Diagnosis of the Transition from Rock to Soil in a Granodiorite

This paper describes an attempt to develop a method to define when granodiorite loses its properties as a rock and transforms into soil material. The research program approached the problem from a mechanistic point of view by comparing the pore water pressures generated during isotropic compression with the apparent—and micro—porosities, in terms of a newly proposed pore size distribution index as well as the compressive strengths of the specimens tested. A site was selected for study where weathering of the rock is not accompanied by the production of clay minerals, thus eliminating the complications introduced by the presence of highly compressive zones in the matrix. Samples tested using various criteria indicated that changes in the pore-size distribution can realistically reflect the transformation sought. It appears that further studies should be directed towards determination of the changes in the pore geometry, whereby a scientific definition of the transformation from rock to soil can be made.