Composite systems comprised of particulate and continuum materials in contact at an interface are frequently encountered in geotechnical engineering. An understanding of the shear behavior occurring at this interface is of considerable importance as it governs the behavior of the entire composite system. Previous research in this area has focused on the behavior involving uniformly sized particulate materials. This is despite the fact that particulate mixtures comprised of differing particle sizes and shapes are most frequently encountered in practice. There is a need for a framework whereby the macroscale response of soil mixtures in contact with an interface can be quantitatively predicted from knowledge of the particulate and interface properties. Such a framework would prove useful in selecting the optimal materials for the design of engineered soils and interfaces. This research explores the case of binary mixtures of quartz sands in contact with smooth geomembrane surfaces. Index properties of the binary sand mixtures have been determined for different particle size ratio combinations. The particle size ratio of the binary mixtures ranges from 2.1 to 6.1 and the size of the particles ranges from 0.13mm to 0.78mm. A series of interface shear tests were conducted to determine the macroscale interface shear behavior and resulting geomembrane wear. The effects of mean particle size and particle size ratio were investigated. Changes to the surface topography of the counterface material are quantified using stylus profilometry and related to the characteristics of the sand mixtures.