DEM-FEM model of the outflow of backfilling sand from behind a seawall under wave motion Lechosław G. Bierawski 1 , Shiro Maeno 2 , Hitoshi Gotoh 3 and Eiji Harada 4 The DEM and FEM techniques are combined to numerically model the collapse of a structure consisting of a fixed vertical revetment and backfilling sand under wave impacts in a vertical domain. The course of events inside the granular material in the vicinity of the wall during its washing out is the object of this research. The study is based on the methods of computational mechanics of sediment transport; hence the sediment and pore water oscillation interaction and the inter-particle collisions are considered. Small-scale laboratory tests were performed to provide verification of the numerical model. Introduction Many cases of coastal structures destruction by waves have been reported. Unfortunately, the discussion on the explanation of the ruination, by H. Ouemeraci (1994), indicates that the present design approaches cannot explain most of the failure modes. Trying to contribute to development of new construction methods we decided to investigate the outflow of backfill from behind a seawall caused by pore water oscillations. The investigated structure could be a seawall, a bulkhead or a revetment, which is narrow and vertical. Its main purpose is to retain or prevent sliding of the land, with the secondary purpose of affording protection to the upland property from damage by wave action (Shore Protection Manual, 1977). Unfortunately, it turns out that even a local scour in the seabed in front of such a structure rapidly increases the risk that the outflow would occur (see Photo 1). This paper treats the mechanism of the collapse of backfilling sand under these conditions. The work is based on studies of many researchers, e.g. Yamamoto (1981), Nago (1982), Zen (1984), and it is continuation of preceding research made by Nago and Maeno (1987 and 1993), Nago et al., (1995) Maeno et al. (1999 and 2002). The dynamic behavior of the sandy bed in the vicinity of a seawall and the possibility for the outflow of the back filling sand was clarified there. The present study is focused on the motion of particular grains around the seawall during the collapsing process, which is still not clarified. To investigate the phenomenon a numerical model has been developed and small-scale experiments conducted. In the proposed numerical model, the FEM (Finite Elements Method) and the DEM (Distinct Elements Method) are coupled. Obtained sequential pore water pressure distributions and related moves of grains provide fine data for discussion of the phenomenon. Additionally, the outflow preventing effect of higher seawall foundations is considered by using the model. Outline of the experiment The experimental apparatus, designed for the small-scale laboratory tests is presented in Fig.1. Its dimensions are: 80 cm in length, 70 cm in height, and 29.5 cm in width. A board of 1 cm thick was fixed as the seawall. The backfill as well as the sandy bed was made of the 1 Doctoral Course Student, Graduated School of Natural Science and Technology, Okayama University, Okayama, Japan, lbier@cc.okayama-u.ac.jp 2 Dr. of Eng., Assoc. Professor, Dept. of Environmental and Civil Eng., Okayama University, 700-8530 Tsushima-Naka, Okayama, Japan, ph. 0862518151, fax 0862518257, maeno@cc.okayama-u.ac.jp 3 Dr. of Eng., Assoc. Professor, Department of Civil Engineering, Kyoto University 4 Dr. of Eng., JSPS Post-Doctoral Research Fellow, Department of Civil Engineering, Kyoto University