Page 116 - Numerical Analysis and Modelling in Geomechanics

WAVE-SEABED-STRUCTURE INTERACTION 97
seabed. This phenomenon is particularly obvious at the sections between seabed
and structure beneath the caisson (i.e. section 4). Its influence will increase as the
degree of saturation increases. Similar trends can be found in fine sand
(Figure 3.23).
Figure 3.23 also indicates the different trend of pore pressure distribution at
S=0.95, compared with other values of degree of saturation (S=0.975 and 1). It is
noted that the results presented in the figure are the real component of the
solution, i.e., the phase change component of pore pressure (i.e., imaginary
component). The trend at S=0.95 is a significant phase change occurring at the
unsaturated seabed, as reported by Okusa (1985) and Hsu and Jeng (1994). The
occurrence of phase lag comes from the flow transfer between different media
(solid and fluid).
Soil type is a dominant factor in the evaluation of the wave-induced pore
pressure (Jeng, 1997a). Two different sandy beds (coarse sand and fine sand) are
considered in this study. The major differences between them are the
permeability and shear modulus.
Comparing Figures 3.19–3.23, a common trend is observed between coarse
sand and fine sand. That is, the effect of other soil parameters on the wave-
induced pore pressure in fine sand is more significant than that in coarse sand.

Effects of geometry of caisson and rubble mound
The geometry of the caisson is an important factor, which must be taken into
account in the design of structures. The width of the caisson may vary from 2 m
to 20 m in engineering practice. In this case, the width of the caisson is
considered to vary between 2 m and 8 m. We intend to examine the effects of the
width of the caisson on the pore pressure distribution.
Figure 3.24 presents the vertical distribution of pore pressure for various
widths of the caisson at section 4 in both coarse and fine sand, respectively. The
figure indicates that the width of the caisson only significantly affects the pore
pressure beneath the caisson (i.e. section 4). It is noted that the width of the
caisson also affects the pore pressure distribution in the rubble mound, unlike
other characteristics.
The geometry of the rubble mound base, including its width (B and B ) and
1
2
height (H ) (as depicted in Figure 3.15) is expected to affect the distribution of
1
the wave-induced pore pressure. After some preliminary parametric tests, the
influences of B 1 and B 2 can be ignored. Thus, we will present the results of
varying H in this section.
1
Figure 3.25 illustrates the vertical distribution of the wave-induced pore
pressure (p/p ) with various heights of the rubble mound base (H ). In general,
1
o
the pore pressure increases as H 1 increases. The effects of the height of the
rubble mound (H ) on the wave-induced pore pressure are significant at sections
1
3 and 4, especially inside the rubble mound base and in the region near the
seabed surface, as shown in Figure 3.25. The figure also demonstrates a greater