Civil Engineering Reference
In-Depth Information
the traditional way for column bases, strip footings or slabs. Nominal shaft diame-
ters range from about 400 to 750mm, and with load capacities similar to those for
traditional piles.
3.3 Methods of pile driving
The methods of driving piles fall into the following categories:
(i) dropping weight
(ii) explosion
(iii) vibration
(iv) jacking against a reaction.
With the exception of the installation of displacement piles by jacking methods, driven
piling is an intrinsically noisy process. Today, as environmental issues become more
important, the need to develop silenced hammers and the use of noise shrouds forms
an important aspect of the technology associated with this form of pile installation.
In the following sections the various means of installation of displacement pile is
discussed, but in any particular application, it should be borne in mind that the
suitability of a particular driving method can be strongly influenced by the envi-
ronmental sensitivity of the site. In keeping with the trend towards instrumentation,
modern piling rigs allow the operator to control the hammer input and such param-
eters as hammer velocity, energy input and drop height can be monitored in the
driver's cab.
3.3.1 Drop hammers
The dropping weight, or drop hammer, is the traditional method of pile driving
and is still employed. A weight approximately equal to that of the pile is raised a
suitable distance in a guide and released to strike the pile head. For single tubu-
lar piles a free-standing timber trestle support is sometimes employed to support
the pile above ground, provided that an internal drop hammer can be used. Alter-
natively, tubular guides (so called 'chimney pots') can serve to align piles during
driving.
For most driven piling, purpose-made tracked rigs will hoist the pile into position,
support it during driving and incorporate a guide for the drop hammer (Figure 3.10).
In the past, it was usual for a crane chassis to be employed as the base machine
for use with a drop hammer, with a pile guide frame, or 'hanging leaders' supported
from the crane jib and strutted from the base of the machine (Figure 3.11). This type
of rig is still used today especially for large driven piles and is almost as adaptable
as a purpose-designed machine so that raking piles or piles of extreme length can be
handled with relative ease. The rig may be winched on skids or rollers, or mounted
on tracks.
In guiding the pile, a balance has to be struck between providing suitable directional
control, and allowing some freedom for slight pile movement within the guides, par-
ticularly at the base of the frame. Twist or slight lateral shifts often occur as a pile
is driven. If this is prevented by a tight-fitting gate at the base of the pile frame,
