AbstractRecently, isolated foundations with cushions have gained more attention in bridge design, especially for cross-sea bridges in deep water 50 meters or more in depth. This foundation has become a good choice for bridges built in seismic active zones. Along with interactions within the cushion layer, interactions between the caisson foundation and composite cushion are crucial for isolated foundations. Unfortunately, the shear characteristics of the cushion–structure interface under monotonic and cyclic loading have not been well evaluated. In this study, a series of monotonic and cyclic large-scale shear tests were conducted to investigate the behavior of the cushion–structure interface using crushed aggregates and pebble gravel as the cushion, respectively. The tests showed that the particle morphology properties have significant correlations with the cushion–structure interface’s shear behavior. Some shear characteristics of the larger grain size’s cushion–structure interface, such as cushion clogging and the effect of particle breakage were analyzed based on the developed tests. The shear stress, isolation effect, shear modulus, and damping ratio of the composite cushion–structure interface were different from that of the single-layer cushion–structure interface, which was primarily due to the pebble gravel. The relative thickness of the pebble gravel layer also influenced the shear characteristics of the composite cushion–structure interface. As the relative thickness of the pebble gravel layer decreased, the isolation effect, shear stress, shear modulus, and damping ratio of the composite cushion–structure interface increase.