Effects of tyre size, inflation pressure and wheel load
We urgently need increased quantitative knowledge on stress transmission in real soils loaded with agricultural machinery. 3D measurements of vertical stresses under tracked wheels were performed in situ in a Stagnic Luvisol (clay content 20 %) continuously cropped with small grain cereals. The tests took place in the spring at field capacity when the topsoil had not been tilled for 1½ year. Two Nokian ELS Radial-ply tyres (800/50R34 and 560/45R22.5) were loaded with two specific loads (30 kN and 60 kN). We used rated tyre inflation pressures for traffic in the field (≤10 km h-1 driving speed). Seven load cells were inserted horizontally from a pit with minimal disturbance of soil in each of three depths (0.3, 0.6 and 0.9 m), covering the width of the wheeled area. The position of the wheel relative to the transducers was recorded using a laser sensor. Finally, the vertical stresses near the soil-tyre interface were measured in separate tests by 17 stress transducers across the width of the tyres. The results showed that the inflation pressure controlled the level of maximum stresses at 0.3 m depth, while the wheel load was correlated to the measured stresses at 0.9 m depth. This supports the principle behind the elasticity theory. However, if fitting the Söhne model to stress measurements in all three depths, the stresses were underestimated at 0.3 and 0.6 m depth, and overestimated at 0.9 m depth. A fit of the model based on data only at 0.3 m depth indicated that stresses were transmitted nearly without down damping through the 0-0.3 m soil layer (average concentration factor of 29.0 across the four treatments). Our results thus qualitatively confirm the principle of elasticity but highlight the need to model arable soil as a two-layer system.