Stress Distribution under Forestry Machinery and Consequences for Soil Stability

Core Ideas Stress impacts of forestry machinery exceed soil stability and cause soil degradation. Special forestry machinery is able to reduce stress impacts significantly. Only weight reduction and cleat tracks combined lead to stable soil conditions in topsoil. The major principal stress (σ 1 ) of modern wheels and cleat‐track forestry machinery was evaluated regarding changes in soil strength during multiple traffic experiments. Four heavy machines up to 32 Mg and three smaller machines up to 6.5 Mg were tested on soils derived from loess and glacial till in Lower Saxony, northwest Germany. The σ 1 was measured at different soil depths by a stress‐state transducer system. Undisturbed soil samples were also taken to identify precompression stress ( P c ) in order to quantify the ratio of P c and actual σ 1 . This ratio is used to describe soil stability responses to specific machinery impacts and offers additional physical soil parameters useful for estimating soil structural changes. The value of σ 1 generally decreased with depth and with an increasing number of traffic events. The findings show considerable differences in σ 1 (up to 43%) following the first and second passes. A specially designed cleat‐track lumber hauler was able to reduce σ 1 by about 64% compared with the typical wheeled forwarder. Although P c /σ 1 ratios developed in the surface soils following traffic by both light and heavy machinery, this ratio dramatically increased for heavy machinery. While on the one hand trafficking during harvesting did not change the soil strength in the subsoil, the negative outcome of these tests on the other hand indicated that only one of the seven tested machines equipped with a special dynamic driving system and containing a lower load was able to maintain σ 1 smaller than the internal strength of the soil at all depths and did not exceed the site‐specific strength history.