Impact of heavy vehicles on low-volume roads.

A recent study sponsored by the South Dakota Department of Transportation evaluated the impact of agricultural equipment on the actual response of low-volume roads. To meet this objective, one gravel pavement section and one blotter pavement section were instrumented in South Dakota and tested under agricultural equipment. Each section was instrumented with pressure cells in the base and subgrade and deflection gauges to measure surface displacement. Field tests were carried out during fall 2000, spring 2001, and summer 2001. Testing in different seasons offered the opportunity to evaluate the impact of heavy equipment on low-volume roads under variable environmental conditions: high and low temperatures and wet and dry conditions. Test vehicles included two Terragators, a grain cart, and a tracked tractor. The field testing program collected the pavement responses under five replicates of each combination of test vehicle and load level and under the 18,000-lb single-axle truck. Data were examined for repeatability, and the average of the most repeatable set of measurements was calculated and used in the analysis. The first part of the research evaluated the relative impact of the different equipment, defined as the ratio of pavement response under each combination of vehicle-load level over the pavement response under the 18,000-lb single-axle truck. Analysis of the pavement response ratios indicated that (a) the tracked tractor is not more damaging than the 18,000-lb single-axle truck, (b) Terragators 8103 and 8144 are more damaging than the 18,000-lb single-axle truck only when they are fully loaded, and (c) the grain cart is more damaging than the 18,000-lb single-axle truck only when it is loaded over the legal load limit. The second part of the research evaluated load equivalency factors for agricultural equipment on gravel and blotter pavements. Analysis of the load equivalency data indicated that an agency can effectively reduce the impact of agricultural equipment on a low-volume road by increasing the thickness of the base layer and keeping the load as close to the legal limit as possible. This paper is also available on CD-ROM (see C 30152 CD-ROM).