Rheological Properties of Wet Soils and Clays under Steady and Oscillatory Stresses

In engineering soil mechanics, it is very common to determine stress–strain relationship of soils (under equiTilled agricultural soils are in a constant state of change induced librium conditions) empirically from simultaneous meaby variations in soil strength due to wetting and drying and compaction by farm implements. Changes in soil structure affect many hydraulic surements of stress and strain, assuming a certain rheoand transport properties; hence their quantification is critical for acculogical model applies to the particular soil deformation. rate hydrological and environmental modeling. This study highlights For example, theory of elasticity is used in Terzaghi’s the role of soil rheology in determining time-dependent stress–strain theory of one-dimensional soil consolidation (Taylor, relationships that are essential for prediction and analysis of structural 1948); and the theory of viscoelasticity is applied for changes in soils. The primary objectives of this study were (i) to study of creep phenomena in soils (McMurdie, 1963). extend a previously proposed aggregate-pair model to prediction of Although very few stress–strain measurements have compaction under external steady or transient stresses and (ii) to been carried out at the so-called tillage range in agriculprovide experimentally determined rheological information for the tural soils, because of the inherent difficulty of obtaining above models. Rheological properties of soils and clay minerals were measurements under field conditions (Keller, 1970), the measured with a rotational rheometer with parallel-plate sensors. These measurements, under controlled steady shear stress application, above empirical methods are often applied for agriculhave shown that wet soils have viscoplastic behavior with well-defined tural soil mechanics with little modification (Koolen, yield stress and nearly constant plastic viscosity. In contrast, rapid 1983). The use of empirical stress–strain relationships transient loading (e.g., passage of a tractor) is often too short for for study of soil deformation, especially for agricultural complete viscous dissipation of applied stress, resulting in an elastic soil mechanics, has several weaknesses. Primarily, the (recoverable) component of deformation (viscoelastic behavior). method is not useful for a priori prediction of magnitude Measured viscoelastic properties were expressed by complex viscosity and mode of strain (Hillel, 1998). Secondly, the apand shear modulus whose components denote viscous energy dissipaproach is based on equilibrium state stress–strain relation, and energy storage (elastic). Results show that for low water tions, while deformations in agricultural soils rarely contents and fast loading (tractor speed), the elastic component of reach equilibrium (Or, 1996), especially when transient deformation increases, whereas with higher water contents, viscosity and shear modulus decrease. Steady and oscillatory stress application and rapid loading by agricultural machinery is considto an aggregate pair model illustrates potential use of rheological ered. Finally, the method is applicable only for describproperties towards obtaining predictions of strains in soils. ing bulk volume changes, but it cannot be used to explain pore-scale evolutionary processes that are very crucial in flow and transport processes. S compaction by externally applied stresses and A viable alternative approach that circumvents these internal capillary tension results in soil structure limitations is to develop pore-scale mechanistic models modifications that are of wide interest because of the that are coupled with intrinsic soil rheological properinfluence of such changes on transport processes, root ties. Recently, we proposed energy balance–based forgrowth, and soil mechanical strength. Soil deformation mulations that provide predictions of soil deformation involves time-dependent reorientation and displacerates, at pore and bulk scales, induced by capillary forces ment of constituents at microscopic and macroscopic of water by considering nonequilibrium stress–strain levels, and it is commonly characterized by the relative rate relations (Ghezzehei and Or, 2000; Or et al., 2000). deformation or strain. For agricultural and environmenThese models address soil deformation induced by intertal applications soil strain at pore scale is important nal capillary forces, where there is no particularly fa[e.g., in the study of evolution of hydraulic properties vored stress direction. This omnidirectional stress is (Or et al., 2000)], while for geotechnical applications, quite different from external stresses that possess strong overall strain and strength properties of the bulk soil directionality; usually vertical stresses are higher than are required. Fundamental concepts of soil rheology, horizontal stresses. Moreover, stresses induced by movwhich describe the flow behavior of wet soils, are useful ing farm implements (e.g., tractor passage) have short for mathematical description of the time-dependent loading duration. As this study shows, there is a fundastress–strain relationships under various loading condimental difference between soil deformation by farm imtions (Mitchell, 1993; Vyalov, 1986). plements (oscillatory stress) and capillary induced Soil strain is relatively large and nonuniformly distribstrains (steady stress) rooted in the inherent soil properuted in space and time and is not easily predictable from ties, manifested in the form of frequency-dependent a given form and magnitude of stress (Hillel, 1998). rheological properties. Hence, the motivation for this study was twofold: (i) to provide independently meaTeamrat A. Ghezzehei and Dani Or, Dep. of Plants, Soils and Biomesured rheological properties for the capillary-induced teorology, Utah State Univ., Logan, UT 84322. Received 27 July 2000. *Corresponding author ([email protected]). Abbreviations: CR, controlled shear rate; CS, controlled stress; OSC, sinusoidal (oscillatory) shear stress. Published in Soil Sci. Soc. Am. J. 65:624–637 (2001).