For an economic design of side slopes lined with geomembranes or protected with geotextiles, the frictional resistance at the soil–geosynthetic interface has to be known. Direct shear testing appears to be an appropriate method by which to determine these frictional resistances. As low normal stresses apply for most typical conditions along side slopes, two simple direct shear box devices were designed for tests at low normal stresses. Soil–geosynthetic interaction was tested for 4 soil types (2 clay and 2 sand types), 19 geotextiles, and 2 geomembranes. It was found that the friction between soils and geosynthetics is less than the angle of shearing resistance for the soil. The nonwoven geotextiles mobilized more than 90% of the sand friction, whereas woven geotextiles had efficiencies between 86 and 76%. For the clays, the efficiencies recorded were approximately 60% for woven geotextiles and 70% for nonwoven geotextiles. It was further observed that for granular materials the shear strength reached after repeated loading and unloading is independent of the degree of compaction. Based on the results obtained in this study and data reported in the literature, design recommendations are suggested for side slopes protected by geotextiles or lined by geomembranes.Key words: geosynthetics, frictional resistance, relative efficiency, soil–geosynthetic interface, low stress levels, direct shear test, side slopes.