Influence of size disparity on small-strain shear modulus of sand-fines mixtures

Characterizing the small-strain shear modulus (G0) of sand with fines is of importance in geotechnical applications since natural sand is usually not clean but contains a certain amount of fines. This paper presents an experimental study to investigate G0 values of several sand-fines mixtures, formed by mixing clean quartz sands of different sizes with crushed silica fines of varying quantity. Focus of the study is on the possible interplay between the influence of particle size disparity and the influence of fines contents for which current understanding is not adequate. By defining the particle size disparity as D50/d50, where D50 is the mean size of base sand and d50 is the mean size of fines, a critical range of size disparity is found to be approximately between 4 and 7. When the size disparity is smaller than 4, the role of fines is manifested mainly by fines content; when the size disparity is beyond 7, the contribution of fines to the load transfer gradually becomes negligible because in this case fine grains tend to roll into the voids. A new concept, referred to as combined size disparity, is proposed to capture the influence of fines content and the influence of size disparity in a collective manner. By adopting this concept, an empirical relationship is proposed for estimating G0 values of sand-fines mixtures. The predictive performance of the relationship is then examined using literature data and a reasonably good agreement between prediction and measurement is obtained.