Civil Engineering Reference
In-Depth Information
where
I
0
is the value of
I
for a rigid pile in an infinitely deep homogeneous
deposit of incompressible soil (
v
=
0
.
5).
R
k
allows for the compressibility of the pile.
R
h
allows for the finite depth of a layer of soil.
R
v
allows for values of Poisson's ratio less than 0.5.
Comparison between the two approaches is generally reasonably good, considering
the simplifying assumptions adopted in the solution developed here. For long com-
pressible piles, the results from Poulos and Davis give higher values of pile stiffness
than obtained using equation (4.45), but this may be partly due relatively coarse dis-
cretization of the very long piles, leading to numerical inaccuracies in the boundary
element solutions.
Two examples are included here to illustrate the approach described above.
Example 1
Estimation of the working settlement of bored pile, 460 mm in diameter, founded at
a depth of 16 m in stiff clay; the upper2mofsoil is made ground, with no shear
transfer assumed. The strength of the clay increases uniformly from 70 kPa just below
the made ground up to 200 kPa at the pile base, with an average value of 135 kPa. The
design load for the pile is 800 kN. Taking a shear modulus for the soil of
G
=
150
c
u
(see section 5.2.4), with
v
=
0
.
2, and assuming the Young's modulus for the pile is
E
p
=
25 000 MPa, we obtain
G
L
=
30 MPa
;
ρ
=
0
.
675
;
l
=
833
For the lower 14 m of pile,
L
/
d
=
30
.
5, thus
2
2
ζ
l
ln
2
r
m
/
d
=
L
d
=
tanh(
μ
L
)
L
d
=
ζ
=
4
.
4
;
μ
L
=
1
.
42
;
19
.
1
μ
L
Thus
2
0
2
π
×
0
.
675
8
+
×
19
.
1
P
t
.
4
.
4
w
t
=
dG
L
×
=
269 kN/mm
1
π
l
8
0
1
+
8
×
19
.
1
.
The deflection at depth of 2 m, for the working load of 800 kN, is thus 800
/
269
=
3
0 mm. In addition, 0.4 mm of compression will occur in the upper2mofthepile,
giving an overall working settlement of 3.4 mm.
.
Example 2
Estimation of the working settlement of a driven cast-in-situ pile, 510 mm in diameter
and driven through low-grade chalk to bear on grade I/II chalk at a depth of 12 m.
Back analyses of earlier pile tests have indicated that the shear modulus for the upper
chalk may be taken as increasing at a rate of 1 MPa for every metre of depth, taking
