Abstract: The change of physical and mechanical properties of the geomaterials can be attributed to the change of microstructures of materials. Three soil samples with different grain shapes were studied by a grain shape scanning device. The influence of the grain shape on the shear strength of coarse sand samples are evaluated by the direct shear tests. The results indicate that, based on their sensitivity, the ranking of the four grain shape parameters is: length to width ratio>flatness ratio>wearing ratio>roundness ratio. When the shape of the coarse sand is more “regular”, the smaller of the internal friction angle for the particle rearrangement. As a result, the internal friction angle with the regularity of the grain particles will reduce. After reaching the peak shear strength, the sliding frictions among particles is smaller for the more “regular shaped” particles and then the smaller residual internal friction angle.

Key words: Coarse Sand, Direct Shear Tests, Grain Shapes, Shear Strength