Magnitude, modeling and significance of swelling and shrinkage processes in clay soils

The dynamic process of swelling and shrinkage in clay soils has significant practical consequences, such as the rapid transport of water and solutes via shrinkage cracks to the subsoil, and the destruction of buildings and roads on clay soils. In order to develop measuring methods and computer simulation models to take swelling and shrinkage processes into account in agricultural and environmental studies, experiments were conducted on soil aggregates, small soil cores, with a lysimeter, and in the field.Determination of shrinkage characteristics of soil aggregates revealed some clay soils from the Netherlands to be among the strongest swelling and shrinking soils of the world with a maximum volume decreases upon drying of 49 %. Normal shrinkage and residual shrinkage are significant within pressure head ranges occurring in field conditions. Shrinkage was isotropic for all horizons of a heavy clay soil, at overburden pressures corresponding to field loads.The shrinkage behaviour of a clay soil in a lysimeter may be explained by the occurrence of structural shrinkage, normal isotropic shrinkage and residual isotropic shrinkage. Field experiments confirmed this picture. A newly developed equation to relate vertical soil movements to water content changes during structural, normal, residual and zero shrinkage, for any shrinkage geometry, was successfully applied in the field.The obtained knowledge about the swelling and shrinkage process was used to develop a computer simulation model, FLOCR, to calculate the water balance of a clay soil, including preferential flow through shrinkage cracks, on a daily basis. Besides, crack volume and surface subsidence are computed. Model calculations for a heavy clay soil during 1985 were in good agreement with field observations. Comparing FLOCR with a rigid-soil model showed a considerable influence of shrinkage cracks on the soil water balance, on the groundwater level and on the topsoil bearing capacity. The simulation model was also applied to predict the effect of drainage on actual swelling and shrinkage processes in a heavy clay soil profile.Finally, the principles of incorporating evaporation from shrinkage cracks, oxygen diffusion in cracking clay soils, and lateral infiltration during preferential flow into the present model approach are presented.