Pile Design and Construction Practice, Fifth edition

156 Resistance of piles to compressive loads
is calculated from the peak value of c u. To allow for the flexibility and slenderness ratio of
the pile it is necessary to reduce the values of � p by a length factor, F, as shown in
Figure 4.6b. Thus total shaft friction:
Q s � F� pc uA s
(4.8)
The slenderness ratio, L/B, influences the mobilization of shaft friction in two ways. First,
a slender pile can ‘whip’ or flutter during driving causing a gap around the pile at a shallow
depth. The second influence is the slip at the interface when the shear stress at transfer from
the pile to the soil exceeds the peak value of shear strength and passes into the lower residual
strength. This is illustrated by the shear/strain curve of the simple shear box test on a
clay. The peak shear strength is reached at a relatively small strain followed by the much
lower residual strength at long strain. It follows that when an axial load is applied to the head
of a long flexible pile the relative movement between the pile and the clay at a shallow depth
can be large enough to reach the stage of low post-peak strength at the interface. Near the
pile toe the relative movement between the compressible pile and the compressible clay may
not have reached the stage of mobilizing the peak shear strength. At some intermediate level
the post-peak condition may have been reached but not the lowest residual condition. It is
therefore evident that calculation of the skin friction resistance from the results of the peak
undrained shear strength, as obtained from unconfined or triaxial compression tests in the
laboratory, may overestimate the available friction resistance of long piles. The length
factors shown in Figure 4.6b are stated by Semple and Rigden to allow both for the flutter
effects and the residual or part-residual shear strength conditions at the interface. The effect
of these conditions on the settlement of single piles is discussed in Section 4.6.
Where an overburden of soft clay is overlying a stiff clay adhesion, factors appropriate to
each type should be selected and the shaft resistance calculated for the portion of the shaft
embedded in each stratum. The length factor in Figure 4.6b is taken on the overall embedded
length.
In marine structures where piles may be subjected to uplift and lateral forces caused by
wave action or the impact of berthing ships, it is frequently necessary to drive the piles to
much greater depths than those necessary to obtain the required resistance to axial
compression loading only. To avoid premature refusal at depths which are insufficient to
obtain the required uplift or lateral resistance, tubular piles are frequently driven with open
ends. At the early stages of driving soil enters the pile when the pile is said to be ‘coring’.
As driving continues shaft friction will build up between the interior soil and the pile wall.
This soil is acted on by inertial forces resulting from the blows of the hammer. At some stage
the inertial forces on the core plus the internal shaft friction will exceed the bearing capacity
of the soil at the pile toe calculated on the cross-sectional area of the open end. The plug is
then carried down by the pile as shown in Figure 4.7a. However, on further driving and when
subjected to the working load, the pile with its soil plug does not behave in the same way as
one driven to its full penetration with the tip closed by a steel plate or concrete plug. This is
because the soil around and beneath the open end is not displaced and consolidated to the
same extent as that beneath a solid-end pile.
Comparative tests on open-end and closed-end piles were made by Rigden et al. (4.6) The
two piles were 457 mm steel tubes driven to a penetration of 9 m into stiff glacial till in
Yorkshire. A clay plug was formed in the open-end pile and carried down to occupy 40% of
the final penetration depth. However, the failure loads of the clay-plugged and steel plate