SOIL DEFORMATION OBSERVATIONS IN A VERTISOL UNDER FIELD TRAFFIC

A procedure based on the point grid method has been used to measure an d visualize soil deformation resulting from tyre loads in a controlled traffic system. Several tests were performed in an irrigated heavy cl ay soil under various moisture conditions and two different geometries of the wheel tracks, recessed and elevated. This represented normal w orking conditions and illustrated that significant deformations were o ccurring. Deformations increased with moisture content and the number of passes. From the point grid displacements, bulk density changes wer e 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 incr ease in bulk density. Soil structural degradation resulting from traff icking this soil at high moisture content should therefore be seen as a different process, such as a change in the orientation of clay parti cles or porosity of natural aggregates, rather than an increase in bul k density. Significant differences between vertical and horizontal dis placements were found for the various tests. Increases in horizontal d isplacements were contributed to an increase in the degree of saturati on. The recessed wheel tracks increased horizontal displacements due t o the geometry of the wheel tracks.