Effect of Soil Moisture Content and Dry Density on Cohesive Soil–Geosynthetic Interactions Using Large Direct Shear Tests

Geosynthetics have increasingly been used to reinforce many earth structures and are now a well-accepted means to improve engineering properties of various types of soil. However, most previous studies and applications of geosynthetic stabilization are confined to noncohesive soils. Few research efforts have been dedicated to the feasibility and benefits of geosynthetic reinforcement on cohesive soils. This paper presents the results of an extensive laboratory testing program using a large direct shear device, in which four different soils (including one sand and three clays of different plasticities) were reinforced by three different geogrids and one woven geotextile. Reinforcement mechanisms were analyzed and the soil–geosynthetic interface parameters were obtained from the testing results. The increase in molding moisture content and/or decrease in dry density caused an appreciable reduction in interface shear resistance, which suggests that it should be more rational to use interface parameters of soils at their 95% maximum dry density and moisture content 2% above their optimum values. This study also provides a basis for future research and modeling the behavior between cohesive soils and geosynthetics.