Keywords: discontinuous deformation analysis, seawall, excess pore pressure, wave load.

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 if 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. The first mode has been widely studied by coastal engineers [1,2], while the second mode has been considered by geotechnical engineers [3]. This study will focus on the mechanism of wave-induced settlement of a seawall through a discontinuous deformation analysis.

In general, a seawall is under a combination of actions induced by waves, currents, differences in water level, seepage flow-induced uplift force and other specific loading. The transfer of these external actions (such as wave loading) through the seawall to the subsoil involves changes in soil response (pore water pressure and effective stress) in the soil layers. Particularly in soft soil, the stress changes will gradually develop during a long period of time. Due to these changes in soil response, the underlying and adjacent soil layers will deform vertically and horizontally while the shear strength of the soil may be reduced. As a consequence, seawalls built on top of the soil layers may deform or even lose its stability. The changes in soil stresses and the associated deformations are not only depending in the loading, but on the geometry (such as slope steepness), the structure weight, permeability, stiffness and shear strength of the soil layers. For this reason, the design of seawall has to be based on an integral approach of the interaction between the structure, wave and soil layers.

It has been well known that dynamic wave pressure at the seabed surface generates pore pressure, which leads to recompression of seabed. Wave-induced instability and settlement of coastal structures has been recognized as an important factor in construction of coastal structures [4,5,6]. In general, the design of a seawall is mainly determined by the hydraulic boundary conditions, the availability of materials and construction methods. The characteristics of soil should also be taken into consideration, because the wave-induced pore-water pressure will affect the functions of the seawall.

In this paper, a discontinuous deformation analysis for the wave-induced soil response in the vicinity of seawall is proposed. The conventional discontinuous deformation analysis will be adopted by including the concept of effective stresses, leading to Discontinous Deformation Analysis-Effective Stress Model (DDA-ESM). With the new model, the wave-induced settlement of a seawall will be examined.

1

H. Oumeraci, "Review and Analysis of Vertical Breakwater Failure � Lessons Learned", Coastal Engineering, 23, 3-29, 1994. doi:10.1016/0378-3839(94)90046-9

2

W.P. Krystian, "Design of seawalls and dikes-including overview of revetments", Proceedings of the Short Course on Coastal Protection, Delft University of Technology, 197-288, 1990.

3

F.B.J. Barends, "Interaction between Ocean Waves and Sea-Bed", Proceedings of the International Conference on Geotechnical Engineering for Coastal Development � Theory and Practice on Soft Ground (GEO-Coast'91), 2, 1091-1108, 1991

4

K. Zen, Y. Umehara, "Analysis of Wave Induced Pore Water Pressure in Sand Layers under Breakwater", Proceedings of Ocean Space Utilization 85, Tokyo, Springer-Varlag, 467-474, 1985.

5

K. Yasuhara, K.H. Anderson, "Post-Cyclic Recompression Settlement in Clay", Soils and Foundation, 31(1), 83-94 (1991)

6

K. Zen, Y. Umehara, W.D.L. Finn, "A Case Study of the Wave-Induced Liquefaction of Sand Layers under Damaged Breakwater", Proceedings 3rd Canadian Conference on Marine Geotechnical Engineering, 505-520,1985.

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