Civil Engineering Reference
In-Depth Information
Seabed after local scour
Sea
Original seabed level
Seabed level after general scour
S
0
p
o
p
o
H
GS
H
GS
H
LS
1
2
H
H
′
3
H
o
Pile
FIGURE 4.10
Relation between scour and lateral soil support.
Reduction in lateral soil support is due to:
1. a lower ultimate lateral pressure caused by decreased vertical effective stress
p
o
, and
2. a decreased initial modulus of subgrade reaction (E
S
).
There is no generally accepted method to allow for scour in the p-y curves
for offshore piles. There is a method for evaluating p
o
and E
S
as a function of
scour depths. In this method, general scour reduces the p
o
profile uniformly
with depth, whereas local scour reduces p linearly with depth to a certain
depth below the base of the scour pit.
Subgrade modulus reaction values (E
S
) may be computed assuming the gen-
eral scour condition only. Other methods, based upon local practice and pre-
vious experience in the same location, may be used instead.
Another area of concern in installing piles for offshore structures is the ade-
quacy of existing hammers to produce the required pile penetration to support
the applied load on the platform. The wave theory is based on the fact that each
hammer blow produces a stress wave that moves along the length of the pile at
the speed of sound. The entire length of the pile is not stressed simultaneously,
in contrast to conventional dynamic formulas.
4.8 PILE WALL THICKNESS
The wall thickness of the pile may vary along its length and may be controlled
at a particular point by any one of several loading conditions or requirements.
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