Civil Engineering Reference
In-Depth Information
a coffer-dam. Where this problem is encountered, and there is no way to reduce the
piling density, pre-boring may be considered as a method of reducing the effect over
the upper part of the pile. At the design stage, if high-density piling is unavoidable
in soils prone to heave such as stiff clays, a low displacement 'H' section pile may be
selected as more suitable. Alternatively, the multi-tube technique described by Cole
(1972) can be employed. All piles within 12 diameters of each other are considered to
form a part of a group, and are driven (and if necessary, re-driven) to final level before
basing out and concreting.
3.2.7.3 Driven expanded piles
Although currently not commercially available, a form of low displacement driven pile,
the Burland 'wedge pile', was developed in the 1990s. It comprises a cruciform cross-
section fabricated from four steel angle sections, tack welded back-to-back at their
edges, to form a closed sleeve. The sleeve is fitted with a shoe and driven (or jacked)
into the ground to the required depth. A tapered cruciform expander mandrel is then
forced down the entire length of the sleeve, splitting the tack welds. An expansion
of about 10% of the width of the pile occurs, which increases the shaft friction to
a significant degree. The mandrel is then withdrawn, leaving a set of spacers behind
that maintain the pile in the expanded condition in order that it may be grouted for
permanence.
3.2.8 Screw cast-in-place displacement piles
Whilst the installation of this type of pile is effected by means of a type of auger,
the process involves compaction rather than removal of the soil and, in this respect;
the piles are of a displacement type. In forming the pile, a heavy-duty single-start
auger head with a short flight is screwed into the ground to the required depth. The
auger head is carried on a hollow stem which transmits the considerable torque and
compressive forces required, and through which the reinforcement cage is inserted
after completion of the installation process. The end of the hollow stem is sealed
with a disposable tip. Following placement of the reinforcement, concrete is placed
through this tube from a hopper at its head. As concrete filling takes place, the auger is
unscrewed and removed, leaving behind a screw-threaded cast-in-place pile. By virtue
of the combined rotation and controlled lifting applied at the extraction stage the
'threads' are of robust dimensions. The sequence of pile construction is shown in
Figure 3.6.
This method of forming a pile is known as the Atlas Piling System, and is marketed
by Cementation Foundations: Skanska Limited in the United Kingdom, in association
with N.V. Franki S.A of Belgium. A purpose-designed, track-mounted rig provides
hydraulic power for auger rotation and the application of downward force and is
fitted with a crane boom for handling reinforcement and concrete skips. For a given
pile size and volume of concrete, pile capacities are greater than for traditionally con-
structed bored piles, although the restricted diameter of the reinforcement cage may
be a disadvantage if the pile is required to resist high bending stresses. The system
does however combine many of the advantages of a displacement pile with the low
noise and vibration characteristics of a bored pile. It will operate in most cohesive
