Civil Engineering Reference
In-Depth Information
mobilized by a driven pile should, in principle, be comparable with that measured
during a cone penetration test. However, effects of scale and of different rates of
testing combine to yield variations in the ratio
q
b
/
q
c
ranging from under 0.5 to over 2.
Unless there is particular evidence to the contrary, it is suggested that the end-bearing
pressure for driven closed-ended piles may be taken equal to that measured in a cone
penetration test, with a reduction of 30% for open-ended or bored piles.
Estimates of the shaft friction for driven piles are often made from friction sleeve
measurements, taking
f
c
. However, caution should be exercised using this
approach, owing to the sensitivity of the value of friction measured to the degree of
consolidation in the soil (a) around the pile and (b) around the friction sleeve. Pile tests
show that the value of shaft friction measured during installation (a similar process
to the continuous penetration of the cone and friction sleeve) may be as low as 20%
of the long-term shaft friction (see section 4.1.5). Estimates of shaft friction directly
from the cone resistance,
q
c
, offer a better guide. However, recommended correlations
range from
τ
s
=
40 (Thorburn
and McVicar, 1971). It is probably more reliable to deduce profiles of undrained shear
strength from the cone penetration tests and then to use the expressions given earlier,
relating shaft friction to the undrained shear strength and to the effective overburden
stress.
As discussed in Chapter 2, results from standard penetration tests in medium to
firm clays and soft rocks may be used to estimate the shear strength of the soil, taking
a ratio for
c
u
/
N
of 4 to 5 kPa for clays of medium plasticity, rising to 6 to 7 kPa
for plasticity indices less than 20 (see Figure 2.4). The values of shear strength may
then be used to provide estimates of end-bearing pressure and of shaft friction in the
normal way.
τ
s
=
q
c
/
10 (Fleming and Thorburn, 1983) down to
τ
s
=
q
c
/
4.1.3 Capacity of piles in rock
Piles driven to refusal on firm or intact rock are often designed as purely end-bearing
piles ignoring any shaft capacity the pile might have, although, in practice, considerable
shear transfer may occur in the upper layers of soil, at working loads. In many cases,
the maximum design load for such a pile will be determined by the stresses in the pile
material itself rather than the permitted bearing pressure on the rock.
Great care should be exercised where steel piles are driven to bedrock, to ensure that
good sound rock contact is achieved. Pile driving may shatter the rock, and indeed it is
good practice to ensure that the piles penetrate a small distance into the rock. This will
help to prevent the piles heaving and coming away from the bearing stratum during
driving of subsequent piles. Alternatively, where access permits, each pile should be
re-tamped at the end of installing the neighbouring piles, to ensure good contact with
the bearing stratum.
In weathered rock, careful monitoring of driving records is essential owing to the
wide variability in strength exhibited by soft rocks such as chalk and marl in a weath-
ered state. Refusal of the piles may not be achieved and the contribution of the pile
shaft to its overall capacity may need to be relied upon. Accurate estimation of the
shaft friction around a pile driven into soft rock is difficult owing to the disruption of
the structure of the rock as the pile penetrates. The shaft friction will depend on the
magnitude of the normal effective stress, which is liable to vary in a haphazard fashion.
