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

A regular set of large scale direct shear tests performed to study gravel particles shape and size effects on shear strength characteristics of sands. The tests conducted using direct shear apparatus with 300 mm x 300 x 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 performed on dry samples in drained condition. Samples prepared in a relative density of 60 percent and tested using three surcharge pressures of 150, 300 and 450 kPa. According to the results, shape and size of oversized particles strongly affect direct shear strength characteristics of sand-gravel mixtures. Mixtures with angular and larger gravel grains show more shear strength and dilation compared to 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 suggested for estimation of mixtureʹs friction angle based on gravel type and content.