Transmission of vertical stress in a real soil profile. Part III: Effect of soil water content

The transmission of stress in soils is extremely sensitive to changes in water content. According to the elasticity theory, for a given load applied to a given soil, an increase in soil water content yields a higher concentration of stresses under the centre of the load and a deeper propagation of stresses. We quantified the effect of soil water content of topsoil/subsoil layers (wet/wet, wet/dry, and dry/dry) on stress transmission. 3D measurements of vertical stresses under a towed wheel (800/50R34) were performed in situ in a Stagnic Luvisol. The tyre was loaded with 60 kN, and we used the recommended tyre inflation pressure for traffic in the field (100 kPa). Seven stress transducers were inserted horizontally from a pit with minimal disturbance of soil at each of three depths (0.3, 0.6 and 0.9 m) and covering the width of the wheeled area. The vertical stresses at the tyre–soil contact area were measured in separate tests. Increase of water content in the topsoil by 114% increased the contact area by 149%, decreased the vertical stresses at the tyre–soil interface by 50%, and decreased the maximum vertical stress at 0.3 and 0.6 m depth by 46 and 63%, respectively. Stress attenuation with depth decreased with an increase in soil water content, yielding approximately equal maximum stresses at 0.9 m depth for the wet/wet and wet/dry treatments, while the dry/dry soil experienced significantly higher stresses than the other treatments (43, 40 and 60 kPa for treatment wet/wet, wet/dry and dry/dry, respectively). The Söhne model underestimated the vertical stresses at all depths, and the model fit was poorest at low water contents. Our data thus support the general characteristics of the elasticity theory, although the simple one-layer Söhne model gave poor quantitative stress predictions.