Page 80 - Numerical Analysis and Modelling in Geomechanics

Chapter 3
A general finite element model for wave-

seabed-structure interaction
D.S.Jeng

Introduction
The mechanism of wave-seabed-structure interaction is an important concern for
designing offshore facilities, such as pipelines, anchors, marine platforms and
breakwaters. In the field, concrete armour blocks at the toes of many marine
structures have been found to subside into the seabed; wave-induced liquefaction
has been identified as the reason for this problem (Silvester and Hsu, 1989).
Moreover, it has been reported that some such structures have possibly failed due
to seabed instability (Lundgren et al., 1989; Silvester and Hsu, 1989).
Waves propagating in shallow water create a significant dynamic pressure on
the sea floor. This pressure field induces a stress field and associated pore water
pressure fluctuations within the seabed. With excess pore-pressure and
diminishing vertical effective stresses, part of the seabed may become unstable
or even liquefied. This causes a quicksand effect in which external loads cannot
be supported. Furthermore, when liquefaction occurs, the soil particles are likely
to be carried away as a fluid by any prevailing bottom current or by mass transport
due to the wave action.
Two mechanisms of wave-induced soil response have been observed in
laboratory experiments and field measurements (Zen and Yamazaki, 1990; Nago
et al., 1993). The first is caused by the progressive development of excess pore
pressure that occurs after a certain number of wave cycles. This type of soil
response is similar to that induced by an earthquake. The second is generated by
the oscillatory nature of the excess pore pressure, which appears periodically
many times during a storm sequence. In most cases, the wave-induced soil
response is oscillatory in nature except for some special cases of non-cohesive
sediments with loose to medium density (Seed and Rahman, 1978). In this study,
we will focus on the wave-induced oscillatory soil response.
Many variables influence the wave-induced soil response in a porous seabed;
anisotropy in soil behaviour is an important issue that must be considered in the
wave-seabed interaction problem. In the field, most marine sediments display a
certain degree of anisotropy, with different elastic properties in different
directions, owing to the manner of their deposition, particle shape and stress