Porosity influence on the shear strength of granular material–clay mixtures

Abstract

Materials forming part of rock fill dams, glacial tills, mudflows, debris flows, solifluction sheets, residual and colluvial soil deposits have a distinct structure, consisting of a mixture of large particles (gravel or hard clay fragments) and a soft matrix of clay. In order to analyze the stability of slopes made of granular material–clay mixtures, a measure of their shear strength is needed. Laboratory tests on these types of mixtures have indicated that their shear strength will depend upon the relative concentrations of the large particles and the clay. If the percentage by weight of the granular material in the granular material–clay mixture was >75%, the shear strength of the mixture was basically that of the granular material alone. When the concentration by weight of the granular material in the mixtures was <40%, the shear strength of the mixtures was basically that of the clay that surrounded the granular material. For the case in which the percentage of the granular materials in the solids mixture was between 40 and 75%, the shear strength of the mixtures was partially controlled by the granular phase. To date, no explanation has been put forward to account for why these limits of granular material in the mixtures exist at which it either controls, partially controls or has no control at all on the shear strength of the mixtures. This study presents an explanation for the existence of these limits. This explanation is based upon the resulting porosity of the mixtures.