Pile Design and Construction Practice, Fifth edition

28 Types of pile

employed. The 89 mm point was machined concave to 12.7 mm depth and embedded in a
chilled cast-iron shoe. Flame treatment of the point was needed after casting into the shoe
to restore the hardness lost during this operation.
Piles may be cast on mass concrete beds using removable side forms of timber or steel
(Figure 2.8). The reinforcing cage is suspended from bearers with spacing forks to maintain
alignment. Spacer blocks to maintain cover are undesirable. The stop ends must be set truly
square with the pile axis to ensure an even distribution of the hammer blow during driving.
Vibrators are used to obtain thorough compaction of the concrete and the concrete between the
steel and the forms should be worked with a slicing tool to eliminate honeycombed patches.
The casting beds must be sited on firm ground in order to prevent bending of the piles during
and soon after casting. After removing the side forms the piles already cast may be used as side
forms for casting another set of piles in between them. If this is done the side forms should be
set to give a trapezoidal cross-section in order to facilitate release. Piles may also be cast in
tiers on top of each other, but a space between them should be maintained to allow air to
circulate (Figure 2.9). Casting in tiers involves a risk of distortion of the piles due to settlement
of the stacks. In addition, the piles which are first to be cast are the last to be lifted which is in
the wrong order, since the most-mature piles should be the first to be lifted and driven.
Where piles are made in a factory, permanent casting beds can be formed in reinforced
concrete with heating elements embedded in them to allow a 24-hour cycle of casting and
lifting from the moulds. This method of construction was used by Soil Mechanics Ltd. to
cast prestressed concrete piles at Drax Power Station in Yorkshire(2.7)where the large num-
ber of piles cast (18 500) justified the establishment on site of an elaborate casting yard such
as would be used in a precast concrete factory. The reinforced concrete formwork is shown
in Figure 2.10. This type, which does not have removable side forms, necessitates the
embedment of lifting plugs or loops into the top of the piles.
The layout of the casting yard at Drax is shown in Figure 2.11. The strand reels were set
on carriers at one end of the four rows of casting beds, with the winches for tensioning the
strand at the opposite end. Each casting bed had five lines of forms. The provision of electric
heating elements enabled the concrete to achieve its release strength of 27.6 N/mm^2 in 40 to
48 hours. An average of 300 piles per week, with a peak of 400 in a week, were manufactured.
Two coats of whitewash were used as a release agent, as it was found that mould oil did not

Spacer bolts

Spacer

blocks

Concrete casting bed

Steel angle

spacers

Steel angle bearer

Timber

sheathing

Timber fillet

Holding down

bolt

Folding

wedges

Figure 2.8Timber formwork for precast concrete piles.