Clay domains are the operational particles in clay soils and their properties can be used as a model with which to interpret soil physical behaviour. The experiments conducted for this thesis were designed to use the proportion of exchangeable sodium on the clay surfaces (sodicity), electrolyte concentration, and mechanical action as tools to examine the nature and stability of clay domains. Two companion studies were undertaken: the first based on permeability measurements and the second on the effect of mechanical energy imparted during the preparation of soil suspensions on domain stability, as measured by the electrolyte concentrations at which the suspensions were flocculated and dispersed.
The permeability experiments demonstrated the nexus between sodicity and electrolyte concentration with respect to permeability and consequently the appropriate role of the DLVO theory for diffuse double layers in interpreting soil physical behaviour. The threshold concentration, at which the first discernible effects on permeability were measured and the turbidity concentration at which dispersed particles appeared in the percolates, were found to be comparable to those of earlier workers. The dispersed particles were characterised in terms of their particle size distribution and specific surface area using laser light scattering (LLS). Their characteristics were relatively uniform over a wide range of conditions and consequently it was concluded that they were, compound particles - clay domains. By comparing their size distribution with the pore diameter calculated from the intrinsic permeability of the soil, it was shown that the reduction in permeability between the threshold and turbidity concentrations is not the result of pore blockage by the released domains. The fundamental mechanism for decreases in soil permeability is the DLVO interactions within the clay domains leading to domain swelling and impairment of the principle water conducting pores.
In contrast to the face-to-face crystal interaction within the clay domains, the flocculation process leads to open card-house structures. It was found that the floccules consisted of clay domains. When circulated for particle size determination by LLS, the floccules released the individual domains. These domains had essentially the same characteristics as those m the percolates, indicating that at sufficiently large electrolyte concentrations the clay domains were able to maintain their identity despite the mechanical energy involved in suspension preparation. At smaller electrolyte concentrations, the DLVO repulsive energy coupled with the mechanical energy causes the domains to breakdown to yield individual clay crystals with a median diameter of 0.26 µm and a surface area of 10 m2 g-1 Since the clay domains have an area of about 1 m2 g-1 this result indicates that the clay domains have about 10% of the total surface area of the clay. This indicates that the domains contain a large number of crystals.
The nature and stability of domains over a wide range of sodicity, electrolyte concentration and mechanical action has been delineated.
A limited number of electron microscope images were taken to provide additional information on the shape and morphology of the domains and crystals. The clay domains are shown to be composed of many clay crystals. They are plate shaped with large aspect ratios, of the order of 50:1, indicating that their growth is principally in the lateral direction. This large aspect ratio is considerably different to the commonly accepted aspect ratios of 10:1 or 15:1 for clay crystals.
The clay domains within a soil are in random array with respect to their principle axis of orientation and as a result, intrinsic discontinuities follow their surfaces. The general discussion draws attention to the fact that these intrinsic discontinuities have special significance with respect to the disposition of soil organic matter and the stabilisation of soil aggregates at the macroscopic level. Attention is also given to the role of the intrinsic discontinuities as the basis of soil swelling and its implications for soil water-suction relations.