Interaction between cohesive-frictional soil and various grid reinforcements

Abstract

A total of 52 large-scale laboratory pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the pullout test, and the maximum pullout resistance was reached within a relatively small pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of resistance mobilization along the reinforcement varies, and the pullout-resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction resistance from the longitudinal member contributed only to about 10% of the total pullout resistance of the grids. The pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the pullout resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a pullout test because of the complicated pullout-resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear resistance can exceed the direct-shear resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.