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Ground Stability, Foundations and Substructures 127
Concrete is fed in
through the pipe at the
back of the rig, through
the pipe and down the
centre of the hollow
stem of the auger
The displacement pile cuts into
the strata, compacting the
surrounding strata
No significant arisings are brought
to the surface
A helical void is formed in
the ground behind the
cutting head
As the CHD bullet head is
removed, concrete is pumped,
under pressure, down the central
pipe
A helical concrete column (pile)
is formed in the ground
Once the drill is fully removed,
the reinforcement cage can be
pushed into the concrete bore
Figure 3.53 CHD piles.
Testing piles
Piles are often tested using dynamic load, static load or sonic integrity methods. These three
methods are described briefly here.
Dynamic load methods
Dynamic load methods of testing are suitable for most types of pile but are more frequently
used on precast concrete or tubular steel piles. The test is usually used on small piling works
where the cost of static load testing cannot be justified. The test determines the loadbearing
capacity of the pile, skin friction and end bearing. Other characteristics such as hammer
energy transfer, pile integrity, pile stresses, driving and load displacement behaviour can
also be determined.
To dynamically test a pile, the pile is struck by a hammer using the piling rig. Two strain
transducers and accelerometers (measures speed and acceleration) are firmly attached to
the face of the pile near to the head of the pile (Photograph 3.10a). As the pile is struck,
the equipment measures the force and acceleration of the pile. The information is relayed
to the monitoring equipment. Once analysed, the data provides models of shaft friction
distribution, bearing capacity and load settlement behaviour.