Abstract: To investigate the influence of soil particle-size gradation on geogrid-sand interface, a series of large-scale direct shear tests, cyclic direct shear tests and post-cyclic direct shear tests is performed on geogrid and sand with three particle-size gradations. The interface shear stress, shear displacement and vertical displacement are analyzed in direct shear tests and cyclic direct shear tests. The influence of cyclic shear history on interface behavior is studied through the comparison and analysis of the results from direct shear tests and post-cyclic direct shear tests. Results show that, in direct shear tests, the shear softening phenomena occur on geogrid-sand interfaces under different soil particle-size gradation, which is more significant under higher vertical normal stresses. In cyclic direct shear tests, the shear hardening phenomena appear on geogrid-sand interfaces under different soil particle-size gradation, and the interface of geogrid-good graded sand has the highest shear strength under the same cyclic number. The apparent interfacial peak adhesion and apparent residual adhesion decrease with the cyclic shear tests, while friction angle increases with the cyclic shear tests. The post-cyclic shear strengths of three types of geogrid-sand interfaces in direct shear tests are higher than those of monotonic direct shear tests.

Key words: geogrid-sand interface, soil particle-size gradation, monotonic direct shear, cyclic direct shear