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In this study, a new tsunami overflow-seepage-coupled centrifuge experimental system was developed and applied to investigate the concurrent processes, and to elucidate the mechanism, of the instability involving the scour of the mound/sandy seabed, bearing capacity failure and flow of the foundation, and the failure of caisson breakwaters, with high-resolution image analysis. A series of experiments were conducted with and without the effects of the seepage by keeping the same overflow conditions (water volume, overflow velocity, fall height and water depth), and with different mound cross-sections. All of the experiments were conducted under 50 gravities and comprised of 3 series. The first series of experiments targeted the instability of the mounds themselves by constraining the caisson movement (sliding and overturning). The second series of experiments then allowed the caisson movement and clarified how the mound scour would affect the overall stability of the caissons. The third series of experiments examined the effect of a countermeasure on the basis of the results from the two series of experiments. The experimental results first demonstrated that the coupled overflow-seepage actions promoted the development of the mound scour significantly. Notably, the scour front developed in the form of progressive slip failure at the vicinity of caissons, regardless of the mound thickness. This stems from the fact that the development of the mound scour shortened the seepage path around the shoulder area of the mound, enhancing the coupling effect of the overflow and seepage. Indeed, the scour stopped far from the caisson toe in the absence of seepage without affecting the stability of the caissons. By contrast, the scour development due to the coupled overflow-seepage approached the caisson toe and brought about bearing capacity failure of the mound, resulting in the total failure of the caisson breakwater, which otherwise remained stable without the coupling effect. The velocity vectors obtained from the high-resolution image analysis illustrated the series of such concurrent scour/bearing-capacity-failure/flow processes leading to the instability of the breakwater. The influence of placing an embankment as a countermeasure was also examined by employing different bank thickness. It is shown that the stability of the breakwaters was significantly improved with decreasing hydraulic gradient that manifested underneath the caissons due to the embankment effect. These findings will facilitate better assessment and improvement of the stability of caisson breakwaters with rubble mound foundations. [ABSTRACT FROM AUTHOR]