Very limited attention was paid on the micro–response of sand as it interacts with geomembrane and the effect of surface hardness on the interaction at the microscopic level. In this study, a coupled finite difference–discrete model was adopted with which to analyze the shear behavior of sand–smooth geomembrane interface. The model was validated using published experimental data. The numerical results show that the effects of normal stress and surface hardness on the interface strength depend on the shear mechanism (sliding or plowing) at the interface. There exists a critical normal stress at which the mechanism transforms from predominant sliding to predominant plowing. There is a high level of coupling effect between normal stress and surface hardness on the interface strength. Micro–topographical analysis of geomembrane provides clear insights into the shear mechanism at the interface, supporting the results obtained from interface shear tests. No shear band is formed during shearing for a sand–smooth geomembrane interface. The shear resistance of sand–smooth geomembrane interface relies on interface indentations and characteristics rather than on the formation of shear band.