Abstract
Soil resistance to penetration and rutting depends on variations in soil texture, density and weather-affected changes in moisture content. It is therefore difficult to know when and where off-road traffic could lead to rutting-induced soil disturbances. To establish some of the empirical means needed to enable the “when” and “where” determinations, an effort was made to model the soil resistance to penetration over time for three contrasting forest locations in Fredericton, New Brunswick: a loam and a clay loam on ablation/ basal till, and a sandy loam on alluvium. Measurements were taken manually with a soil moisture probe and a cone penetrometer from spring to fall at weekly intervals. Soil moisture was measured at 7.5 cm soil depth, and modelled at 15, 30, 45 and 60 cm depth using the Forest Hydrology Model (ForHyM). Cone penetration in the form of the cone index (CI) was determined at the same depths. These determinations were not only correlated with measured soil moisture but were also affected by soil density (or pore space), texture, and coarse fragment and organic matter content (R2 = 0.54; all locations and soil depths). The resulting regression-derived CI model was used to emulate how CI would generally change at each of the three locations based on daily weather records for rain, snow, and air temperature. This was done through location-initialized and calibrated hydrological and geospatial modelling. For practical interpretation purposes, the resulting CI projections were transformed into rut-depth estimates regarding multi-pass off-road all-terrain vehicle traffic.
Highlights
The soil cone index (CI), a measure of a soil’s resistance to penetration (MPa), is a commonly used soil mechanical property to determine soil strength [1] [2]
The high springtime levels for MCv within the top 15-cm soil at the UNB and SM locations are due to high Ah-layer OM content, which—according to Equation (1)—lowers Db and enhances the soil-filled pore space (PS) between the coarse fragments
While the plot-by-plot determinations of this study are limited to three contrasting forest locations, they are at least representative of how soil moisture, CI, and rutting depth vary by soil properties, season and topographic position, as demonstrated through daily and spatial modelling
Summary
The soil cone index (CI), a measure of a soil’s resistance to penetration (MPa), is a commonly used soil mechanical property to determine soil strength [1] [2]. This strength generally increases with increasing clay, coarse fragment (CF), and soil density (Db), or reduced pore space (PS), but decreases with increasing soil moisture (MC) and organic matter content (OM, %) [3] [4] [5] [6]. The resistance of soils to rutting is directly proportional to the ratio between tire footprint pressure and CI [20] [21] [22]. Rut depths further increase from single to multiple passes, and with slope-induced tire spinning [23]
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