Pile Design and Construction Practice, Fifth edition

esistance of cylindrical augered footings and a 30% to 50% reduction in belled footings in
clay when sustained loads were carried over a period of 3 to 4 months. It was considered that
the reduction in uplift was due to a loss of suction beneath the pile base and the dissipation
of negative pore pressures set up at the initial loading stage. These authors pointed out that
such reductions are unlikely for piles where the depth/width ratio is greater than 5.
The ICP method (4.30) can be used to determine the tension capacity of driven piles
carrying tension loading. For piles in clay the method does not differentiate between shaft
resistance in compression or tension, i.e. equations 4.20 to 4.24 can be used without modification
for either type of loading. Conditions are different for piles in sands where the
degradation of the soil particles at the pile–soil interface has a greater effect on stability.
Also in the case of tubular steel piles the radial contraction across the diameter under tension
loads is a further weakening effect on frictional resistance, particularly for open end
piles. Accordingly, equation 4.27 is modified to become
� f � (0.8 �� rc � ��� rd)tan �� cv
�� rc
(6.1)
where and are calculated as described for compression loading in Section 4.3.7. For
open end piles in tension �f as calculated by equation 6.1 is reduced by a factor of 0.9.
Cyclic loading generally results in a weakening of shaft capacity. The reduction can be
significant for offshore structures where piles are subjected to repetitive loading from wave
action. The degree of reduction depends on the amplitude of shear strain at the pile–soil
interface, the susceptibility of the soil grains to attrition, and the number and direction of the
load-cycles, i.e. one-way or two-way loading. The amplitude of the shear strain depends in
turn on the ratio of the applied load to the ultimate shaft capacity. In clays the repeated load
applications increase the tendency for the soil particles to become re-aligned in a direction
parallel to the pile axis at the interface which may eventually result in residual shear
conditions with a correspondingly low value of . In sands, it is evident that the greater the
number of load-cycles the greater the degree of degradation, although the residual silt-sized
particles produced by a silica sand will have an appreciable frictional resistance.
Degradation, both in sands and clays, takes place initially in the region of the soil-line
where the amplitude of the tensile strain is a maximum; it then decreases progressively down
the shaft but may not reach the pile toe if the applied load is a relatively small proportion of
the ultimate shaft capacity.
Jardine et al. (4.30) �cv recommend cyclic shear tests in the laboratory using the site-specific
materials as a means of quantifying the reduction in friction capacity. In clays the interface
shear is likely to occur in undrained conditions; accordingly, the laboratory testing programme
should provide for simple cyclic undrained shear tests. An alternative to laboratory
testing suggested by Jardine et al. is to simulate the relative movement between pile and soil
under repetitive loading by finite element or t–z analyses (Section 4.6).
EC7 adopts a criterion for avoiding the ultimate limit state for single piles or pile groups
in tension by the expression similar to that for compression loading, that is
F td � R td
��� rd
Piles to resist uplift and lateral loading 309
(6.2)
where F td is the design value for actions in tension on a pile or pile group and R td is the
design value of resistance in tension of the pile or the foundation. Partial factors for actions
are as shown for compression piles in Table 4.1.