Civil Engineering Reference
In-Depth Information
With segmental pile systems special precautions are necessary to ensure that the
ends of the segments are square. Out-of-squareness will cause difficulty in making the
joints and will cause eccentric transfer of load across the joints. A lack of fit of the
joints will rapidly become unacceptable on driving, owing to the slender geometry of
pile segments. It is suggested the limit on out-of-squareness should be less than 1 in
300, in good practice. The first section driven determines the direction of pile motion
and following sections must follow the same path. Unless the surrounding soil is very
weak the straightening of the pile caused by soil forces will cause cracks in the section
adjacent to the joints.
The design andmanufacture of steel H-piles is quite straightforward. If heavy driving
conditions are expected, the pile may be fitted with a shoe or a rock point. Prepara-
tion of tubular or hexagonal piles may involve a considerable amount of welding.
As far as possible, the weld should be carried out in the fabrication shop under the
supervision of suitably qualified personnel. Pile driving will subject the welds to high
repetitive stresses and any defects can easily lead to the propagation of brittle frac-
tures. It is important that the pile sections are correctly aligned, as lack of straightness
will cause problems on driving and result in high bending stresses being locked into
the pile.
Buckling of slender piles is not usually a critical design consideration, as even soft
alluvium provides sufficient lateral restraint to prevent buckling of the pile. However,
the selection of unduly light sections can cause problems in handling and pitching of
the piles.
Similarly there are few problems with the manufacture of timber piles, provided the
correct species of wood is selected for the job in hand. The piles should be fitted with
a steel or cast-steel shoe to protect the toe of the pile. A steel band is normally fitted
to the head of the pile to prevent 'brooming' during driving.
7.1.2 Installation of driven piles
7.1.2.1 Pile driving
The most obvious cause of damage during driving of concrete piles is distortion or
spalling of the head of the pile (Figure 7.1). Such damage is frequently the result of
insistence on driving to a predetermined length. The driving of piles may be controlled
by a specified final set, and driving to a length may entail the adoption of expedients
such as jetting or pre-boring.
Depending on the soil profile, heavy driving may be necessary to penetrate thin dense
layers. The piles selected should be capable of accepting this heavy driving, indeed it
may be considered necessary to drive test piles to check the performance of the pile in
this respect. Steel piles will accept heavy driving as they rarely break. However, severe
distortion of the pile head may occur. Damage to the pile toe may also occur and a pile
may appear to be driving normally although the set may not build up as expected. If
this is suspected the piles should be load tested and/or extracted for examination (see
Figures 7.2 and 7.3).
It is not usually practicable to inspect the toe of a driven pile, except in the case of
tubular piles. If there is any doubt as to the ability of the pile toe to penetrate the various
