Civil Engineering Reference
In-Depth Information
0.4
Driven piles
w
b
/
d
=
0.2
0.3
w
b
/
d
=
0.15
w
b
/
d
=
0.1
0.2
0.1
Bored pile (Lee & Salgado, 1999)
0
0
0.2
0.4
0.6
0.8
1
Relative density,
I
D
Figure 4.7
Normalized end-bearing pressures for driven open-ended piles and bored piles.
base displacement and relative density, as shown in Figure 4.7. Allowing for some
densification of the soil below the pile base and within the soil plug, a reasonable
design limit of 0.2 for
q
b
/
q
c
is probably realistic. Figure 4.7 also shows a design curve
for bored piles (Lee and Salgado, 1999), which lies close to the curve for open-ended
driven piles at a similar normalized displacement.
Lehane
et al.
(2005) reviewed the capacity of driven piles with particular reference to
offshore design, and noted that more careful averaging of the cone resistance, allowing
for limited penetration into strong founding layers, led to an improved correlation,
with an end-bearing ratio
q
b
/
q
c
of 0.6 for closed-ended piles, rather than the value
of 0.4 suggested by Figure 4.6. For open-ended piles, the end-bearing ratio reduced
linearly with effective area ratio,
A
r
(ratio of net to gross area of pile, but allowing for
partial plugging where this can be estimated) according to
q
b
=
(0
.
15
+
0
.
45
A
r
)
q
c
(4.7)
This gives an end-bearing ratio of 0.6 for closed-ended piles, reducing to 0.2 for typical
open-ended steel piles with an area ratio of around 0.1.
Correlations have also been developed for estimating
q
b
directly from the results
of standard penetration testing. The ratio
q
b
/
N
) where
N
is the number
of blows per 0.3 m penetration, varies with the composition of non-cohesive soil.
As has been discussed in Chapter 2 (see Figure 2.7), there is an approximately linear
relationship between
q
c
/
N
(or
q
c
/
N
and average particle size of the soil, with the ratio varying
from as high as 0.8 MPa for medium-size gravel, down to 0.25 MPa for silt.
Values of end-bearing pressure estimated from results of standard penetration tests
should be assessed by means of equation (4.2), adopting estimated values of the various
soil parameters where necessary, before being used in design. Correlations of results
from standard penetration tests, with the relative density of the sand and effective
overburden stress, may be used for this purpose, as indicated in Chapter 2 (Figure 2.9).
