There are limitations in experimental studies on sand-gravel mixtures due to the small size of testing
specimens. Due to this problem, many researchers have worked on prediction of the shear strength of mixture by testing
the sandy fraction of soil alone and developed empirical relationships. Most of the previous relationships have been
determined for low surcharge pressures in which particle breakage does not affect the shear strength parameters.
However, the particle breakage affects the relationships in higher confinements. At the present study, the results of
large scale direct shear tests on sand and sand-gravel mixtures was used to investigate the shear behavior and
dilatancy characteristics in a wider range of surcharge pressures. The gravel content, relative density, surcharge
pressure and gravel grain size were considered as variables in testing program. The relationships between shear
strength characteristics of sand and sand-gravel mixtures were determined considering dilation characteristics of the
soil. In this regard, the minimum void ratio was found as a useful indirect index that relates uniquely to the critical
state friction angle independent of soil gradation. The relations between critical state or peak friction angles of the
mixture with minimum void ratio were determined as a function of surcharge pressure. The correlations could be useful
for determination of the strength parameters of sand-gravel composites by testing sandy fraction of mixture.
