slurry which is adequate to smear the wall of the borehole and give it the necessary short-term
support. After drilling with this support through the granular overburden, the casing is lowered
in one or more lengths and pushed down to seal it into the stiff fine-grained soil below. The
thrust is provided either by the hydraulically operated crowd mechanism on the kelly-bar of
the drilling machine or by means of a vibrator (see Section 3.1.5) mounted on the casing.
This technique is known as ‘mudding-in’the casing.
The use of a bentonite slurry to aid drilling with or without temporary lining tubes may
cause some difficulties when placing concrete in the pile. The nature of these problems and
the means of overcoming them are described in Section 3.4.8, and the effects of a bentonite
slurry on shaft friction and end-bearing resistance of piles are discussed in Sections 4.2.3
and 4.3.6. For example, if the slurry becomes overloaded with solids from the excavation, a
thick filter cake will be formed and may not be removed by scouring during concreting. In
such cases it may be necessary to use a mechanical scraper to remove the excess filter cake
prior to concreting. Reese et al.(3.13)recommend a minimum diameter of 600 mm for piles
installed using slurry techniques, to avoid some of the problems associated with the method.
Polymer support fluids are more expensive than bentonite as an initial cost, but because
they can be recycled without the frequent de-sanding required for bentonite, polymers can be
economical for use on large projects and congested sites. Also the filter cake is much thinner
and more easily scoured when placing concrete. When used for piling work on land or in river
works, waste bentonite slurry has to be treated as ‘hazardous’under pollution control regula-
tions and disposed of accordingly, whereas polymers can be neutralized and, subject to
desanding and approval from the water company, can be disposed of to existing drains.
3.3.9 Base and shaft grouting of bored and cast-in-place piles
When bored and cast-in-place piles are installed in granular soils, the drilling operation may
loosen the soil surrounding the shaft and beneath the base of the pile borehole. Such loos-
ening below the base can cause excessive working load settlements when the majority of the
load is carried by end bearing. Base grouting is a means of restoring the original in-situ
density and reducing settlements. Bolognesi and Moretto(3.14)described the use of stage
grouting to compress the soil beneath the toes of 1.00 to 2.00 m bored piles supporting two
bridges over the Parana River in Brazil, the piles being drilled with the aid of a bentonite
slurry. The soil beneath the pile toes loosened by the drilling operations was subjected to a
grouting pressure of up to 10 MN/m^2. The cement grout was introduced through a cylindrical
metal basket pierced by a number of holes and filled with uniform gravel (Figure 3.37). The
basket, with its upper surface covered by a rubber sheet, was lowered into the borehole
suspended from the pile reinforcing cage. The pile was then concreted, followed by the
injection of the grout into the basket through a 38 mm pipe set in the concrete of the shaft.
The uplift caused by the grouting pressure was usually resisted by the shaft friction in the
pile shaft, but in some cases the pile cap was constructed to provide additional dead-load
resistance. Although Bolognesi and Moretto did not mention any weakening at the pile toe
caused by the entrapment of bentonite slurry, as described by Reese et al.(3.13), the stage-
grouting technique would be a useful method of expelling any slurry from beneath the toe
of a pile.
The ‘flat-jack’method of pressure grouting to compact soil beneath the base of a bored
pile is similar. After completing the drilling, which can be performed underwater in
favourable conditions, the reinforcing cage with a circular plate welded to the base is
Piling equipment and methods 121