Effects of Interface Roughness, Particle Geometry, and Gradation on the Sand–Steel Interface Friction Angle

Determination of interface shear strength is crucial in the design of many geotechnical structures. To study the effect of interface roughness, particle geometry (size and shape), and sand gradation on the interface friction angle, direct interface shear tests were performed for 10 sands with varying particle sizes, shapes, and gradations and four steel surfaces with different levels of rusting. When sheared along the same interface, the interface friction angle was greater for sands with smaller particle sizes and more angular or elongated particle shapes. For a given sand, the interface friction angle increases with increasing surface roughness. For sands with uniform particle size, a unique relationship was found between the normalized surface roughness and the ratio of the sand–steel critical-state interface friction angle to the internal critical-state friction angle of the sand. Given the same surface roughness and mean particle size, smaller critical-state interface friction angles were mobilized for graded sands than for sands with uniform particle sizes.