Rheological Parameters as Affected by Water Tension in Subtropical Soils

Patricia Pértile,José Miguel Reichert,Dörthe Holthusen,Paulo Ivonir Gubiani,André Da Costa

doi:10.1590/18069657rbcs20150286

Abstract

Rheological parameters have been used to study the interaction between particles and the structural strength of soils subjected to mechanical stresses, in which soil composition and water content most strongly affect soil resistance to deformation. Our objective was to evaluate the effect of water tension on rheological parameters of soils with different mineralogical, physical, and chemical composition. Surface and subsurface horizons of four Oxisols, two Ultisols, one Alfisol, and one Vertisol were physically and chemically characterized; their rheological parameters were obtained from amplitude sweep tests under oscillatory shear on disturbed soil samples that were saturated and subjected to water tension of 1, 3, 6, and 10 kPa. In these samples, the rheological parameters linear viscoelastic deformation limit (γL), maximum shear stress (τmax), and integral z were determined. By simple regression analysis of the rheological parameters as a function of soil water tension, we observed increased mechanical strength with increasing water tension up to at least 6 kPa, primarily due to increased capillary forces in the soil. However, increased elasticity assessed by γL was not as expressive as the increase in structural rigidity assessed by τmax and integral z. Elastic deformation of the soil (γL) increases with the increase in the number of bonds among particles, which depend on the clay, total carbon, expansive clay mineral, and cation contents; however, maximum shear resistance (τmax) and structural stiffness (integral z) mainly increase with clay, kaolinite, and oxide content by increasing the strength of interparticle bonds. A decrease in mechanical strength occurs for water tension of 10 kPa (the lowest water content evaluated) in sandy horizons or in horizons with a high proportion of resistant microaggregates (pseudosand), when associated with low bulk density, due to fewer points of contact between soil particles and therefore lower capillary force.

Highlights

Soil physical degradation from external stresses increases with decreasing structural strength of soil (Horn, 2003)
Soil mechanical strength increased with increasing water tension up to 6 kPa, primarily due to increased capillary forces in the soil
Increased bonding between particles promotes soil elastic deformation, and γL increased with higher amounts of clay, total carbon, expansive clay, and cations

Summary

Introduction

Soil physical degradation from external stresses increases with decreasing structural strength of soil (Horn, 2003) Traditional techniques such as direct shear and static compressibility are widely used for determining mechanical parameters by stress-strain relations correlated with structural properties on the scale of aggregates, such as soil density and porosity and aggregate stability. As these properties depend on soil composition, such as particle size distribution, mineralogy, and organic matter content (Horn and Peth, 2011), the traditional techniques are limited in their ability to clearly determine soil behavior caused by mechanisms acting on a particle scale (Markgraf et al, 2006). A decrease in soil water content strengthens the bonds among soil particles by an increase in cohesive forces, friction between particles, and capillary forces (adhesion) due to water surface tension and the number and curvature of menisci (Kemper and Rosenau, 1984; Gallipoli et al, 2003; Mitchell and Soga, 2005; Lourenço et al, 2012)

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Conclusion