Effect of Surface Roughness on Soil-Geogrid/Geotextile Interface Shear Strengths

To get some insights on the effect of surface roughness on the interface shear strength, interface direct shear tests were conducted using two sandy soils (M1 and M2) and 4 geogrids (GG) and 5 geotextiles (GT) with difference surface roughness. The normalized surface roughness (R_n) and the asperity ratio (AR) are used as indexes for surface roughness. R_n is the ratio between average asperity height and D₅₀ of a soil, AR is the ratio of the half spacing between two adjacent asperities and D₅₀. For interface friction angles (δ), the test results indicate that R_n (> 0.06) had a positive effect on increase δ and resulting in δ > ϕ (internal friction angle of a soil). There is no clear tendency of the effect of AR on δ. For (apparent) interface adhesion (c_a), there are different tendencies for M1-GG and M2-GT interfaces. For M2-GT interfaces, there is a tendency of increase c_a with increase of R_n value. While, for M1-GG interfaces, for the conditions tested, there is a tendency of increase c_a with decrease of AR value. Further, in terms of δ/ϕ and c_a/c (cohesion of a soil) ratios, generally, the average values for M2-GT interfaces are higher than that of M1-GG interfaces. The reason considered is the interaction between soil particles and the surface fibers of the GTs, i.e. the soil particles plowed on the surface of GTs.