The reinforcement-soil interface in the reinforced soil structure not only suffers from the horizontal cyclic shear

but also the compression of the cyclic normal load. In this study

a large-scale direct shear apparatus was used to analyze the cyclic shear characteristics of the aggregate-geogrid interface under bidirectional cyclic loading. The influences of various cyclic normal load frequencies

horizontal cyclic shear frequencies and waveform of normal loading on the interface shear strength were investigated. The results showed that when the cyclic normal load frequency and the horizontal cyclic shear frequency were synchronized

the peak shear stress and the maximum vertical displacement increased with increasing the frequency

and the time shift between shear stress and the displacement curve at the front and back half cycle was the same. On the contrary

the relationship between shear stress-displacement and the vertical displacement showed two different morphological characteristics under bidirectional cyclic loading. The largest values of shear stress and vertical displacement were observed under the sinusoidal waveform of cyclic normal load. But the hysteresis loop and vertical displacement curve under harmonics and square waveform were different from those under the triangular and sinusoidal waveform. The shear stiffness increased with increasing the frequency of the bidirectional cyclic load. The interface shear stiffness is the largest under the sine wave and the smallest under the ramp wave.