Subplasticity In Australian Soils. IV. Plasticity And Structure Related To Clay Cementation
Subplastic behaviour apparently arises from some form of cementation between clay particles and between groups of particles. Five red-brown earth subsoils from the Riverina area of New South Wales, with subplasticity ratings from SP0 to SPIII+, have been examined here in order to relate plasticity, and other physical properties which reflect structure and stability of structure, to an appropriate laboratory measure, or index, of cementation. The soils were so chosen as to minimize the number of other properties that vary among them. The properties and indexing of the soils in the laboratory show broad consistence with their range of subplasticity as described in the field. When it has been released from cementation, the clay in the more subplastic soils is highly active, having properties similar to those of a montmorillonite with calcium and sodium on the exchange surface. The structure and the high total clay content of the subplastics require a high degree of orientation of individual clay particles into packets or domains. These packets are stabilized by cementation, largely at their edges, and at the same time attached to neighbouring non-parallel packets. Water readily reaches most of the pore space and the material, although very stable, continues to display some properties which are consistent only with a large content of clay particles. Consequently it seems likely that much of the planar surface of the individual clay particle is not contaminated with a cement.