Civil Engineering Reference
In-Depth Information
The response of the piles to constant rate of penetration tests may be matched by
adopting a shear modulus of
G
45
z
MPa, taking Poisson's ratio
equal to 0.5. Equation (4.47) then gives the pile stiffness as
P
t
/
=
250
c
u
=
1
.
5
+
0
.
90 kN/mm. The
local movement to mobilize full shaft friction may be estimated from equation (4.52)
as 2.5 mm. Thus, slip between pile and soil will start at the top of the pile when the
applied load reaches 90
w
t
=
225 kN. Figure 4.30 gives the measured pile responses,
showing both piles having the same initial stiffness, with an essentially linear response
up to loads of about 500 kN.
The complete load settlement curve may be estimated by calculating the load and
corresponding settlement for a given depth of slip between pile and soil. Thus, consider
the stage at which full shaft friction has been mobilized over the top l0 m of the pile.
The load shed over the top 10 m may be calculated from the given profile of shear
strength, as 150 kN. For the bottom part of the pile, the load settlement ratio may
be calculated from equation (4.45) as
P
×
2
.
5
=
112 kN/mm. Thus the load level at
the transition point (where the settlement is 2.5 mm) is 280 kN. The overall load at
the pile head is then 280
/
w
=
430 kN. The pile compression over the top 10 m
of the pile may now be calculated as 2.3 mm, giving an overall settlement at the pile
head of 4.8 mm.
The above process may be repeated for different depths of slip between pile and
soil to give the two estimated load settlement curves shown in Figure 4.30. In practice,
adequate estimates of these curves may be obtained merely from the initial slopes of the
load settlement curves, and estimates of the pile settlements when full shaft capacity
has been mobilized. Thus for the shorter pile, the compression of the pile for the shaft
+
150
=
Figure 4.30
Calculated and measured load settlement curves for two long piles.
