HEAVY SOIL LOADING AND ITS CONSEQUENCE FOR SOIL STRUCTURE, STRENGTH AND
DEFORMATION OF ARABLE SOILS
R. Horn1, S. Peth1, B. G. Richards2
1Institute
of Plant Nutrition and Soil Science, Christian Albrechts University of Kiel, Hermann Rodewald
Str. 2, 24118 Kiel, Germany; e-mail: rhorn@soils.uni-kiel.de
2Richards & Associates, Geotechnical Engineering, 55 Logan Place, QLD 4069, Australia
The alteration of mechanical soil properties by a single stress application exceeding all previously
applied stresses is analysed for a conventionally tilled and a conservational managed stagnic Luvisol
derived from loess. At the conventionally tilled site a pronounced plough pan layer with high internal
soil strength and a platy structure was encountered. Despite its compactness, this plough layer turned
out to be less rigid as the "relictic" plough layer of the formerly conventionally and since 1992
conservation managed site. We assume that wheeling with a sugar beet harvester (rear wheel 140
kPa, front wheel 110 kPa, total mass 37Mg), which significantly exceeds all former applied loads,
resulted in a breakup of the plough pan.
This was most obvious in the conventionally tilled soil whereas under conservation tillage the plough
pan seemed to resist the induced forces. The fragmentation into smaller plates is accompanied by the
formation of a new soil structure with altered mechanical properties.
Our results suggest that the breakup of the compact plough layer and the subsequent rearrangement
of the newly formed fragments results in a smaller mechanical stability of the deformed soil. Soil
structural changes within the plough pan are also indicated by the alteration of the anisotropy of
cohesion and precompression stress, respectively.
Anisotropy of mechanical parameters is due to the horizontal alignment of platy minerals (clay
minerals) under vectored loads, which is most obvious in plough pans. Breaking up the plough pan by
a load heavier than previous ones also resulted in a change of mechanical anisotropy ratios. Altered
mechanical properties induced by heavy soil loading affects the soil response to subsequent loading
events, which was shown by finite element simulations of stress- strain properties. The simulations
showed that a decrease in soil stiffness reduces the stress attenuation within the plough pan causing
compressive and shear stresses to be transmitted into deeper soil levels, while at the same time shear
strain increased.