Dynamic stress measurements and the impact of timber harvesting on physical soil properties

Abstract

ABSTRACTThe stress impacts of forestry machinery were determined and compared with internal soil strength to evaluate soil stability and identify soil compaction processes. In order to simulate a typical timber harvest, we analysed the soil deformation processes of one harvester (HA; 24 tonnes (Mg)) and one timber hauler (TH; 28 tonnes (Mg)) at three different sites in Germany, on soils derived from loess. The 1st pass was carried out with the HA and the 2nd to 5th passes were carried out with the TH. To determine stress impacts, the major principal stress (σ1) was measured using a stress-state transducer system installed at three different depths (20, 40 and 60 cm). Before and after wheeling, soil samples were taken to determine the precompression stress (PC)—as the fundamental parameter for internal soil strength—and the hydraulic parameters saturated hydraulic conductivity (Ks) and air capacity (AC). The combination of stresses at the various depths and the internal soil strength forms the basis for the proposed quantification of induced soil deformation processes. Soil stability can thus be derived from both parameters, and negative consequences for soil functions can be predicted if the ratio PC/σ1 is <0.8). These changes were quantified by comparing changes in Ks, AC and PC. Results for stress impacts revealed decreasing values of σ1 with increasing soil depth, and maximum values (σ1 > 600 kPa) during the 1st pass with HA. The PC/σ1 ratio described unstable soil conditions for almost all soil horizons, including that at 60 cm. PC was significantly increased in the topsoil after the 1st pass with HA and even more so after the 5th pass with TH, whereas both Ks and AC values decreased. Significant decreases in Ks were determined for almost all measurements at all depths, as well as for AC in topsoil and after the 5th pass with TH at 40 cm soil depth.