Pile Design and Construction Practice, Fifth edition

14 Types of pile
previous experience of the ground conditions in a particular locality is important in assessing
the likely pile length on which to base a tender. Experience is also an important factor in determining
the extent and cost of a preliminary test piling programme. This preliminary work can
be omitted if a piling contractor can give a warranty based on in-house knowledge of the site
conditions that the engineer’s requirements for load/settlement criteria can be met. The cost of
test piling can then be limited to that of proof-loading selected working piles. In well-defined
ground conditions and relatively light structural loads, the client may rely solely on the
contractor’s comprehensive warranty that the working piles meet the load-carrying requirement
with an appropriate safety factor. It is a precept in EC7 that pile design should be related
directly or indirectly to the results of static load tests and in certain cases such tests are mandatory.
Where analytical calculations or interpretations of dynamic tests are used for design, the
methods must have been validated against previous static load tests ‘in comparable conditions’.
EC7 introduces design by the ‘observational method’ in which the design is reviewed during
construction and in response to monitoring during performance. This is not relevant to pile
design, but a design method based on observed performance of comparable piled foundations
is acceptable provided that it is ‘supported by ground investigation and ground testing’.
In any case, preliminary test piling may be necessary to prove the feasibility of the
contractor’s installation method and to determine the load–settlement relationship for a
given pile diameter and penetration depth. If a particular piling system is shown to be
impracticable, or if the settlements are shown by the test loading to be excessive, then
considerable time and money can be expended in changing to another piling system or
adopting larger-diameter or longer piles. During the period of this preliminary work the
main contractor continues to incur overhead costs and may well claim reimbursement of
these costs if the test-piling work extends beyond the time allowed in the constructional
programme. To avoid such claims it is essential to carry out a thorough ground investigation
(as BS 5930 and EC7-Part 2 Ground investigation and testing), and it is desirable to conduct
the preliminary test piling before the main contractor commences work on the site.
Finally, a piling contractor’s resources for supplying additional rigs and skilled operatives
to make up time lost due to unforeseen difficulties and technical ability in overcoming these
difficulties are factors which may influence the choice of a particular piling system.
2.2 Driven displacement piles
2.2.1 Timber piles
In many ways, timber is an ideal material for piling. It has a high strength to weight ratio, it
is easy to handle, it is readily cut to length and trimmed after driving, and in favourable
conditions of exposure durable species have an almost indefinite life. Timber piles used in
their most economical form consist of round untrimmed logs which are driven butt uppermost.
The traditional British practice of using squared timber may have become established
because of the purchase for piling work of imported timber which had been squared for
general structural purposes in the sawmills of the country of origin. The practice of squaring
the timber can be detrimental to its durability since it removes the outer sapwood which is
absorptive to creosote or some other liquid preservative. The less absorptive heartwood is
thus exposed and instead of a pile being encased by a thick layer of well-impregnated
sapwood, there is only a thin layer of treated timber which can be penetrated by the hooks
or slings used in handling the piles or stripped off by obstructions in the ground.