in "dry" boreholes is usually a straightforward procedure. On completion
of the borehole, the reinforcement is placed with suitable spacers to
locate it centrally in the pile shaft. During concreting, the casing is
withdrawn, always maintaining a head of concrete within the casing. A
hopper and tube should be used to ensure that the mix is not directed
towards the reinforcement.
method of concreting is often employed when casting under water . This
is a practical method of forming a pile shaft provided certain precautions
are taken. It is essential that a slump mix greater than 175 mm is used,
and that the tremie pipe is always kept well below the water/concrete
interface during concreting. The tremie pipe and hopper connections should
be watertight and in clean condition to permit free flow of the concrete.
A minimum internal diameter for the tremie pipe of 150 mm is suggested
for use with concrete having a maximum aggregate size of 20 mm, increasing
for larger aggregates. A plastic balloon or poly-styrene plug should be
used in the tube between the water and first batch of concrete, and immediately
before placing the tremie pipe in the pile bore, a check on sedimentation
should be made. Excess sediments should be removed by air-lifting. When
the pile cut-off level is above ground, concrete should overflow from
the pile head on completion. The upper part of the pile consists usually
of weak concrete and may also be contaminated from boring detritus. In
the case of a pile cut-off below ground, the concrete level should be
raised to allow for around 0.5 to 1.0 m for trimming off the weak concrete.
in concrete level, as the casing is withdrawn, should be avoided and such
incidents be noted. This could indicate that the soil strength is too
low to withstand the weight of the concrete column in the pile shaft.
In soils, such as alluvial clays or loose water-saturated sands, it is
possible for the fluid pressure of the wet concrete in the borehole to
fail the soil, and a bulge can occur in the pile shaft. An undrained shear
strength of about 15 kN/m2 has been observed to result in this
effect. As a result, excessive flow of concrete can occur, resulting in
a zone of concrete over-break and/or necking of the pile shaft. Also distortion
of pile reinforcement can occur, whereby the steel cage is moved downwards
at the zone of over-break. Consequently, contaminated or segregated concrete
can occur in the pile shaft, a feature which sometimes is mistakenly ascribed
to the cross-flow of ground water. To prevent this situation to occur
in unstable ground, the bore hole should be cased and boring ahead of
the pile casing should be avoided.
of soil failure can be reduced by pulling the casing at a steady rate,
rather than in short rapid intervals. Limiting the head of concrete in
the casing is adisable, provided this can be done without risk of the
concrete slumping below the bottom of the casing in the bore.
a self-compacting concrete mix, it is not necessary to vibrate the concrete
in the borehole during casting. A vibrator should not be used inside the
pile casing, as this can affect the stability of the soil surrounding
the pile shaft. It is sometimes considered beneficial, however, to vibrate
the casing while being withdrawn, but this should be done with suitable
equipment, otherwise mixing of the soft concrete and the soil can occur.
In a stratigraphic
succession in which water-bearing gravel overlies a stiff clay, a temporary
casing is used to support the borehole through the gravel, and is then
driven a short distance into the clay in order to seal off the water.
Boring then continues under dry conditions.