Civil Engineering Reference
In-Depth Information
can be pressed into the ground in an almost vibration-free and quiet manner
sometimes using the weight of the structure to be supported as reaction. If appro-
priate instrumentation is provided, the micro-piles are effectively load-tested as
they are installed.
This type of system is available from companies such as Roger Bullivant in
the United Kingdom. In a displacement system evolved by Ménard Techniques
Ltd., a tapered steel mandrel is driven into the ground by vibratory methods. The
pre-formed hole is then filled with a dry concrete mix and the mandrel re-driven.
Some expansion of the pile occurs against the soil during this operation and a pile
'skin' develops. Filling with concrete, and finally inserting the reinforcement cage
completes pile installation.
(ii)
Down-hole hammer methods
. It is common practice to formmicro-piles by driving
thin-walled steel tubes into the ground using a down-hole percussive hammer.
Typical pile diameters for this form of pile installation range between 100 and
250mm, with wall thicknesses of 3mm. A 2 or 3m long tubular lead section with
a crimped cruciformpoint is first set vertically at the pile location and plumbed into
position with the down-hole hammer in place. No special guidance equipment is
necessary. The hammer is usually air operated from a small compressor and bears
onto a plug of dry-mix concrete in the base of the pile. The hammer is set into action
to advance the tube until a further open-ended length is added. Each tube is slightly
expanded over a length about a diameter at the upper open end so that the next
tube can be lightly tapped inside and welded in place. The tubes may be left open
or cement-grouted, in which case a structural steel section can be incorporated in
order to improve the bending resistance of the piles (see Figure 3.41).
Maximum lengths of driving are about 10m and the pile can equally be employed
at a raking angle. Unless the pile is simply driven to a prescribed depth, empirical
means of determining a suitable 'set' are employed, which may be in the form of a
maximum penetration per timed interval, e.g. no more than 6mm in 10 seconds. The
down-hole system with permanent steel casing is referred to as the 'Grundomat' pile,
and in the United Kingdom is operated by companies such as Systems Geotechnique
Limited and has proved an adaptable system that can be used in temporary works to
provide support for access platforms for heavier rigs or as lateral support to excava-
tions in clays. In one application by the author, because of the simple means by which
Grundomat piles can be installed, thin-walled 100mm diameter grouted micro-piles
were used as a means of installing inclinometers into a landslip where there was very
difficult rig access. The pile casing was not found to impede the mass movement of
the soil and whilst it was accepted that a minor vertical displacement of the true shear
zone could be recorded, a reliable record of the slip movement was achieved.
It should be noted that the installation of displacement micro-piles is not practicable
in ground with significant obstructions. Where they are used in underpinning and inter-
cept existing foundations, a hole in the footing has to be drilled as a separate process.
3.6.3 Sizes of micro-piles and load capacities
Many types of proprietary micro- and 'mini-' piles are available, in a range of sizes,
from approximately 75mm to (say) 300mm in diameter. Design load capacities are
