Page 101 - Numerical Analysis and Modelling in Geomechanics

82 D.S.JENG
more coarse material such as gravel (Figure 3.12(c)), there is a further increase in
pore pressure. Thus, it can be concluded that the coarser the material present is
within the cover layer, the greater the pore pressure will be. The pore pressure
increases proportionally when a coarser material is introduced in the cover layer
as there is an increase in permeability. An evaluation of both the width and depth
of the cover layer (W and B) will now proceed and a comparison will be drawn
between coarse sand and gravel.
Figure 3.13 illustrates the distribution of the wave-induced pore pressure (p/
p ) around a buried pipeline for various values of the depth of the cover layer (B).
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In this example, the width, W has been fixed at 2.0 m. It can be seen that with an
increase in the depth (B), there is a reduction in the pore pressure around the
pipeline. Relatively speaking, it would be expected that there would be an
increase in the pore pressure. However, this is not only the case with an increase
in the depth (B) but also with an increase in the burial depth. This increase in the
burial depth comes about because the pipe is always situated in the centre of the
cover layer. A higher pore pressure is expected to be present around the pipeline
when gravel is the medium used in the cover layer (see Figure 13(b)). The figure
clearly demonstrates that the pore pressure around the pipeline is much greater in
gravel and there is much less variation around the pipeline, which is logical.
An evaluation of the effects of pore pressure (p/p ) by varying the width of the
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cover layer (W) can also be made. Figure 3.14 shows the difference in pore
pressure with increasing width (W), where both coarse sand and gravel were
again both evaluated. The depth of the cover layer has been fixed at 2.0 m in the
example and the width (W) increases from 2.0 m to 4.0 m. Unlike when varying
the depth of the cover layer B, where there was no direct increase in pore
pressure due to a relative increase in burial depth, b, an increase in pore pressure
was experienced with an increase in the width of the cover layer (W).
Figure 3.14 also indicates that an increase in pore pressure was achieved, as
more water was able to penetrate horizontally in towards the pipeline. Again, a
much higher pore pressure was experienced around the pipeline when gravel was
introduced into the cover layer because of the higher permeability. The range of
pore pressure around the pipeline decreased with an increase in the width of the
cover layer, however the pore pressure was much higher. There was also a much
greater difference in pore pressure (p/p ) on the underside of the pipeline
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compared with the top of the pipeline. Greater water penetration was thus being
achieved in the upper extent of the cover layer around the pipeline.
When gravel was present in the cover layer the resulting pore pressures on the
lower half of the pipe were a little ambiguous. It could be seen that when coarse
sand was present in the cover layer and there was a successive increase in the
width (W), there was also an increase in the pore pressure (p/p ). However, in the
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gravel there was actually a minor decrease in the pore pressure with successive
increases in the width (W) and around the lower extent of the pipe the pore
pressure varied quite strangely. However, it should be noted that the pore
pressure was actually much higher, which was to be expected. Also, the