Changes in soil stress during repeated wheeling: A comparison of measured and simulated values

Agricultural machinery traffic is one of the main causes of soil compaction in modern agriculture. Soils with weak inherent soil structural stability already have low bearing capacity and, when subjected to intensive tillage with a high frequency of traffic, are susceptible to severe soil compaction. In this study, repeated wheeling experiments were carried out on an Iranian clay soil prepared at two water contents (corresponding to 0.9 and 1.35 × water content at the lower plastic limit), two wheel loads (light and heavy rear wheel loads of a two-wheel-drive tractor) and two vehicle travel speeds (0.5 and 1 m s–1). The experiments tested whether the stress variations due to repeated wheeling are mainly due to variations in rut depth with repeated tyre passes and whether traffic at a higher travel speed has a smaller compaction effect. Mean normal stress was measured at three depths (0.15, 0.25 and 0.35 m) beneath the centre of tyres using cylindrical Bolling probes. Rut depth and cone index were measured after each pass. The results showed a linear increase in rut depth with consecutive tractor passes, with a greater increase on wet soil. However, bulk density increased more in dry soil than in wet soil at 0.15 and 0.25 m depth, most likely due to soil water content being close to the optimum Proctor water content. At 0.35 m depth, the bulk density increase was larger for wet soil, with obvious impacts of wheel load and travel speed (greater increase for slower speed and heavier wheel). Cone index generally increased with repeated tractor passes, with the greatest increase at 0.35 m depth in wet soil under heavy rear wheel traffic. Stress generally increased with increasing rut depth due to repeated wheeling. Reduced distance between the soil–tyre interface and the Bolling probes with increasing rut depth was investigated as a potential reason using analytical stress simulations, but could not fully explain the increase in stress with rut depth. Therefore, additional factors (e.g. soil strength) must have contributed to the stress increase with increasing number of tractor passes.