A numerical simulation on seepage-induced collapse of caisson breakwater using a stabilized ISPH method *Tetsuro Goda 1) and Mitsuteru Asai 2) 1), 2) Department of Civil and Structural Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka city, Fukuoka 8190395, Japan 2) asai@doc.kyushu-u.ac.jp ABSTRACT The clarification on the collapse mechanism of a caisson breakwater has been an urgent task in order to reduce damages caused by next millennium tsunamis. In this study, piping destruction of a mound induced by seepage flow is taken into consideration solely. A stabilized ISPH method proposed by Asai (2012) is adopted with some modification associated with the extended Darcy law (Akbari 2013, 2014) to simulate both seepage flow and surface-flow phenomenon. The numerical results in piezo water head show quantitative agreement with hydraulic experimental results obtained by Kasama (2013). Furthermore, initiation of the mound collapse behavior can be predicted by the critical hydraulic gradient calculated from our numerical solutions. 1. INTRODUCTION The huge tsunami induced by the Tohoku-Kanto earthquake caused great damages to the port structures including breakwaters. In order to reduce the expected damages induced by next millennium tsunamis, the clarification on the collapse mechanism of breakwaters has been an urgent task. A variety of research on a caisson-typed breakwater has been done, and the following three causes have been found: (i) horizontal force due to the water-level difference between the front and back of a caisson, (ii) scour induced by tsunami overtopping in the rear of a caisson, (iii) seepage-induced piping caused by the bearing capacity degradation of a mound. The interaction of these three causes has not been clarified yet because of the complexity associated with the failure of breakwaters. The development of analysis methods for each cause will lead to a robust analysis scheme which is capable of handling these different phenomena simultaneously. In this paper, we only focus on the cause (iii), seepage-induced piping caused by the bearing capacity degradation of a mound, as a fundamental study. 1) Graduate Student 2) Associate Professor