Macro-void structure and permeability of Auckland residual clay

The properties of the residual clays derived from in situ weathering of the Waitemata group sandstones and siltstones, found around Auckland, are known to be surprisingly variable. In addition, it is known that the permeability of the material decreases rapidly as the effective consolidation pressure is increased. A possible contribution to both the variability and the rapid decrease in permeability might be the void structure of the soil. The advent of micro-CT (computed tomography) scanning equipment provides an opportunity to examine how the shapes and sizes of the voids change with increasing consolidation pressure. This note presents a discussion on some preliminary results on small specimens of Auckland residual clay. These indicate that at low effective confining pressures there are some quite large macrovoids. These are found to be greatly diminished in size when the effective confining pressure reaches about 200 kPa. From very low effective consolidation pressures up to about 200 kPa the permeability decreases markedly.