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 D-50 of a soil, AR is the ratio of the half spacing between two adjacent asperities and D-50. For interface friction angles (delta), the test results indicate that R-n (> 0.06) had a positive effect on increase delta and resulting in delta > phi (internal friction angle of a soil). There is no clear tendency of the effect of AR on delta. 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 delta/phi 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.