Pile Design and Construction Practice, Fifth edition

The penetration depths in Figure 4.14 have been limited to 20 m for dense sands. This is
because the pile capacity as determined by the base resistance alone exceeds the value to
which the pile can be driven without causing excessive compression stress in the pile shaft.
For example, taking a heavy section tubular pile with a wall thickness of 25 mm in high-
yield steel and limiting the compression stress to twice the value given by the allowable
working stress of 0.3 times the yield stress, the ultimate pile load is 9.7 MN. This is exceeded
at 12 and 20 m penetration using the Berezantsev and Kulhawy factors respectively. The
high base resistances which can be obtained in dense sands often make it impossible to drive
piles for marine structures to a sufficient depth to obtain the required resistances to uplift
and lateral loading. This necessitates using open-end piles, possibly with a diaphragm across
the pile at a calculated height above the toe as described in Section 2.2.4.
The second term in equation 4.16 is used for calculating the friction on the pile shaft. The
value of Ksis critical to the evaluation of the shaft friction and is the most difficult to deter-
mine reliably because it is dependent on the stress history of the soil and the changes which
take place during installation of the pile. In the case of driven piles displacement of the soil
increases the horizontal soil stress from the original Kovalue. Drilling for bored piles can
loosen a dense sand, and thereby reduce the horizontal stress.
When piles are driven into coarse-grained soils (gravels, sands and sandy silts) massive
changes take place around the pile shaft and beneath its base. Loose soils are readily
displaced in a radial direction away from the shaft. If the loose soils are water-bearing, vibrations
from the pile hammer cause the soils to become ‘quick’and the pile slips down easily. The

168 Resistance of piles to compressive loads

200

150

Bearing capacity factor (

N

)q

100

50

0

25 30

Angle of shearing resistance (f)

35

70

Depth/least width ratio

20

D/B=5

D/B=2

0

D/B

= 5

40 45

Berezantsev

Brinch Hansen

Figure 4.13Bearing capacity factors of Berezantsev et al.(4.21)and Brinch Hansen(5.4).