Experiments were conducted in a large-scale direct shear apparatus to study the mechanical behavior at the sand-geogrid (S-GG) and sand-geotextile (S-GT) interface. The results show that a certain relationship exists between peak/residual shear strength and shear dilatancy of interface. That is

peak shear stress will occur at a stage when relative shear compression finishes

while residual shear stress will occur when relative shear dilatancy finishes. The shear stress obvious reduction with increasing shear displacement is known to be caused by geosynthetics polishing

folding and rupture

which should not be neglected in interface behavior studying. Thus

a combined constitutive model has been presented for predicting sand-geogrid interface behavior. The model is composed of four relationships: (1) The peak/residual shear strength envelope. (2) A hyperbolic relationship model of shear behavior in pre-peak region. (3) A displacement-softening model for post-peak region. (4) Modeling for shear dilatancy of sand-geogrid interface. The predictions made by the proposed model are found to be in good agreement with the results of direct shear tests

so as to prove that the model is reasonable.