عنوان مقاله [English]
There are always limitations for appropriate borrow materials in common engineering activities which results in wide usage of mixed soils with larger particles up to boulders. Using these types of materials need correct knowledge of their behavior. For example, shear strength characteristics of mixed soils are usually required for stability analysis of slopes, landslides and earth dams. However, there are limitations in experimental studies on soil mixtures due to the small size of testing specimens. In the present study, a regular set of large scale direct shear tests performed to investigate gravel particles shape and size effects on shear strength characteristics of fine sands. The tests conducted using direct shear apparatus with 300 mm × 300 mm shear box. Poor graded fine sand was used as the base material and mixed with zero, 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 three different relative densities of 35, 60 and 85 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, surcharge pressure or relative density increases the effects of oversized particle on shear behavior of mixture.