SOIL DEFORMATION OBSERVATIONS IN A VERTISOL UNDER FIELD TRAFFIC

A procedure based on the point grid method has been used to measure and visualize soil deformation resulting from tyre loads in a controlled traffic system. Several tests were performed in an irrigated heavy clay soil under various moisture conditions and two different geometries of the wheel tracks, recessed and elevated. This represented normal working conditions and illustrated that significant deformations were occurring. Deformations increased with moisture content and the number of passes. From the point grid displacements, bulk density changes were calculated which corresponded with the measured increases in density obtained from cores. Large displacements were observed when the soil was trafficked at high moisture content with little corresponding increase in bulk density. Soil structural degradation resulting from trafficking this soil at high moisture content should therefore be seen as a different process, such as a change in the orientation of clay particles or porosity of natural aggregates, rather than an increase in bulk density. Significant differences between vertical and horizontal displacements were found for the various tests. Increases in horizontal displacements were contributed to an increase in the degree of saturation. The recessed wheel tracks increased horizontal displacements due to the geometry of the wheel tracks.