The interface shear behaviour between the geomembranes and soils was studied. Sand/bentonite (80/20), crushed sand, river sand, crushed gravel, and river gravel were used in this study. Polyvinyl chlorides were cured in the 0.5 molar saltwater and high-density polyethylene was cured in municipal solid waste leachate for eight months. Direct shear experiments were performed using cured GMs. This study recommends the use of crushed gravel in projects that use polyvinyl chloride and high-density polyethylene. The interface friction angles, which were exposed to the effects of saltwater and municipal solid waste leachate, decreased even after eight months, and this reduction effect should be considered in future projects. When designing projects involving GMs exposed to MSW leachate, particularly in landfills, potential damage over time should be considered, and appropriate design parameters should be selected. Failure to do so can lead to disasters that cause the loss of life and property.