The partial factor in Table 4.2 is applied to obtain , and Nqis derived from using
the relationship with SPT or CPT tests. The mean and minimum values of the standard pen-
etration or cone resistance should be plotted against depth rather than basing the calculations
on individual test results. The design values obtained from the test profiles should be
divided by the calibration factor of 1.05. The correlation factor depends on the number of
test profiles (Table 4.6).
Nqfactors obtained from the Brinch Hansen relationship with (Figure 4.13) should be
multiplied by the shape factor of 1.3 as previously noted. The partial factorsbandsfor
driven piles are shown in Table 4.3.
All set combinations using the DA1 approach should be analysed to obtain the design pile
penetration depth until the engineer is sufficiently familiar with the EC7 procedures to be
able to appreciate the combination critical to the particular problem. The M sets are not used
because the (and tan (^) r) are derived from in-situ tests.
4.3.3 Piles with open ends driven into coarse-grained soils
It was noted in Section 4.3.1 that it is frequently necessary to drive piles supporting off-shore
petroleum production platforms to a very great depth below the sea bed in order to obtain
the required resistance to uplift loading by shaft friction. Driving tubular piles with open
ends is usually necessary to achieve the required penetration depth. Driving is relatively
easy, even through dense soils, because with each blow of the hammer the overall pile diameter
increases slightly thereby pushing the soil away from the shaft. When the hammer is
operating with a rapid succession of blows the soil does not return to full contact with the
pile. A partial gap is found around each side of the pile wall allowing the pile to slip
down. Flexure of the pile in the stick-up length above sea bed also causes low resistance to
penetration.
At some stage during driving a plug of soil tends to form at the pile toe after which the
plug is carried down with the pile. At this stage the base resistance increases sharply from
that provided by the net cross-sectional area of the pile shoe to some proportion (not 100%)
of the gross cross-sectional area.
The stage when a soil plug forms is uncertain; it may form and then yield as denser soil
layers are penetrated. It was noted in Section 2.2.4 that 1067 mm steel tube piles showed
little indication of a plug moving down with the pile when they were driven to a depth of
22.6 m through loose becoming medium dense to dense silty sands and gravels in Cromarty
Firth. No plugging, even at great penetration depths, may occur in uncemented or weakly
cemented calcareous soils. Dutt et al.(4.23)described experiences when driving 1.55 m diameter
steel piles with open ends into carbonate soils derived from coral detritus. The piles fell
freely to a depth of 21 m below sea bed when tapped by a hammer with an 18-tonne ram.
At 73 m the driving resistance was only 15 blows/0.3 m.
It should not be assumed that a solidly plugged pile will mobilize the same base resistance
as one with a closed end. In order to mobilize the full resistance developed in friction on the
inside face the relative pile–soil movement at the top of the plug must be of the order of^1 ⁄ 2 %
to 1% of the pile diameter. Thus with a large-diameter pile and a long plug a considerable set-
tlement at the toe will be needed to mobilize a total pile resistance equivalent to that of a
closed-end pile. Another uncertain factor is the ability of the soil plug to achieve sufficient
resistance to yielding by arching of the plug across the pile interior. Research has shown that
vok Resistance of piles to compressive loads 173