Modeling the process of rut formation during sequential interaction of forwarder wheels with solid obstacles on a skid trail taking into account soil moisture

A mathematical model of rut formation during sequential interaction of forwarder wheel sets with solid obstacles (stumps, stones) on a skidding trail is presented, taking into account soil moisture. The model is based on the laws of energy conservation, contact mechanics, and the Mohr–Coulomb shear failure criterion of soil. It has been established that the dynamic pressure on the soil when a wheel hits an obstacle can be 2 or more times higher than the static pressure. Based on field surveys of a skidding trail in a cowberry pine forest, it was revealed that the depth of deformations after solid obstacles is greater towards the loaded travel direction of the forwarder, and the resulting “comb” type rut leads to increased fuel consumption, increased dynamic loads, and reduced operator comfort. It is shown that obstacle height and soil moisture are critical factors: at moisture content above the plasticity limit of loam (W > 28–30 %) and obstacle height h ≥ 0.3 m for a three-axle forwarder, the relative rut depth reaches λ = 1.3, which corresponds to the bottom contacting the ground surface and loss of trafficability. Technical measures to reduce rut depth are proposed and quantitatively substantiated: equipping wheel sets with flexible track belts (single-track wheels) and optimizing axle load. For a four-axle forwarder, the installation of single-track wheels allows skidding on wet soils with obstacles up to 0.4 m high without reducing load capacity. The developed approach makes it possible to predict rut depth for given geotechnical conditions and to reasonably apply measures to reduce dynamic loads while maintaining the rated productivity of the machines.