Rocking behavior of shallow foundations for bridge piers has emerged as an effective method to reduce the seismic load on the superstructure. The design philosophy that uses the rocking behavior of shallow foundation during an earthquake is called "rocking foundation". Previous studies on rocking foundation have focused on the verification of rocking effect to reduce the seismic load. Nevertheless, the rocking foundation is not used in practice due to the lack of understanding on the mechanism of rocking behavior. In this study, slow cyclic tests were performed using a geotechnical centrifuge with different slenderness ratios of structures to investigate the rocking mechanism of shallow foundation. From the variations of the pressure below the shallow foundation, "soil rounding" phenomenon was observed. In addition, the soil rounding effect, the key mechanism of rocking foundation, was described in two parts. Before yielding, a relation between overturning moment in the shallow foundation and the cyclic load direction was observed. The overturning moment converged to the ultimate moment capacity of shallow foundation after yielding. Therefore, the rocking mechanism of the shallow foundation was investigated using the soil rounding phenomena.
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