Date of Award:

8-2001

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Soil Science

Committee Chair(s)

Dani Or

Committee

Dani Or

Committee

James A. Bay

Committee

Stephen E. Bialkowski

Committee

Lynn M. Dudley

Committee

Lawrence E. Hipps

Abstract

Tillage modifies the soil structure to create conditions favorable for plant growth. However, the resulting loose structure is susceptible to collapse by internal capillary forces and external compactive stresses with concurrent changes in soil hydraulic properties. Presently, limited understanding of these complex processes often leads to consideration of the soil plow-layer as a static porous medium. The objective of this dissertation was to develop physically based modeling of soil structural dynamics at the pore scale, based on soil mechanical and rheological properties. The basic geometrical framework of the models was based on regular packing of monosized spherical aggregates forming unit-cells. A distinction was made between steady and transient stresses based on the rate of change of stress relative to strain rate. Capillary forces, external constant stress, and overburden were considered as steady stresses, because the relative rate of change of interaggregate contact stress is slow compared to the associated strain rate. In contrast, stress due to passage of farm implements was considered as transient, because the relative rate of change of interaggregate stress was comparable with the strain rate. Rheological properties of soil determined by application of steady and oscillatory shear stress were used for the calculations of strains under steady and transient interaggregate stresses, respectively. Under steady stress, the process of soil aggregate rejoining was modeled by considering the rate of energy dissipation due to viscous deformation of wet soil and the corresponding rate of work done by steady stress. Under transient stress, the soil deformation process involves elastic (recoverable) and viscous (irrecoverable) components. The elastic part in soil aggregates was attributed to bulging of the aggregates and was calculated using modified Hertzian theory. The viscous component was attributed to welding of soil aggregates at their contacts. The unit cell calculations were upscaled to soil aggregate bed by considering a one-dimensional stack of unit cells. Stress transmission rules were proposed that comply with the soil rheological properties under the given stresses. Evolution of the soil pore size distribution (and associated hydraulic functions) was modeled as a flow of probability density function using the Fokker-Planck equation (FPE). The coefficients of the FPE that determine the rates of change of mean, variance, and total sum of pore sizes were linked to the physical process models of soil aggregate deformation. Application of the proposed models was demonstrated using illustrative examples. Comparison of selected examples with experimental measurements from the literature showed reasonable qualitative and quantitative agreement.

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