Civil Engineering Reference
In-Depth Information
Various methods of reducing vibration transmission from piling have been
advocated with conflicting claims as to their success. It may for example help to use
low-displacement piles rather than piles of large displacement, but in the last resort
it may be necessary to change the piling equipment and adopt a different piling tech-
nique. This is a costly exercise when the problems of the particular site are discovered
too late.
The incidence of damage due to piling vibrations in the United Kingdom is very
low, but it is clear that because of human sensitivity, many people confronted with
the problem for the first time become understandably fearful for the safety of their
property. Confidence will often be restored by undertaking a programme of monitoring
and programming work so as to minimize the periods of annoyance.
One of the main considerations in recent years particularly in the United Kingdom
has been the disposal of arisings associated with non-displacement piling and costs
associated with its removal to landfill sites. If pile bore arisings are in any way con-
taminated the costs dramatically escalate. This has led to soil displacement piling
methods becoming more popular and development of piling plant to install piles by
displacement means (other than simple pile driving). These generally involve screw-
ing an auger into the ground rather than using the auger to remove soil in forming
a pile bore. There may be a slight increase in environmental damage from the plant
generally necessary to install a screw type of pile of comparable capacity as it will use
more power and produce more emissions but this is not particularly significant and
in balance is probably less if the transport of spoil from an augered pile and its sub-
sequent handling is taken into account. As an aside, the disposal of bentonite slurries
used in temporary support of pile bores is also becoming more problematic and the
use of polymer muds has advantages in this respect.
10.5 Cost considerations
It must be obvious that there are so many variations of pile type, ground conditions,
structural requirements and other factors involved, that there is no simple way of deter-
mining cost effectiveness in a particular case, apart from competitive price comparisons
and perhaps several trial designs.
In the United Kingdom the piling market is fairly evenly divided between driven
or displacement piles and bored or replacement types, although the ground condi-
tions of some areas favour one type more than another. Elsewhere in the world much
depends on areas with predominating ground conditions and even with traditional
piling processes.
Where soft strata overlie a definite granular or rock stratum and there is a well
defined depth at which substantial end bearing capacities can be achieved, a driven
pile will usually prove most attractive for conventional uses, provided environmental
or other unusual circumstances do not intervene. If the bearing stratum is within say
25 m of ground surface, then a simple unlined concrete cast-in-place type of pile is
often the least expensive choice, and although it increases cost, some systems of this
kind make provision for possible base enlargement. Expansion of the pile foot is often
not necessary, but where it is effective, it can sometimes lead to significant saving in
pile length and hence overall economy may be improved if there is a stratum within a
suitable depth range.
