Transmission of vertical stress in a real soil profile. Part I: Site description, evaluation of the Sِhne model, and the effect of topsoil tillage

Transmission of stresses in the soil profile is one crucial ingredient of the ‘chain of cause and effect’ leading to either persistent deformation or elastic deformation. This study is the first of a series of three, where effects of contact stress and soil conditions on the stress distribution in the soil profile were evaluated. Measurements of vertical stresses were performed in an undisturbed Stagnic luvisol in three dimensions during one pass of one wheel (in depth, across and along the driving direction). In the present study, we tested the effect of loosening the topsoil by ∼20 cm ploughing on the stress distribution in the soil profile. The reference soil had not been ploughed or otherwise tilled for 18 months. The distribution of vertical stress near the tyre–soil interface was fitted by a model combining a power function and an exponential function for the stresses, respectively, along and across the driving direction (FRIDA model). The tillage-induced reduction in topsoil strength lead to more even stress distribution at the tyre–soil interface but did not significantly affect the measured vertical stresses at 0.3, 0.6 and 0.9 m depth. The vertical stresses at 0.3 m depth were equivalent to the peak stresses measured in the contact area between tyre and soil (approx. 200 kPa) and appeared more scattered when the top 0.2 m had been recently ploughed. Taking the FRIDA-estimated vertical stresses in the tyre–soil interface as input to the analytical Söhne model, vertical stresses in the subsoil were underestimated for both treatments. The precompression stress of the topsoil for this arable soil was much lower than the subsoil even for the treatment not tilled for 18 months. Hence, the vertical heterogeneity of soil for both treatments did not obey the model assumptions of isotropic soil properties. However, the model performed well from 0.3 to 0.9 m depth. Hence, an alternative to the model of Söhne is needed for the calculation of stress transmission in the frequently tilled topsoil of arable soils from the tyre–soil interface.