The Operational Strength of Fissured Clays

Synopsis In clays that are generally, but not necessarily, stiff fissures or joints exist with varying intensity. As the shear strengths along these discontinuities are much less than those of the intact material, they constitute planes of weakness, with the result that the strength measured depends on the size of the sample used. Previous theories on the strength–size relation are reviewed and their limitations discussed. An equation is proposed to describe this behaviour and a method of determining the parameters involved in the equation is developed. The results of field and laboratory tests on block samples of a stiff fissured clay, and data reported in the literature, were analysed. It was shown that the strength–size relation may be represented adequately by the proposed equation and that the strength of the clay mass operative in the field may be predicted. By taking the effect of sample size, anisotropy and time to failure into account, the conventional φ= 0 analysis, hitherto considered unreliabl...