Traditional investigation of soil-structure interfaces has been limited to global measurements of force and displacement. As a result, interpretation of soil behavior at the particle scale has focused primarily on global response and the assessment of local soil behavior in terms of strain has not been possible. This paper presents a series of monotonic and cyclic tests carried out using a modified direct interface shear device that enables digital imaging of soil behavior within the test specimen throughout shear. Particle Image Velocimetry (PIV) is implemented to analyze the image sequence and quantify the development of volumetric and shear strains within the interface shear zone. Results indicate that initial void ratio and confinement conditions (normal stress and normal stiffness) influence the development of interface shear zone characteristics. Although PIV analysis compared well with global displacement measurements, complex interfacial soil behavior cannot be fully characterized solely with global measurements. The implications of the lack of local response characterization inherent in global measurements are illustrated in a critical state soil mechanics framework modified for soil-structure interfaces.