vehicles on these roads results in dust emissions. As the number of passing vehicles increases, the soil surface gradually degrades under shearing caused by the tires. This enhances emissions and reduces visibility, increasing health and environmental related issues as well as the risk of collisions. The present study was conducted to quantify the effect of surface degradation on dust emissions from several types of vehicles travelling on different soils and assess the evolution of security risks in the vicinity of unpaved roads. Three types of vehicles (a passenger car, a 4WD vehicle and a truck) have been run on an experimental road covered by four different types of soil particles. Each test consisted in driving one of the vehicles at a given speed (30, 45 or 60 km.h(-1)) on one of the types of particles previously spread on the road at a given mass per unit area (200, 400 or 600 g.m(-2)). The mass concentration of resuspended particles was measured from the roadside using an Optical Particle Counter (OPC) while the visibility reduction was simultaneously recorded at the same location. The results show that PM10 emission factors increase linearly with vehicle momentum for vehicles without mud flaps. The presence of such appendices can reduce dust emissions by a factor of 7. Moreover, although not considered in conventional models, the state of degradation of unpaved roads may be a key factor regarding dust emissions. The study has also shown that soils with similar silt content can result in very different emission factors. An emission model has then been developed, based on the clay content of the particles. Indeed, this geotechnical parameter is more relevant to characterize the fine soil propensity to generate dust. PM10 emissions are correlated with the reduction of visibility according to a power law for the truck tests. An interesting finding of this work is the fact that the present results can be extended to earthwork sites where visibility problems require spraying of water on the traffic lanes. Improving the modeling of the mechanisms generating dust lift and the associated safety risks is indeed essential for a more efficient use of water on construction sites.