Keywords: liquefaction, shear failure, scour, wave loading, seabed response.

Protection of the coastal environment is vital for countries like Australia, where 86% of the total population is concentrated around the coast. Marine structures such as caissons and seawalls are commonly adopted for such protection. Although the protection of marine structures has been extensively studied in recent years, understanding of their interaction with waves and the seabed is far from complete. Damage of marine structures still occurs from time to time, with two general failure modes evident. The first mode is that of structural failure, caused by wave forces acting on and damaging the structure itself. The second mode is that of foundation failure, caused by liquefaction or erosion of the seabed in the vicinity of the structure, resulting in collapse of the structure as a whole. This study will focus on the mechanism of wave-induced seabed instability in the vicinity of a breakwater.

The wave phenomenon in the vicinity of a breakwater or seawall is complicated, which normally involves with wave interactions and wave-structure interactions. The simples case is a fully reflection in the direction normal to the wall (i.e, two-dimensional standing waves), which has been widely studied in the past [1]. In fact, the wave interactions in the vicinity of a breakwater is a three-dimensional short-crested waves, which could be an interaction of two or more wave trains. This three-dimensional wave system has been studied by numerous researchers [2].

In general, the wave-induced seabed instability in the vicinity of a breakwater can be classified into three categories: scour, liquefaction and shear failure. Among these, scouring has been studied by coastal engineers, while liquefaction and shear failure have been studied by geotechnical engineers in the past. The phenomenon of scouring around a coastal structure has been extensively studied in the past, and numerous empirical formulas have been proposed in the past . Basically, the scouring depth is related to the mass transportation and velocity distribution, and the onset of scouring and scouring depth are two main concerns. Liquefaction occurs when the wave-induced excess pore pressure is large enough to overcome the self-weight of the soil particles, which is generally occurs in finer seabeds [3]. Shear failure is caused by the wave-induced shear forces acting on the soil particles, which can be determined by Mohr -Columbia criteria [4] Basically, liquefaction is a kind of vertical movement of the soil particles, while shear failure is a horizontal movement. In this study, we will focus on the wave-induced liquefaction and shear failure in the vicinity of a breakwater.

Since the estimation of the wave-induced liquefaction and shear failure is based on the wave-induced pore pressure and effective stresses within the seabed, the wave-induced soil response will be an important parameter. Most previous investigations for wave-seabed interaction have been based on quasi-static approach, i.e., the conventional consolidation equation [3], in which the acceleration due to soil and pore fluid motions are excluded. Some research has considered a more advanced approach by considering the acceleration of soil motion such as u-p approximation. a full dynamic soil behavior has also been considered recently. Coulomb-damping friction, which is particularly important for sandy and clay seabeds, has also been considered in some models [5]. A comparison between various existing models for the wave-seabed-interaction has been performed in the authors' recent papers [6]. However, it is only limited to two-dimensional case and wave-induced soil response, not three-dimensional cases and wave-induced seabed instability.

In this paper, an analytical solution for wave-induced seabed response in a Columb-damping poroelastic seabed in the vicinity of a breakwater is derived. Based on the analytical solution, we examine (1) the wave-induced soil response at different location; (2) the maximum liquefaction and shear failure depth in coarse and fine sand; (3) the effects of reflection coefficients; and (4) the added stresses due to the self-weight of the breakwater.

1

I. Irie and K. Ndaoka, "Laboratory reproduction of seabed scour in front of a breakwaters", Proceedings of 19th International Conference on Coastal Engineering, A.S.C.E., 2, 1715-1731, 1984.

2

J.R.C. Hsu, Y. Tsuchiya, R. Silvester, "Third-order approximation to short-crested waves", Journal of Fluid Mechanics, 90, 179-196, 1979. doi:10.1017/S0022112079002135

3

D.-S. Jeng, Wave-Induced Seabed Response in front of a Breakwater. PhD Thesis, The University of Western Australia, 297pp, 1997.

4

D.V. Griffith, "Some theoretical observation on conical failure criteria in principal stress space", International Journal of Solids and Structures, 22, 553-565, 1986. doi:10.1016/0020-7683(86)90044-2

5

T. Yamamoto, "On the response of a Coulomb-damped poroelastic bed to water waves", Marine Geotechnology, 5(2), 93-127, 1982. doi:10.1080/10641198309379839

6

M. Lin and D.-S. Jeng, "Comparison of existing poroelastic models for wave damping in a porous seabed", Ocean Engineering, 30(11), 1335-1352, 2003. doi:10.1016/S0029-8018(02)00137-3

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