Void structure of Auckland residual soil using X-ray CT scanning

In this paper, the properties of undisturbed Auckland residual soil are related to visualisation of the void structure obtained by X-ray CT scanning. The soil investigated was derived from in situ weathering of the Waitemata group sandstones and siltstones, a process which produces soil with surprisingly variable properties. We performed consolidation tests in a triaxial cell to understand better the origin of this variability. Specimens were consolidated in several stages and after each stage the specimen was allowed to swell in stages back to zero consolidation pressure. It was then taken to another piece of apparatus and X-ray CT scanning performed. Processing of the CT data revealed that the void structure of the residual soil was inhomogeneous. From the images obtained, it was observed that the large voids (macro-voids) present at low consolidation pressures were gradually closed with increasing effective confining pressure. The decreasing size of the large voids is suggested as the cause of the decreasing values for the permeability coefficient with increasing consolidation pressure.