accompanied by settlement of the ground surface around the pile borehole. Rocks are drilled
by chiselling and using a baler to raise the debris.
Tripod rigs are not as suitable as the spiral-plate or continuous auger types for drilling
small-diameter piles in clays, except in situations where low headroom or difficult access
would prevent the deployment of lorry-mounted or track-mounted augers. Methods of
operating tripod rigs have been described by North-Lewis and Scott(3.12).
3.3.8 Drilling for piles with bentonite slurry
Lining tubes or casings to support the sides of pile boreholes are a requirement for most of
the bored-pile installation methods using equipment described in Sections 3.3.1 to 3.3.7.
Even in stiff fine-grained soils it is desirable to use casings for support since these soils are
frequently fissured or may contain pockets of sand which can collapse into the boreholes,
resulting in accumulations of loose soil at the pile toe, or discontinuities in the shaft.
Casings may be avoided by providing support to the pile borehole in the form of a slurry
of bentonite clay. However, BSEN 1536 requires that an excavation under support fluid shall
be protected by a lead-in tube or guide wall (for a barrette); a length of casing will also be
required to carry a pile-top rig. Bentonite, or other montmorillonite clay with similar
characteristics, has the property of remaining in suspension in water to form a stiff ‘gel’
when allowed to become static. When agitated by stirring or pumping, however, it has a
mobile fluid consistency – i.e. it is ‘thixotropic’. In a granular soil, the slurry penetrates the
walls of the borehole and gels there to form a strong and stable ‘filter-cake’. In a clay soil
there is no penetration of the slurry but the hydrostatic pressure of the fluid, which has a
density of around 1 040 kg/m^3 , prevents collapse where the soil is weakened by fissures.
Recommended rheological characteristics are provided in BSEN 1536.
When used in conjunction with auger or grab-type rigs the slurry is maintained in a state
of agitation by the rotating or vertical motion of the drilling tools. When it becomes heavily
contaminated with soil or diluted by groundwater it can be replaced by pumping-in fresh or
reconditioned slurry. Toothed or bladed augers with double helix configurations and a flap
in the carriage area help to retain spoil as the auger is withdrawn through the bentonite.
Bentonite slurry is used most efficiently in conjunction with reverse-circulation rigs (see
Section 3.3.5). The slurry is pumped into the outer annulus and the slurry–soil mixture that
is discharged from the airlift riser pipe is allowed to settle in lagoons or tanks to remove soil
particles. It is then further cleaned in a cyclone, and chemicals to aid gelling are added
before the reconditioned slurry is pumped into a holding tank and then returned to the pile
borehole.
Bentonite slurry is used in a simple and rather crude way in conjunction with rotary auger
equipment when drilling pile boreholes through sands and gravels to obtain deeper penetration
into stiff fine-grained soils. The hole must be augered through the sands and gravels without
support, and then the casing is lowered down. It is uneconomical to provide screwed joints
in large-diameter lining tubes and all joints are made by welding. To save time and cost in
welding, the holes are drilled to the maximum possible depth before installing the first
length of casing. In these conditions support may be provided while drilling by means of a
bentonite slurry. Where the depth of coarse-grained soil is relatively small it is uneconomical
to bring in high-speed mixers, slurry tanks, pumps and reconditioning plant for the normal
employment of bentonite techniques. Instead, a few bags of the dry bentonite are dumped
into the pile borehole and mixed with the groundwater or by adding water to form a crude
120 Piling equipment and methods