Pavement Friction Reduction Due to Fine-Grained Earth Contaminants

Mining operations utilize haul trucks to transport raw earth materials for processing or disposal. When loaded haul trucks travel on or across a public roadway, some spillage of their load may occur. Clay soils, rock crusher dust, lime powder, or other such fine-grained earth materials that are hauled from mining operations, may contaminate a paved roadway and reduce the friction available to a braking vehicle. Skid tests using accelerometer-instrumented vehicles were conducted at 2 sites in separate states where surface mining operations had contaminated the road surface with fine-grained materials. At one site, representative material from the mining operation was spread on a section of pavement, wetted, tested 6 times, cleaned from the pavement by scraping with a front-end loader, then wetted and tested twice more. At the other site, the contaminated surface was tested in place during both wet and dry days. At both locations, a significant reduction in friction was observed on the wet contaminated surfaces as compared to the non-contaminated surfaces. Two mechanisms are postulated to account for this friction reduction. If the contaminant layer is sufficiently thick, all contact at the tire-pavement interface will be lost and the available friction must consist of only that between the tire and contaminant. Perhaps more commonly, the fine-grained contaminants may only partially fill the depressions between aggregates. In such a case, the pavement texture is effectively reduced, resulting in a smoother surface with less friction potential. Scraping the pavement surface using a front-end loader or other such scraping technique is not an effective method of removing fine-grained contaminants from the pavement texture.