Quantifying vertical stress transmission and compaction-induced soil structure using sensor mat and X-ray computed tomography

Accurate estimation of stress transmission in soil and quantification of compaction-induced soil pore structure is important for efficient soil use and management. Continuum mechanics have so far mostly been applied for agricultural soils, even if topsoil structure is aggregated due to regular tillage. In this study, partially confined uniaxial compression tests were carried out on intact topsoil columns placed on subsoil columns. Two methods were employed for estimation of stress transmission in soil: (i) soil deformation patterns were quantified using X-ray CT and converted to stress distributions, and (ii) a tactile sensor mat was employed for measuring stresses at the interface of the topsoil and subsoil columns. The resulting soil pore structure under applied stresses was quantified using X-ray CT and by air-permeability measurements. In topsoil discrete stress transmission patterns were observed at 275 kPa applied stress, whereas continuum-like stress transmission was observed at 620 kPa. At the interface of topsoil and subsoil, discrete stress transmission patterns were observed at all applied stresses ranging from 68 to 620 kPa, but it was less discrete as we moved from lower to higher applied stresses. Our results imply that at lower stresses the stress transmission in arable soil was discrete because the applied load was mainly transmitted through chain of aggregates. At higher applied stresses the soil] aggregates deformed and to a larger degree resembled a continuous material where continuum-like stress transmission patterns were observed. We found that continuum stress transmission patterns were well simulated with models based on the elasticity theory (e.g., Boussinesq, 1885) compared to discrete stress transmission patterns. The soil pore structure was affected by increasing applied stresses. Total porosity was reduced 5-16% and macroporosity (pores > 0.5 mm) 50-85% at 620 kPa for topsoils. For subsoils - serving here as the material underlying the topsoil tests columns - only a small decrease was observed in both total porosity and macroporosity. Air permeability was reduced 55-80% for topsoils and 10-20% for subsoils at 620 kPa stress. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.