Dynamics of soil aggregate coalescence governed by capillary and rheological processes

The desired soil structure following tillage of agricultural soils is often unstable and susceptible to coalescence of aggregates and reduction of int eraggregate porosity due to wetting and drying cycles. This process of aggr egate rejoining was modeled by equating the rate of work done by liquid-vap or menisci, to the rate of energy dissipation due to viscous deformation of a pair of spherical aggregates. The nonlinearity of wet soil viscous flow behavior was accounted for by introducing a Bingham rheological model. A na tural outcome of the analysis was the formulation of a mathematical conditi on for the onset and termination of coalescence based on soil strength at s pecified water content. The condition states that sufficient energy in exce ss of soil strength (yield stress) must be available for coalescence to pro ceed. The rate of aggregate coalescence is proportional to available energy and is inversely related to the coefficient of plastic viscosity. Transpor t of wet soil to the periphery of the interaggregate contact by viscous flo w leads to smoothing of the neck, resulting in pore closure, on the one han d, and restricting the minimum matric potential that can be achieved, on th e other. The interplay between rheology and geometry prevent coalescence fr om proceeding indefinitely. Independently determined soil rheological prope rties were used to illustrate the use of the model. Coalescence under const ant water content and during wetting-drying cycles was calculated. Comparis on of data from experiments on one-dimensional, aggregate bed settlement ha s shown reasonable agreement with the model predictions.