Interfaces

which are commonly introduced into the rock filling dams constructed with different materials bearing distinct mechanical properties

according to the results of previous study

can exert evident influences on the performance of the whole dam.Using the NGI-type simple shear apparatus

cyclic simple shear tests on the interface between granular material and grave-clays were conducted to study the mechanical behaviors.Observations and conclusions are summarized based on the test results as follows: （1） The direction of initial shear stress significantly influences the mode of relative displacement of interface

i.e.relative displacement in the direction of initial shear stress is always greater than that in the opposite direction.（2） The volumetric deformation is regular during any semi-cycle in such a manner that at the loading stage of first 1/4 cycle

the specimen shrinks dominantly at low shear stress but dilates at high shear stress level

whereas only shrinkage deformation takes place at the next 1/4 unloading stage;after each cycle

the negative volumetric deformation dominates.（3） Shear strength of the interface increases as the shear cycle increases and is approximately linear proportional to the normal stress level on the interface.