Basic piling methods - Piling Engineering - page 28

intended pile position. In practice, the operator 'sees' a stable target pile position
on the in-cab display derived from the brief instants of time when the tall oscillating
mast is momentarily vertical. Hence the pile is located in the correct position with-
out attempting to steady the rig to an unattainable degree ∗ (Stent Piling - 'SAPPAR'
system). In a further advance of this system, the final positioning of the rig can be
carried out automatically. Telemetry of one kind or another can also ensure that much
of the monitoring of the piling operation is available on-line in 'real time' whether in
the site office or back at a Company office.
3.1 Pile types
It used to be possible to categorize the various types of pile and their method of
installation, using a simple division into 'driven' or 'bored' piles. This is adequate in
many situations, but does not satisfactorily cope with the many different forms of pile
now in use. A more rigorous division into 'displacement' or 'non-displacement' piles
overcomes this difficulty to some extent, but some piles are installed by a combination
of these methods and their description may require qualification.
In the displacement (generally driven) pile, soil is displaced radially as the pile shaft
penetrates the ground. There may also be a component of movement of the soil in
the vertical direction. Granular soils tend to become compacted by the displacement
process, and clay soils may heave, with little immediate volume change as the clay is
displaced.
Piles of relatively small cross-sectional area, such as steel 'H' section piles or open
pipe piles, are termed 'low displacement piles', and the effects of compaction or soil
heave are reduced. This can be advantageous if long lengths of pile are to be driven
through granular deposits, if the piles are at close centres, or if clay heave is a problem.
In the non-displacement (generally bored) pile, lateral stresses in the ground are
reduced during excavation and only partly reinstated by concreting. Problems result-
ing from soil displacement are therefore eliminated, but the benefit of compaction in
granular soils is lost and in all soils spoil is produced which may be costly to remove
from a site, especially if it is contaminated.
The displacement of the soil by a pile during installation is therefore a fundamental
property, and its recognition in any classification of pile type is clearly advantageous.
Little-used types such as pre-formed screw piles can also be covered by the (low)
displacement classification, whereas they could not be correctly termed 'driven piles'.
In a further development of the screw pile that is becoming more frequently employed,
especially on contaminated sites where it reduces or eliminates the production of spoil,
a hollow screw-form auger is rotated into the ground and the bore filled with concrete
as it is back-rotated out or retracted without rotation.
The two main categories of pile types may be classified further according to whether
pre-formed units are used, and whether the pre-formed unit is used as temporary
support for the ground and withdrawn during concreting or left in place. For non-
displacement piles, factors such as pile diameter and underreaming are introduced to
the classification, as they have a bearing on the method of installation, and particularly
∗ Original Patent by A Weltman 1995; transferred to BICC 1996.