Pile Design and Construction Practice, Fifth edition

188 Resistance of piles to compressive loads
the same type of soil and similar structures with particular reference to local information
(Clauses 7.6.2.3 and 1.5.2.2). The overall safety factor for DA1 approach is mainly influenced
by the correlation factors derived from the number of in-situ tests made on the site.
Jardine et al. (4.30) do not offer any recommendations for applying EC7 procedures to their
design methods.
4.3.8 Time effects for piles in coarse-grained soils
The engineer should be aware of a possible reduction in capacity where piles are driven into
fine sands and silts. Peck et al. (4.18) stated that ‘If the fine sand or silt is dense, it may be
highly resistant to penetration of piles because of the tendency for dilatancy and the
development of negative pore pressures during the shearing displacements associated with
insertion of the piles. Analysis of the driving records by means of the wave equation may
indicate high dynamic capacity but instead of freeze, large relaxations may occur.’
An example of this phenomenon was provided by the experiences of driving large diameter
tubular steel piles into dense sandy clayey silts for the foundations of the new Galata Bridge
in Istanbul (4.36) . The relaxation in capacity of the 2 m OD piles in terms of blows per 250 mm
penetration is shown in Figure 4.23. The magnitude of the reduction in driving resistance was
not related to the period of time between cessation and resumption of driving. It is likely that
most of the reduction occurred within a period of 24 hours after completing a stage of
driving. The widely varying time periods shown in Figure 4.23 were due to the operational
movements of the piling barge from one pile location or group to another.
Correlation of blow count figures with tests made with the dynamic pile analyser
(Section 7.3) showed a markedly smaller reduction in dynamic soil resistance than indicated
by the reduction in blow count after the delay period.
These experiences emphasize the need to make re-driving tests after a minimum period of
24 hours has elapsed after completing the initial drive. Loading tests should not be made on
piles in sands until at least seven days after driving. Where piles are driven into laminated fine
sands, silts and clays, special preliminary trial piling should be undertaken to investigate time
effects on driving resistance. These trials should include tests with the pile driving analyser.
Increases in shaft capacity similar to those described above are not expected with
bored piles.
4.4 Piles in soils intermediate between sands and clays
Where piles are installed in sandy clays or clayey sands which are sufficiently permeable to
allow dissipation of excess pore pressure caused by application of load to the pile, the base
and shaft resistance can be calculated for the case of drained loading using equation 4.16.
The angle of shearing resistance used for obtaining the bearing capacity factor Nq should be
the effective angle �� obtained from unconsolidated drained triaxial compression tests. In a
uniform soil deposit, equation 4.16 gives a linear relationship for the increase of base resistance
with depth. Therefore, the base resistance should not exceed the peak value of 11 MN/m2 unless pile loading tests show that higher ultimate values can be obtained. The effective
overburden pressure, ��vo, in equation 4.16 is the total overburden pressure minus the pore
water pressure at the pile toe level. It is important to distinguish between uniform c � �
soils and layered c and � soils, as sometimes the layering is not detected in a poorly executed
soil investigation.