Society calls for protection of agricultural soils in order to sustain the production of
foods for a growing population. Compaction of subsoil layers is an increasing problem
in modern agriculture and a cause of serious concern because of the poor resilience
in natural amelioration. The concept of soil precompression stress has been adapted
from civil engineering, although in soil science it is applied to unsaturated soils that
have developed a secondary structure from the action of weather, biota and tillage.
It assumes strain is elastic at loads up to the precompression stress, while plastic deformation is expected at higher stresses. To determine this threshold we performed
uniaxial, confined compression tests for a total of 584 minimally disturbed soil cores
sampled at three subsoil layers on nine Danish soils ranging in clay content from
0.02 to 0.38 kg kg1
. The cores were drained to either of three matric potentials
(50, 100 or300 hPa) prior to loading. Stress was applied by a constant-strain rate
method. We estimated the point of maximum curvature of the strain-log10(normal
stress) relation by a numerical procedure. This point is considered here as a compactive
stress threshold, typically labeled the soil precompression stress, σpc. The preload suction stress (PSS) was calculated as the product of initial (i.e., before loading) water
suction and initial degree of pore water saturation. Multiple regressions were performed
to evaluate the effect of soil properties (textural classes, volumetric water content, bulk
density (BD), soil organic matter (SOM), and PSS) on σpc. The best model explained 39%
of the variation in σpc, and indicated that σpc increases with increasing PSS, BD and SOM.
For a given combination of clay, BD and SOM, PSS affected σpc negatively. We recommend our regression model for use in risk assessment tools for estimating sustainable
traffic on agricultural soils. The model was validated by five independent data sets
from the literature. Our study shows that caution should be applied when regarding
σpc as a fixed threshold for compressive strength. We hypothesize that plastic deformation is initiated over a range of stress rather than at a distinctive single value.
Further studies are needed to better understand—and potentially quantify—to what
extent the predicted σpc can be regarded a central estimate of allowable stress for a
given soil.
