H-section piles are not economical for carrying high compression loading when driven
into sands. Plugging of the sand does not occur in the area between the web and flanges. The
base resistance is low because of the small cross-sectional area. Accordingly the pile must
be driven deeply to obtain worthwhile shaft friction. The latter is calculated on the total sur-
face of the web and flanges in contact with the soil. At Nigg in Scotland soil displacements
of only a few centimetres were observed on each side of the flanges of H-piles driven about
15 m into silty sand, indicating that no plugging had occurred over the full depth of the pile
shaft. The base resistance of these piles can be increased by welding short stubs or wings
(see Figure 2.19a) at the toe. Some shaft friction is lost on the portion of the shaft above
these base enlargements.
The exponential distribution of interface friction shown in Figure 4.15 has been shown by
the Imperial College research to be a function of the length to diameter ratio, or in the terms
of the researchers to the ratio of the height above the toe to the pile radius (h/R). It follows
that it is more advantageous to use a large-diameter pile with a relatively short embedment
depth, rather than a small diameter with a deep penetration, but in some circumstances,
however, it may be necessary to drive deeply to obtain the required resistance to uplift or
lateral loading.
The maximum working stress on proprietary types of precast concrete jointed piles is in
the range of 10 to 17 MN/m^2. Therefore, if the peak design ultimate resistance of 11 MN/m^2
is adopted the piles will have to develop substantial shaft friction to enable the maximum working
load to be utilized. This is feasible in loose to medium-dense sands but impracticable in dense
sands or medium-dense to dense sandy gravels. In the latter case peak base resistance values
higher than 11 MN/m^2 may be feasible, particularly in flint gravels.
When using EC7 rules to determine the ULS resistance of piles driven into coarse-
grained soils design approach DA1 in equation 4.16 becomes
Rcd (4.17)Nq vokAb
b0.5Ks vok tan (^) r As
s
172 Resistance of piles to compressive loads
Table 4.11 Values of the angle of pile to soil friction for various
interface conditions
Pile/soil interface condition Angle of pile/soil friction,
Smooth (coated) steel/sand 0.5 –0.7
Rough (corrugated) steel/sand 0.7 –0.9
Precast concrete/sand 0.8 –1.0
Cast-in-place concrete/sand 1.0
Timber/sand 0.8–0.9
Table 4.10 Values of the coefficient of horizontal soil stress,Ks
Installation method Ks/Ko
Driven piles, large displacement 1 – 2
Driven piles, small displacement 0.75–1.25
Bored and cast-in-place piles 0.70– 1
Jetted piles 0.50–0.7