Contribution of fines to the compressive strength of mixed soils

The concept of granular void ratio, in which the volume of fines is treated as voids, was introduced to characterise mixed soils that contain both coarse (sand-size and above) and fine (clay and silt-size) soil particles. In using this concept, fines are assumed to make no contribution to the mechanical properties of the mixed soil. In this paper, this assumption is assessed through the analyses of a number of published results as well as data from tests conducted specifically for this study. The assessment shows that, in most instances, the relation between un-drained shear strength and granular void ratio of the mixed soil is not the same as that of the host coarse material. At the same granular void ratio plastic fines generally cause a reduction in undrained strength and, at best, act like voids, whereas non-plastic fines increase the undrained strength and, at worst, act like voids. A recently introduced equivalent granular void ratio, which makes use of a modification factor b to account for the contribution of fines, was evaluated. The equivalent granular void ratio was found to be able to synthesise the various results. The factor b has a different range of values, depending on the hardness of the fines, and reflecting the different ways these fines contribute to the behaviour. For non-plastic and thus relatively harder fines, b depends on the ratio of the average size of the fines to the average pore size space of the host coarse material. For plastic fines, this factor depends on the pore space of the host coarse material and the stress history, such as the overconsolidation ratio.