In order to explain the mechanism of breakwater failure destroyed by the massive tsunami triggered by the 2011 off the Pacific coast of Tohoku Earthquake, a failure mode is presented from the view of geotechnical engineering by considering the seepage force rather than the failure modes by using the existing failure mechanisms released in Coastal Engineering. It is proposed that a pseudo-steady state flow condition inside of the rubble mound can be created by high hydraulic head difference, which becomes possible due to the behaviour of the long period of tsunami wave. At first, a piping initiated local failure model is proposed and the phenomenon of piping initiated failure by laboratory model experiments is re-enacted. Also, the piping-initiated local failure of a breakwater is re-enacted by DDA simulation. And then, DDA is applied to the stability analysis of breakwater under seepage flow with new proposed displacement based failure criteria which can consider both global and local failure. A relationship between the height of a tsunami with the safety factor of a breakwater has been obtained. The results have been validated by both the laboratory model experiments and comparison with simplified Bishop Method.

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