Effects of Grain Size and Moisture Content on the Strength of Geogrid-Reinforced Sand in Direct Shear Mode

Abstract In this study, the effects of grain size and moisture content on the interface shear strength of sandy soils against a polypropylene biaxial geogrid were evaluated using a large-scale shearing device. Soil specimens with various maximum grain sizes (2.0–13.2 mm) and moisture contents (5%–11%) were sheared against the geogrid at normal stress values of 250–750 kPa. The interface shear strength was primarily influenced by the soil’s internal shear strength, the friction between the soil and the geosynthetic plane surface, and the resistance provided by the transverse ribs. The results indicated that the shear strength of the soil–geogrid interface decreased as the maximum grain size of the soil decreased. The lower interface shear strength was primarily a consequence of the reduced internal shear strength and friction. The internal shear strength contributed more than 50% of the shear strength of the soil–geogrid interface, whereas the contributions from the friction or transverse ribs were less than 30%. The transverse ribs provided effective resistance when the median of the particle size distribution was greater than the thickness of the transverse ribs. Increasing the soil moisture content reduced the interface shear strength, especially when the moisture content exceeded the optimal moisture content. Increasing the moisture content also negatively impacted the bearing resistance of the transverse ribs.