Civil Engineering Reference
In-Depth Information
allow for stress relief on excavation of the pile shaft, or a direct 'total stress' approach
may be used. Assuming that little change in the in-situ effective stress state takes place
during construction of the pile, Meyerhof's (1976) observation of a
K
value close to
1
5
K
0
would indicate that the shaft friction for a bored pile may be estimated as about
70% of that for a driven displacement pile. This figure may be conservative for piles
in soil with low in-situ values of
K
0
, since some increase in lateral stress may occur
during the process of casting the pile.
For bored piles in stiff, heavily overconsolidated clay, an
.
value of 0.45 was sug-
gested in order to estimate the shaft capacity (Skempton, 1959). This approach is
based largely on experience in London Clay, with shear strengths measured on small
(38 mm diameter) samples. In fissured clays, such as London clay, the shear strength
is affected by sample size and the modern use of 100 mm diameter samples leads to
lower shear strengths, and thus a higher value of
α
α
will be appropriate. Recent reviews
suggest
values of 0.5 for conventional bored piles, and 0.6 for CFA piles, based on
shear strengths determined from unconsolidated undrained triaxial tests on 100 mm
diameter samples. Care should be taken when applying the same value of
α
to clays
where the strength ratio,
c
u
/σ
v
, is higher than for London Clay, since an effective stress
approach may well indicate substantially lower values of shaft friction than the total
stress approach based on
α
values in the range 0.5 to 0.6.
Weltman and Healy (1978) analyzed a number of pile tests in boulder clays and
other glacial tills, and suggested values of
α
varying with the undrained shear strength
of the soil as shown in Figure 4.14. The ratio of values for bored and driven piles is just
over 80%, but both curves are derived from data showing considerable scatter. The
figure also shows a separate curve for very short, stubby piles driven into till overlain
by soft clay. For such piles, the ratio of
c
u
/σ
v
will be high and the resulting low
α
α
values are consistent with the effective stress approach outlined earlier.
4.1.2.6 Correlations with field tests
Comparisons of measured pile capacities with results of cone penetration or standard
penetration tests show a much greater scatter than for non-cohesive soils. The drainage
conditions during a penetration test will be rather different from those existing during
a pile test, and this may account for some of the variation. The end-bearing pressure
Figure 4.14
Variation of
α
with shear strength of glacial till (Weltman and Healy, 1978).
