Page 93 - Numerical Analysis and Modelling in Geomechanics

74 D.S.JENG
leads to higher pore pressure. In the same manner, with a decrease in E the soil
z
will be weaker in the vertical direction than in the horizontal direction. This
encourages greater seepage in the vertical direction, which again leads to higher
pore pressure.
Figure 3.7 illustrates the effect of changing the cross-anisotropic constant, m,
in both coarse and fine sand. With an increase in m there is a subsequent increase
in the pore pressure, p/p . With an increase in the shear modulus, G , or a
z
o
decrease in the Young’s modulus, E , there is an increase in the cross-anisotropic
z
constant, m. As a result of m increasing, there is an increase in the pore pressure.
The pore pressure increases as the water will be able to penetrate the seabed
easier in the vertical direction than in the horizontal direction. The pore pressure
in coarse sand is greater, however there is less variation with the increase in m
(Figure 3.7(a)). As shown in Figure 3.7(b), the increase in m has a great
influence on the pore pressure. Thus, the value of m can be said to be more
critical in fine sand.
It is common to find gas within marine sediments. For example, samples taken
from Mississippi Delta sediments, equilibrating when they are exposed to
atmospheric pressure, have a degree of saturation between 75% and 95% (Esrig
and Kirby, 1977). It is believed that most marine sediments have degrees of
saturation very close to unity, implying nearly full saturation (Pietruszczak and
Pande, 1996). However, it is rare that full saturation can be attained in field or
laboratory conditions, except for an ideal condition.
Figure 3.8 highlights the influence of a varying degree of saturation on the
pore pressure (p/p ) around the buried pipeline. Obviously, with a higher degree
o
of saturation the pore pressure should be larger. The pore pressure around the
pipe is much higher in coarse sand (Figure 3.8(a)), compared with that in fine
sand (Figure 3.8(b)).

Effects of geometry of the pipe
Besides the influences of soil characteristics, the geometry of the buried pipe is
another important factor that must be considered in the analysis of the wave-
seabed-pipe interaction problem. The geometry of the pipe (including the burial
depth and pipe radius) is particularly important for the design of the pipeline with
respect to economic concerns.
Burial depth is an important factor that must be taken into consideration for
the design of the pipeline. The burial depth of the pipeline (b) is defined as the
distance from the seabed surface to the top of the pipeline. The influence of the
burial depth (b) on the wave-induced pore pressure against the polar angles
around the pipe surface is illustrated in Figure 3.9. In the figure, the pore
pressure (p/p ) in coarse sand is slightly higher than in fine sand. This would be
o
expected as coarse sand is more porous than fine sand, as larger voids are
present. However, it should be noted that the pore pressure is only marginally