Effect of Gravel Grains on Direct Shear Strength Characteristics of Sandy Soils

A regular set of large scale direct shear tests were performed to study gravel particle effects on shear strength characteristics of sands. The tests were conducted using direct shear apparatus with 300 mm × 300 × 170 mm shear box. Uniform, fine sand was used as the base material and mixed with 20, 40 and 60 percent of angular and rounded gravel grains with maximum size of 12.5 and 25.4 mm as oversized particles. All tests were performed on dry samples in drained condition. Samples were prepared in a relative density of 60 percent and tested using three surcharge pressures of 150, 300 and 450 kPa. According to the results, mixtures with angular and larger gravel grains show more shear strength and dilation compared to the mixtures contain rounded and smaller oversized particles. Enhancement of gravel content increases the effects of oversized particle on shear behavior of mixture. In this regard, some correlations for estimation of mixture's friction angle and shear strength were suggested based on gravel type and gravel content.