Abstract: Direct shear tests were conducted on soil-grid interfaces composed of 4 types of geogrids and 2 types of soils to investigate the characteristics of geogrid-soil interface. The influencing factors including the initial state of soils (water content and dry density) and shearing rate on the shearing strength parameters and shearing stiffness were examined. The results indicated that: (1) With regard to geogrid-sandy gravel interfaces, higher impaction degree or shearing rate resulted in larger shear strength. (2) Both the shear strength parameters and shear stiffness of geogrid-sandy gravel interface were significantly higher than those of geogrid-clay interface. (3) Both the cohesion and the internal friction angle of geogrid-clay interface decrease remarkably with the increase of water content. The strength of geogrid-clay interface is more sensitive to water content than clay itself. (4) The recommended value of friction factor ratio (0.8) in current codes is suitable. But according to literatures it was found that the shear strength of the interface obtained from pull-out test was much smaller than that from direct shear test. (5) All the factors including the increasing of vertical stress and degree of compaction, the decreasing of water content and shearing rate would result in the apparent increase of shear stiffness. (6) The shear stiffness of the interface between uniaxial geogrid with short longitudinal ribs and sandy gravel was the largest, and the other three geogrids showed similar interface shear stiffness.

Key words: geogrid, filling, interface shear strength, shear stiffness