Rheological properties of wet soils and clays under steady and oscillatorystresses

Tilled agricultural soils are in a constant state of change induced by vari ations in soil strength due to wetting and drying and compaction by farm im plements. Changes in soil structure affect many hydraulic and transport pro perties; hence their quantification is critical for accurate hydrological a nd environmental modeling, This study highlights the role of soil theology in determining time-dependent stress-strain relationships that are essentia l for prediction and analysis of structural changes in soils. The primary o bjectives of this study were (i) to extend a previously proposed aggregate- pair model to prediction of compaction under external steady or transient s tresses and (ii) to provide experimentally determined theological informati on for the above models, Rheological properties of soils and clay minerals were measured with a rotational rheometer with parallel-plate sensors, Thes e measurements, under controlled steady shear stress application, have show n 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 dissi pation of applied stress, resulting in an elastic (recoverable) component o f deformation (viscoelastic behavior). Measured viscoelastic properties wer e 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 compo nent of deformation increases, whereas with higher water contents, viscosit y 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.