Rheological properties of wet soils and clays under steady and oscillatory stresses

Tilled agricultural soils are in a constant state of change induced by variations in soil strength due to wetting and drying and compaction by farm implements. Changes in soil structure affect many hydraulic and transport properties; hence their quantification is critical for accurate hydrological and environmental modeling, This study highlights the role of soil theology in determining time-dependent stress-strain relationships that are essential for prediction and analysis of structural changes in soils. The primary objectives of this study were (i) to extend a previously proposed aggregate-pair model to prediction of compaction under external steady or transient stresses and (ii) to provide experimentally determined theological information for the above models, Rheological properties of soils and clay minerals were measured with a rotational rheometer with parallel-plate sensors, These measurements, under controlled steady shear stress application, have shown that wet soils have viscoplastic behavior with well-defined yield stress and nearly constant plastic viscosity, In contrast, rapid transient loading (e,g,, passage of a tractor) is often too short for complete viscous dissipation of applied stress, resulting in an elastic (recoverable) component of deformation (viscoelastic behavior). Measured viscoelastic properties were expressed by complex viscosity and shear modulus whose components denote viscous energy dissipation, and energy storage (elastic). Results show that for low water contents and fast loading (tractor speed), the elastic component of deformation increases, whereas with higher water contents, viscosity and shear modulus decrease, Steady and oscillatory stress application to an aggregate pair model illustrates potential use of theological properties towards obtaining predictions of strains in soils.