Civil Engineering Reference
In-Depth Information
shells can sometimes be competitive with the simpler forms of driven cast-in-place
pile. The use of such shells is now infrequent but they can be treated appropriately in
aggressive ground or where down-drag is anticipated, and they may be manufactured
in short lengths, thus permitting low headroom conditions.
In so far as conventional bored piles are concerned, in general they find their best use
in clay soils where open bore holes can be formed without the use of long temporary
casings. In terms of cost per metre, they can seldom achieve the low prices of standard
driven cast-in-place piles of similar diameter, when the driven pile is used in its ideal
conditions. They are less efficient than driven piles as load transfer units but they
can be installed rapidly, they are efficient in terms of installation energy use, they are
relatively vibration-free particularly when a rotary system is used, and they have the
advantage that the soil is at least seen and its properties can be appreciated during the
process of installation.
As the length of casing needed to stabilize upper soft soils increases, so also does
the cost of any given diameter of pile, and where there is a need for very deep casings
or in those circumstances where casing has to be abandoned in favour of bentonite or
polymer suspension, the cost per metre run of pile will rise rapidly to perhaps twice
the cost of the simplest unlined similar pile.
Piles bored by percussion methods cannot generally be constructed as fast as when
rotary boring equipment is used, and are therefore more expensive per metre of com-
parable size, but nevertheless these piles still find substantial use on small sites. The
large-diameter version of the percussion bored pile had merit in the construction of
secant walls when required but equipment for this purpose has now become rare. It
had good characteristics for producing truly vertical piles in particular.
In the United Kingdom continuous flight auger (CFA) piling has become a dominant
force in the market for normal land based piling. Its use is generally regulated by
the ICE Specification for Piling and Embedded Retaining Walls (1996) which is now
widely applied. Specifically this calls for electronic monitoring in a detailed way and
although certain of its requirements may need minor amendment in the light of the
commentary provided by the Federation of Piling Specialists, it has been a great force
for good in the production of better quality work. In this field of work it should be
recognized that small diameter piles and machines of generous power involve much
less risk than machines of meagre power attempting to construct piles on which they
struggle. A key issue is to restrict the auger turns per metre of depth advanced to as
near a minimum as possible, particularly if the auger has passed through upper sandy
soils, avoiding rotating augers unnecessarily and drawing sand laterally onto the auger
with associated loosening effects. Well constructed continuous flight auger piles can
still be constructed rapidly, are sensibly predictable in performance and are relatively
inexpensive.
There is therefore a wide range of pile prices that cannot be generalized because of
the different circumstances of use. Any variation from standard procedures in the con-
struction of piles in general increases the cost. Raking piles are notably more expensive
than vertical piles, permanently lined piles than unlined piles, and heavily reinforced
than nominally reinforced piles.
As stated previously, the costs of establishing large equipment on site are much
greater than for small equipment, and the choice of plant depends very much on the
volume of work to be carried out. However, having chosen the best equipment and
