Basic piling methods - Piling Engineering - page 31

Some micro-piles are also included in this category. A summary of the main types of
pre-formed displacement pile is given below.

Totally pre-formed displacement piles

(a) Pre-cast concrete : full length reinforced (pre-stressed)

: jointed (reinforced)
: hollow (tubular) section

(b) steel : 'H' section

: tubular section
: other (e.g. screw-form piles)

3.2.2 Pre-cast reinforced concrete piles

Pre-cast concrete piles are now usually of the jointed type, unless a large contract with
a more or less constant depth of piling makes it economical to pre-cast the piles on
site, thus overcoming a potential difficulty in transport. Pre-cast non-jointed piles are
generally of square section and may be up to 600

×

600mm to work at loads up to
approximetely 3000 kN in suitable ground. Typical sizes and capacities are given in
Table 3.1. Extending pre-cast piles that do not have pre-formed joints is a lengthy
process, involving breaking down the projecting pile head to provide a suitable lap
for the steel and casting concrete to form a joining surface. The pile sections are
then butted together in a steel sleeve using an epoxy cement, or joined by inserting
steel dowel bars into drilled holes and using an epoxy cement to fix them in place.
Good alignment of the pile sections is required to prevent excessive bending stresses
developing on subsequent re-driving.
There are some benefits from pre-stressing concrete piles. Tensile stresses which can
be set up in a pile during driving are better resisted, and the pile is less likely to be
damaged during handling in the casting yard and when being pitched. Bending stresses
which can occur during driving are also less likely to produce cracking. However, the
ultimate strength in axial compression is decreased as the level of pre-stress is increased,
and pre-stressed piles are therefore more vulnerable to damage from striking obstruc-
tions during driving. They are also difficult to shorten and special techniques have to
be employed. As a result they are most suitable for a constant-length application.

Table 3.1 Typical load capacities for pre-cast
concrete piles for normal driving conditions

Square pile
section (mm)

Typical working
capacity (kN)

200 × 200 400

250 × 250 500

275 × 275 750

300 × 300 900

350 × 350 1200

Note
Characteristic strength of concrete 50N/mm 2 at
28 days. Piles driven witha4to5tonne hammer.
Courtesy of Teranto De Pol.