using standard penetration tests or static cone tests. Normally consolidated soils show low
penetration values at the surface increasing roughly linearly with depth. Over-consolidated
soils show high values at shallow depths, sometimes decreasing at the lower levels.
When calculating in equation 4.6, the factor Kin sands and other coarse-grained soils
is denoted by Kswhich is related to Ko, to the type of pile and to the installation method.
Some typical values are shown in Table 4.10.
The angle of interface friction (^) rin equation 4.6 is obtained by applying a factor to
the average effective angle of shearing resistance ( ) of the soil as determined from its
relationship with SPT or CPT values as shown in Figure 4.10 and 4.11. The factor to
obtain (^) rfrom the design depends on the surface material of the pile. Factors established
by Kulhawy(4.22)are shown in Table 4.11. They apply both to driven and bored piles. In the
latter case depends on the extent to which the soil has been loosened by the drilling
process (Section 4.3.6). The CFAtype of bored pile (Section 2.4.2) is advantageous in
this respect.
Use of the Ks/Korelationship in Table 4.10 to determine the shaft resistance of a pile
driven into sand when using equation 4.16 does not reflect the exponential distribution of
intergranular friction shown in Figure 4.15. A semi-empirical method based on cone resist-
ance values has been developed at Imperial College, London. It is particularly suitable for
piles driven to a deep penetration and is described in Section 4.3.6.
EC7 requires that the base resistance of tubular piles driven with open ends having an
internal diameter greater than 500 mm should be the lesser of the shearing resistance
between the soil plug and the pile interior, and the base resistance of the cross-sectional area
of the pile at the toe.
4.3.2 Driven piles in coarse-grained soils
Driving piles into loose sands densifies the soil around the pile shaft and beneath the base.
Increase in shaft friction can be allowed by using the higher values of Ksrelated to Kofrom
Table 4.10. However, it is not usual to allow any increase in the values and hence the
bearing capacity factor Nqcaused by soil compaction beneath the pile toe. The reduction in
the rate of increase in end-bearing resistance with increasing depth has been noted above. A
further reduction is given when piles are driven into soils consisting of weak friable particles
such as calcareous soils consisting of carbonate particles derived from disintegrated corals
and shells. The soil tends to degrade under the impact of hammer blows to a silt-sized
material with a marked reduction in the angle of shearing resistance.
Because of these factors, published records for driven piles which have been observed
from instrumented tests have not shown values of the ultimate base resistance much higher
than 11 MN/m^2. The authors use this figure for closed-end piles as a practical peak
value for ordinary design purposes but recognize that higher resistances up to a peak of
22 MN/m^2 may be possible when driving a pile into a dense soil consisting of hard angular
particles. Such high values should not be adopted for design purposes unless proved by
loading tests. Figure 4.14 shows that the base resistance of a closed-end pile driven into a
dense sand can reach the maximum compressive stress to which the pile can be subjected
during driving at a relatively short penetration. Therefore, if the peak base resistance of
11 MN/m^2 is used for design there is no advantage in attempting to drive piles deeply into
medium-dense to dense soils with the risk of pile breakage in order to gain a small increase
in shaft friction.
sResistance of piles to compressive loads 171