Traction and repeated wheeling – effects on contact area characteristics and stresses in the upper subsoil

Abstract

Reducing wheel loads has long been the key advice to reduce the risk of subsoil deformation, but this disregards other machinery-soil interactions such as the effects of traction and repeated wheeling. We conducted a field experiment to disentangle the effects of traction (described as drawbar pull) and repeated wheeling on the contact area characteristics and stresses in the upper subsoil. Experimental traffic comprised a tractor (static load 11 Mg) with activated 4 WD towing a trailer (static load 17 Mg or 24 Mg) and took place on a sandy loam with stubble of oats at soil water content near field capacity. Measurements included drawbar pull, contact area and vertical contact stress, and horizontal (in the driving direction) and vertical stresses at ∼0.36 m depth in an undisturbed soil profile. Drawbar pull was significantly higher (9.13 kN compared to 6.46 kN) for the high trailer load, but no differences were observed between the two steering modes. The contact area of the tractor’s rear tyres increased for the high drawbar pull but with no significant differences in length or width. The maximum vertical stress in the tractor rear tyre’s contact area then tended to be lower, despite the increase in the dynamic wheel load (from 3.5 Mg to 4.1 Mg). Whereas high drawbar pull improved the stress distribution in the driving direction, the effect across the tyre was complicated. We found evidence of different tyre-soil interaction for tyres with and without traction. No significant effect of repeated wheeling with a single towed tyre (5.5 Mg) on the contact area characteristics were found. For the towed tyres, horizontal soil stress increased linearly with vertical soil stress, and we suggest that this increase is intrinsic to the soil (at these experimental conditions). Traction does then influence the ratio of horizontal to vertical stress. The results confirm the importance of considering dynamic aspects of field traffic such as traction and dynamic wheel load.