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5345 results in Thermal-fluids engineering

Page 61 of 268

  • 2 - Basic Seabed Mechanisms

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 34-108

  • Summary

    Introduction

    Considerable efforts from researchers in the field of marine geotechnics have been devoted to the phenomenon of the wave-induced soil response since the 1970s. One reason for the growing interest is that many coastal/offshore structures (such as vertical walls, caissons, offshore monopiles and pipelines) have been damaged by the waveinduced seabed response, rather than from construction deficiencies (Christian et al. 1974; Smith & Gordon 1983; Lundgren et al. 1989).

    As mentioned in Chapter 1, two mechanisms of the wave-induced soil response have been reported in the literature. Both oscillatory (transient) and residual mechanisms of the wave-induced soil response have been intensively studied since the 1970s with two different approaches: the Yamamoto–Madsen model (Madsen 1978; Yamamoto et al. 1978) for the oscillatory mechanism and the Seed–Rahman model (Seed & Rahman 1978) for the residual mechanism. Following these approaches, numerous models have been proposed in the literature. In this chapter, the basic concepts and some key results of the previous models for wave-seabed interaction will be summarised.

    Oscillatory Mechanism

    Basically, previous studies of the wave-induced oscillatory soil response can be classified into three categories: the Yamamoto–Madsen model, the boundary-layer approximation and the dynamic model. We outline the contribution of several key publications available in the literature here.

  • (a) Yamamoto–Madsen model (YM model, or Biot's consolidation model): Yamamoto et al. (1978) proposed an analytical solution for an infinite seabed with hydraulic isotropy, while Madsen (1978) derived an analytical solution for a similar problem but with hydraulic anisotropy (i.e., permeabilities in all directions are different). Different seabed conditions, such as a seabed of finite thickness (Yamamoto 1977) and a layered seabed (Yamamoto 1981), were considered. Based on the plane stress conditions, Okusa (1985b) further reduced the sixth-order governing equation in the YM model to a fourth-order governing equation. Later, the YM model was further extended to more complicated wave conditions such as three-dimensional (3D) short-crested wave systems (Hsu et al. 1993, 1995; Hsu & Jeng 1994) or seabed conditions such as cross-anisotropic soil behaviour (Jeng 1997b) or a non-homogeneous seabed profile (Jeng & Seymour 1997a; Kitano & Mase 2001). Some numerical models have been developed with this framework, which were reviewed in Chapter 1.

  • (b) Boundary-layer approximation: Based on the mixture theory, Mei & Foda (1981) proposed the boundary-layer approximation for the wave-induced soil response.

  • 4 - Integrated Model of Wave-Seabed Interactions around Caisson-Type Breakwaters

  • Dong-Sheng Jeng, Griffith University, Queensland
  • Book:
    Mechanics of Wave-Seabed-Structure Interactions
    Published online:
    28 April 2018
    Print publication:
    26 April 2018, pp 129-176

  • Summary

    Introduction

    Breakwaters are commonly adopted to protect and enhance the utility of coastlines. For example, the total length of all breakwaters in Japan is 4,143 km – one-fifth of its coastline (Hsu, Uda & Silvester 2000). In countries such as the United Kingdom and Japan, coastline protection is a national priority. The construction of new breakwaters and the expansion of existing breakwaters involve major investment. Worldwide, the combined costs for building new breakwaters and maintaining the existing ones are on the order of tens of billions of pounds a year.

    Breakwaters are vulnerable to liquefaction of the seabed foundation, a process that can often lead to significant degradations of the foundation in as little as a few years after construction and can sometimes even result in total collapse (Zen et al. 1985; Lundgren et al. 1989; Franco 1994; Zhang & Ge 1996; Sumer & Fredsøe 2002; Chung, Kim, Kang, Im & Prasad 2006). Inappropriate design or maintenance of breakwaters can lead to catastrophic coastal disaster. A recent example of coastal tragedy caused by the failure of breakwaters is that of New Orleans during Hurricane Katrina, which caused deaths and personal and economic chaos (Travis 2005).

    The phenomenon of wave-seabed-structure interactions (WSSIs) has a major bearing on this issue and is central to the design of coastal structures such as breakwaters, pipelines and platforms. There have been numerous investigations of wave-seabed interactions around marine structures based on Biot's poro-elastic theory. Among these, Mase et al. (1994) developed a finite element method (FEM) numerical model to investigate wave-induced pore-water pressures and effective stresses under standing waves in a sand seabed and in the rubble-mound foundation of a composite caisson-type breakwater based on Biot's consolidation equations. Later, Mizutani & Mostafa (1998) and Mostafa et al. (1999) developed a boundary element method–FEM combination numerical model to investigate the wave-seabed-structure interaction. In their models, Poisson's equation is used to govern the irrotational wave field for an incompressible, inviscid fluid, and Biot's poro-elastic consolidation equations are used to govern the porous seabed and structures. Jeng, Cha, Lin & Hu (2001) developed a two-dimensional (2D) generalised FEM numerical model (GFEM-WSSI) to investigate the wave-induced pore pressure under a linear wave around a composite breakwater located at a finite, isotropic and homogeneous seabed.

Page 61 of 268