Pile Design and Construction Practice, Fifth edition

Types of pile 31

Timber bearers should be placed between the piles in the stacks to allow air to circulate

around them. They should be protected against too-rapid drying in hot weather by covering

the stack with a tarpaulin or polyethylene sheeting. Care must be taken to place the bearers

only at the lifting positions. If they are misplaced there could be a risk of excessive bending

stresses developing and cracking occurring, as shown in Figure 2.12.

Prestressed concrete piles of hollow cylindrical section are manufactured by centrifugal

spinning in diameters ranging from 400 to 1800 mm in lengths up to 40 m.

The precautions for driving precast concrete piles are described in Section 3.4.2, and the

procedures for bonding piles to caps and ground beams and lengthening piles are described

in Sections 7.6 and 7.7.

One of the principal problems associated with precast concrete piles is unseen breakage

due to hard driving conditions. These conditions are experienced in Sweden where the

widely used jointed or unjointed precast concrete piles are driven through soft or loose soils

onto hard rock. On some sites the rock surface may slope steeply, causing the piles to deviate

from a true line and break into short sections near the toe. Accumulations of boulders over

bedrock can also cause the piles to be deflected with consequent breakage. Because of these

experiences the Swedish piling code recommended the provision of a central inspection hole

in test piles and sometimes in a proportion of the working piles. A check for deviation of the

pile from line can be made by lowering a steel tube down the hole. If the tube can be lowered

to the bottom of the hole under its own weight the pile should not be bent to a radius which

would impair its structural integrity. If the tube jams in the hole, it is the usual practice to

bring an inclinometer to the site to record the actual deviation, and hence to decide whether

or not the pile should be rejected and replaced. The testing tube also detects deviations in

the position or alignment of a jointed pile.

Breakages are due either to tensile forces caused by driving with too light a hammer in

soft or loose soils, or to compressive forces caused by driving with too great a hammer drop

on to a pile seated on a hard stratum; in both cases the damage occurs in the buried portion

of the pile. In the case of compression failure it occurs by crushing or splitting near the pile toe.

Such damage is not indicated by any form of cracking in the undriven portion of the pile above

ground level. The provision of a central test hole will again enable crushing of the pile due

to failure in compression to be detected.

2.2.3 Jointed precast concrete piles

The disadvantages of having to adjust the lengths of precast concrete piles either by cutting

off the surplus or casting on additional lengths to accommodate variations in the depth to a

Cracks in pile due to excessive

bending stresses

Correctly placed

Lifting holes bearers

Misplaced bearers

Figure 2.12Misplaced packing in stacks of precast concrete piles.