Civil Engineering Reference
In-Depth Information
4.2.4 Behaviour of long slender piles
It was shown earlier that as the length of a pile is increased, the stiffness of the pile
response approaches a limiting value. If piles are being used to reduce settlements,
it would clearly be illogical to design piles of such a geometry that this limit was
reached or even approached. However, there are many cases where piles are used
mainly to provide sufficient bearing capacity, for example, to support storage tanks
on soft ground, or for offshore oil production platforms. In these situations, it may
prove economically advantageous to use very long slender piles, and, in many cases,
compression of the pile may be sufficient to cause relative slip between pile and soil in
the upper part of the pile, under working conditions.
The load level at which slip starts to occur between pile and soil close to the ground
surface may be estimated by combining equations (4.34) and (4.47). Thus, the local
movement of the pile to mobilize full shaft friction is obtained by substituting
τ
s
for
τ
0
in equation (4.34) to give
(
w
s
)
slip
=
ζ
d
τ
s
/
2
G
(4.52)
The ratio
G
/τ
s
varies from between 100 to 400 for piles in clay, to over 1000 for piles
in sand. Thus the movement for slip to occur will be 0.5 to 2% of the pile diameter
in clay, and as low as 0.2% of the diameter in sand. Substitution of equation (4.52)
into equation (4.47) leads to an expression for the pile load,
P
slip
at which slip starts
to occur at ground level (where the pile movement is greatest), given by
ζ
2
d
2
L
P
slip
/
Q
s
=
1
/μ
L
=
(4.53)
where
Q
s
τ
s
.Itisnot
uncommon for this ratio to be as low as 0.2, particularly for piles used offshore, in
which case considerable slip will occur at working loads, where
P
is the ultimate shaft capacity of the pile, which equals
π
dL
/
Q
s
may be of the
order of 0.5 to 0.6.
It is clear that an elastic solution for the pile load settlement ratio becomes inap-
propriate at load levels greater than that given by the expression above. Estimation of
the pile settlement must then allow for the region of slip between pile and soil. This
may be accomplished in the same manner as for a pile in a layered soil profile, essen-
tially treating the upper part of the pile (where full shaft friction has been mobilized)
independently from the lower part. Some iteration will be required, since, for a given
applied load at the pile head, the transition point between the two regions must be
estimated initially. This procedure will be illustrated by an example, taken from pile
load tests reported by Thorburn
et al
. (1983).
The two piles in question were precast concrete, 250 mm square, driven into silty
clay to penetrations of 29 m and 32 m. The shear strength varied with depth according
to
c
u
=
9 for the clay, which
was lightly overconsolidated, the shaft capacities of the two piles may be estimated as
0.84 MN and 1.00 MN respectively. In calculating the load settlement ratio for the
piles, a cylindrical pile of radius 141 mm (giving the same cross-sectional area) will be
taken. The short-term Young's modulus of the piles was
E
p
=
6
+
1
.
8
z
kPa (where
z
is the depth in m). Taking
α
=
0
.
26 500 MPa.
