Pile Design and Construction Practice, Fifth edition

134 Piling equipment and methods
concrete by tremie pipe. This is because the water in a bored pile is rarely clean, and the silt
stirred up by dumping the aggregate tends to get dispersed on to the surface of the stones.
It is then displaced by the rising column of grout and tends to form layers or pockets of
muddy laitance.
The procedure for drilling pile boreholes with support by a bentonite slurry is described in
Section 3.3.8 and in CIRIA Report PG3 (3.24) . Problems can be caused when placing concrete in
a bentonite-filled hole. A tremie pipe must be used, and there must be a sufficient hydrostatic
pressure of concrete in the pipe above bentonite level to overcome the external head of the
slurry, to rupture the gel and to overcome friction in the tremie pipe. Sometimes a dispersing
agent is added to the bentonite to break down the gel before placing the concrete. Where the
mud becomes flocculated and heavily charged with sand (i.e. has a density greater than 1350
to 1400 kg/m 3 ) it should be replaced by a lighter mud before placing the concrete and the base
of the pile cleaned of any debris. Circumferential steel should be kept to a minimum. The concrete
in the piled foundations for the Wuya Bridge, Nigeria (3.25) was placed under bentonite.
The piles were 18 to 21 m deep and a mud density of 1600 kg/m 3 was necessary to prevent
the sides from collapsing. The concrete failed to displace the gel which was stiffened by the
high ground temperatures and jamming occurred, especially when placing was suspended to
remove each section of the tremie pipe. The problem was finally overcome by increasing the
workability of the concrete by means of a plasticizer together with a retarder. The tremie pipe
was lifted out as a single unit to avoid the delays in breaking the pipe joints.
3.4.9 The installation of bored and cast-in-place piles by
grabbing, vibratory, and reverse-circulation rigs
The use of either grabbing, vibratory or reverse-circulation machines for drilling pile
boreholes can involve continuous support by lining tubes which may or may not be withdrawn
after placing the concrete. In all three methods the tubes may have to follow closely
behind the drilling in order to prevent the collapse of the sides and the consequent weakening
of shaft friction. The boreholes must be kept topped up with water in order to avoid
‘blowing’ of the pile bottom as a result of the upward flow of the groundwater. This is
particularly necessary when drilling through water-bearing sand layers interbedded with
impervious clays.
Grabbing in weak rocks can cause large accumulations of slurry in the boreholes which
make it difficult to assess the required termination level of the pile in sound rock. The slurry
should be removed from time to time by baling or by airlift pump with a final cleaning-up
before placing the concrete.
The techniques of placing concrete in ‘dry’ holes or under water, are exactly the same as
described in Sections 3.4.6 and 3.4.8.
3.4.10 The installation of bored and cast-in-place piles
by tripod rigs
Pile boreholes in clays are drilled by a clay cutter operated from a tripod rig. Water should
not be poured down the hole to soften a stiff clay, or used to aid removal of the clay from
the cutter as this causes a reduction in shaft friction. When drilling in granular soils the
lining tubes should follow closely behind the drilling to avoid overbreak, and the addition of
water is needed to prevent ‘blowing’ and to facilitate the operation of the baler or shell.