<p>The glass fiber-reinforced polymer (GFRP) composite is widely recognized as an optimal and extensively employed alternative for structural elements. However, a comprehensive understanding of the interface shearing mechanism between GFRP and sand, considering the variability in the roughness of sand-epoxied GFRP and the deformation mechanism of soil at the sand–GFRP interface, remains elusive. To enhance the understanding of the shear mechanism at the sand–GFRP interface, this study investigated the influence of GFRP surface type and roughness on both shear and volumetric behavior at the sand–GFRP interface and examined variations in surface roughness. Three types of GFRP were utilized: smooth surface finish, sand-epoxied, and rough surface finish GFRP. The effectiveness of GFRP was evaluated along with the impact of particle size, initial relative density, and proportion of sand. The results demonstrated that increased dosages of epoxied sand effectively roughen the GFRP surface, enhancing interlocking at the interface and thereby increasing shear strength. In contrast, GFRP specimens featuring lower coating ratios of epoxied silica sand exhibited greater slippage and reduced particle rotation. Based on relative roughness and interface friction angle criteria, interface roughness was categorized into distinct classifications. Simplified design charts were proposed to provide a framework for performance-based interface design by estimating interface friction angles and informing the optimization of sand–epoxy coatings for targeted interface behavior.</p>