The torsional ring shear test is widely employed to assess soil-pile interface friction angle, which is influenced by factors such as soil gradation, particle morphology, surface roughness, and stress state. A comprehensive under-standing of the physical mechanisms governing interface shear behavior of granular soils, such as sand, is essential for offshore soil-structure applications. This study investigates the effects of normal stress, surface roughness, and particle shape on the residual interface friction angle using natural sand particles and synthesized glass beads as an idealized granular material. A wide range of surface roughness values (spanning two orders of magnitude) and normal stresses (10–50 kPa) were selected to identify transitional effects and underlying mechanisms. Results revealed a critical normalized surface roughness where the residual friction angle approaches the internal friction angle. Glass beads, characterized by higher sphericity and smoother surfaces, exhibited lower interface friction angles than sub-angular sand particles. As surface roughness increased, a transition from sliding to rolling motion in the granular particles likely occurred. An empirical relationship was proposed to predict residual interface friction angles, accounting for particle size, normal stress, and interface roughness.