Soil density and oscillation frequency effects on viscoelasticity and shear resistance of subtropical Oxisols with varying clay content

Soil density and oscillation frequency are some of the main factors affecting soil micromechanical behavior, but their role in the oscillatory amplitude sweep test (AST) is not yet well understood. Current studies use either a reference density, i.e. field density or a density based on texture and/or organic matter content or opt for a common density for all samples, while oscillation frequency may be relevant in simulating agricultural traffic speeds and affects coupled hydraulic-mechanical processes. We analyzed the effect of soil density and AST oscillation frequency at different matric potentials of three weathered Oxisols from southern Brazil that were repacked to different densities, namely different levels of compaction, no compaction and field density. Higher density, probably due to increased cohesion and friction, increased rheological shear stress parameters: shear stresses at the end of the linear viscoelastic range (LVR), tau(L); and at the yield point (YP), tau(YP); and storage and loss moduli (G' and G, respectively) at the YP, G', G(YP). However, viscoelasticity parameters: strain at the end of LVR gamma(L); strain at the YP gamma(YP); and the integrated zone I-z, were negatively affected at low densities with increasing magnitude at higher density levels. Uncompacted soil resulted in densities similar to field density, and delimitated a soil state where density does not interfere with friction. This suggests a methodology for the comparison of distinct soils or where information on field density is missing or unavailable. Frequency alterations from 0.25 to 1.0 Hz had low impact and only increased few rheological parameters, mainly in some soils drained at -6 kPa, but neither reduced data accuracy nor affected the samples' evaporation. Hence, rheological tests can be executed at a higher frequency than 0.5 Hz (at least up to 1.0 Hz) to increase sample output. For both density and frequency, (i) drainage increased soils micromechanical resistance due to water menisci; and (ii) soil resistance showed the order: Typic Hapludox (clayey brown-reddish Oxisol)> Rhodic Kandiudox (clayey red Oxisol)> Rhodic Hapludox (sandy red Oxisol), mainly due to the cohesive effect of clay and Fe oxides, as well as the aggregating effect of soil organic matter.