By quantifying the evolution of void ratio in granular soils of different angularity under different normal stress levels adjacent to geomembranes of varying roughness at different stages of shearing, the effect of geomembrane surface roughness on the interface shear mechanisms between granular materials and geomembranes was investigated. The extent of the shear zone thickness at the interfaces between combinations of round/angular particles and smooth/textured geomembrane surfaces were evaluated. For smooth geomembranes, the interface shear strength is developed by sliding and plowing of sand particles, while for textured geomembranes, the strength results from the interlocking and dilation of sand particles. Study on the effect of particle angularity showed that more angular sand particles resulted in larger plowing effects with smooth geomembranes which in turn yielded higher residual friction angles than with rounded to sub-rounded sand. Higher normal stress produced larger plowing effects for a given sand sheared against a smooth geomembrane and reduced the amount of dilation adjacent to textured geomembranes.