An Assessment Scheme for Soil Degradation Caused by Forestry Machinery on Skid Trails in Germany

The worldwide demand for wood products has led to an intensification of forestry and to higher weights of forestry machinery. In Germany but also in other countries, timber harvesting is limited to skid trails that minimize soil compaction and degradation. To preserve the trafficability of skid trails and to prevent the additional degradation of unloaded soils it is important to understand soil mechanical and physical behavior. We developed an assessment scheme to link visible deformations and soil physical processes caused by forestry machinery. For this reason five different machine types and additional equipment were tested at six study sites in Germany, namely in Lower Saxony between 2012 and 2015. During these studies, disturbed and undisturbed soil samples were taken at soil depths of 20, 40, and 60 cm to determine various physical soil parameters. Furthermore, major principal stress (sigma(1)) and shear stress (tau(s)), caused by the tested forestry machinery, were measured with the stress-state transducer system (SST) in the same depths. Based on these measurements, an assessment scheme was developed, which can be divided into two levels. The first level distinguishes internal and external soil parameters influencing soil stability. Internal soil parameters are processes and elements which describe soil strength and soil stability parameters (P-c and tau(r)), for example clay migration and matric potential. External parameters are wheel load or contact area, resulting in stress impacts (describing parameters: sigma(1) and tau(s)). The comparison between internal and external soil parameters led to the second level of this assessment scheme, describing four processes of soil degradation and deformation: elastic deformation, plastic deformation, failure of bearing capacity, and homogenization of soil structure.