Pile Design and Construction Practice, Fifth edition

Types of pile 25
Table 2.5 BS 8004 requirements for longitudinal steel reinforcement, hoops, and links
in precast piles
Longitudinal steel Volume of steel at head Volume of steel in body Other requirements
and toe of pile of pile
To provide for 0.6% gross volume over 0.2% of gross volume Lapping of short bars
lifting, handling, and distance of 3 � pile spaced at not more with main reinforcement
superstructure loads width from each end than 1 ⁄2 � pile width to be arranged to avoid
and for tensile forces sudden discontinuity
caused by ground heave
BS EN 12794 states that longitudinal reinforcement shall be a minimum diameter of 8 mm
with at least one bar placed in the corner of square piles; circular section piles shall have at
least six bars placed around the periphery. Transverse reinforcement must be at least 4 mm
in diameter depending on the pile diameter and the pile head must have a minimum of nine
links in 500 mm. BS EN 12794 refers to BS EN 13369 for the quality of reinforcement and
prestressing steel to be used, which in turn refers to other ENs, such as BS EN 10080 and to
the national standards in the countries where the products are to be used. BS 4449 Steel for
reinforcement of concrete, has been revised for use with BS EN 10080. Notwithstanding the
new standards, users of reinforcing steel are advised to obtain third party certification such
as the CARES scheme in the UK.
The proportion of main reinforcing steel (Table 2.5) in the form of longitudinal bars is
determined by the bending moments induced when the pile is lifted from its casting bed to
the stacking area. The magnitude of the bending moments depends on the number and positioning
of the lifting points. Design data for various lifting conditions are dealt with in 7.2.
In some cases the size of the externally applied lateral or uplift loads may necessitate more
main steel than is required by lifting considerations. Lateral steel in the form of hoops and
links is provided to prevent shattering or splitting of the pile during driving. In hard driving
conditions it is advantageous to place additional lateral steel in the form of a helix at the
head of the pile. The helix should be about two pile widths in length with a pitch equal to
the spacing of the link steel at the head. It can have zero cover where the pile head is to be
cut down for bonding to the cap. A design for a precast concrete pile to comply with BS 8004
for easy driving conditions is shown in Figure 2.5a. A design for a longer octagonal pile suitable
for driving to end bearing on rock is shown in Figure 2.5b. The design of a prestressed
concrete pile in accordance with the recommendations of BS 8110 is shown in Figure 2.6.
Prestressed concrete piles have certain advantages over those of ordinary reinforced
concrete. Their principal advantage is in their higher strength to weight ratio, enabling long
slender units to be lifted and driven. However, slenderness is not always advantageous since
a large cross-sectional area may be needed to mobilize sufficient resistance in shaft friction
and end bearing. The second main advantage is the effect of the prestressing in closing up
cracks caused during handling and driving. This effect, combined with the high-quality
concrete necessary for economic employment of prestressing, gives the prestressed pile
increased durability which is advantageous in marine structures and corrosive soils.
Prestressed concrete piles should be made with designed concrete mixes of at least
C35/45, but as noted above, account should be taken of the special exposure conditions
quoted in EC2-1-1 and BS 8500 when deciding on the concrete class to be used. Minimum
percentages of prestressing steel stipulated in BS EN 12794 are 0.1% of cross-sectional area