Pile Design and Construction Practice, Fifth edition

Structural design of piles and pile groups 383
A heavier hammer can be used to achieve the necessary penetration but this may involve a
risk of overstressing the pile.
7.4 The effects on bending of piles below ground level
Slender steel tubular piles and H-piles may deviate appreciably off line during driving.
These effects were described in Section 2.2.4 where it was noted that, whereas the ill-effects
of bending or buckling of tubular piles below ground level could be overcome by inserting
a reinforcing cage and filling the pile with concrete, such a procedure could not be adopted
with H-piles. Therefore, where long H-piles are to be driven in ground conditions giving rise
to bending or buckling, a limiting value must be placed on their curvature.
It is not usual to take any special precautions against the deviation of reinforced concrete
piles other than to ensure that the joints between elements of jointed pile systems (see
Section 2.2.3) are capable of developing the same bending strength as the adjacent unjointed
sections. Reinforced concrete piles without joints cannot in any case be driven to
very long lengths in soil conditions which give rise to excessive curvature. It is, of course,
possible to inspect hollow prestressed concrete piles internally and to adopt the necessary
strengthening by placing in-situ concrete if they are buckled.
It is impossible to drive a pile with a sufficient control of the alignment such that the pile
is truly vertical (or at the intended rake) and that the head finishes exactly at the designed
position. Tolerances specified in various codes of practice are given in Section 3.4.12. If the
specified deviations are exceeded, to an extent detrimental to the performance of the piles
under working conditions, the misaligned piles must be pulled out for redriving or additional
piles driven. Calculations may show that minor excesses from the specified tolerances do
not cause excessive bending stresses as a result of the eccentric loading. In the case of driven
and cast-in-place or bored and cast-in-place piles it may be possible to provide extra reinforcement
in the upper part of the pile to withstand these bending stresses. For this reason
Fleming and Lane (7.6) recommend that checks on the positional accuracy of in-situ forms of
piling should be made before the concrete is placed. The methods described in Section 6.3.9
can be used to calculate the bending stresses caused by eccentric loading. The effect of the
deviation is expressed as a bending moment Pe, where the load P deviates by a distance e
from the vertical axis of the pile.
7.5 The design of axially loaded piles as columns
Buckling of axially loaded piles terminating at ground level in a pile cap or ground beam
cannot occur if the piles are loaded to within the permissible working stresses on the pile
material. Thus such piles need not be considered as long columns for the purpose of
structural design. However, it is necessary to consider the column strength of piles projecting
above the soil line, as in jetties or piled trestles.
BS 8004 recommends that the depth below ground surface to the point of contraflexure
should be taken as 1 m in firm ground and as much as one-half of the penetration depth but
not necessarily more than 3 m in a weak ground such as soft clay or silt. A stratum of liquid
mud should be treated as if it were water. The column strength of the pile is then calculated
as for a short column and a reduction factor is applied to the calculated ultimate load to
allow for the slenderness of the column, where the slenderness is defined as the ratio of the
effective length to the breadth or radius of gyration.