Pile Design and Construction Practice, Fifth edition

Types of pile 59
The simplest form of construction consists of drilling an unlined hole and filling it with
concrete. However, complications may arise such as difficult ground conditions, the presence
of groundwater, or restricted access. Such complications have led to the development of
specialist piling plant for drilling holes and handling lining tubes, but unlike the driven and
cast-in-place piles, very few proprietary piling systems have been promoted. This is because
the specialist drilling machines are available on sale or hire to any organization which may
have occasion to use them. The resulting pile as formed in the ground is more or less the
same no matter which machine, or method of using the machine, is employed. There have
been proprietary systems such as the Prestcore pile, which incorporates precast units
installed in the pile borehole, but these methods are largely obsolete.
There are two principal types of replacement pile. These are bored and cast-in-place piles,
and drilled-in tubular (including caisson) piles. A general description of the two types now
follows. Mechanical plant for installing the piles and methods of construction are described
in Section 3.3.
2.4.2 Bored and cast-in-place piles
In stable ground an unlined hole can be drilled by hand or mechanical auger. If reinforcement
is required, a light cage is then placed in the hole, followed by the concrete. In loose or
water-bearing soils and in broken rocks casing is needed to support the sides of the borehole,
this casing being withdrawn during or after placing the concrete. In stiff to hard clays
and in weak rocks an enlarged base can be formed to increase the end-bearing resistance of
the piles (Figure 2.30). The enlargement is formed by a rotating expanding tool. Hand
excavation is now uneconomic because of stringent statutory health and safety regulations,
even in piles with a large shaft diameter. A sufficient cover of stable fine-grained soil must
be left over the top of the enlargement in order to avoid a ‘run’ of loose or weak soil into the
unlined cavity, as shown in Figure 2.30.
Bored piles drilled by hand auger are limited in diameter to about 355 mm and in depth
to about 5 m. They can be used for light buildings such as dwelling houses, but even for
these light structures hand methods are used only in situations where mechanical augers, as
described in Section 3.3.1, are not available.
60°
Unstable soil
1 m (3.3 ft)
Stable soil
Figure 2.30 Under-reamed base enlargement to a bored and cast-in-place pile.