Pile Design and Construction Practice, Fifth edition

Chapter 7
Some aspects of the structural
design of piles and pile groups
7.1 General design requirements
Piles must be designed to withstand stresses caused during their installation, and subsequently
when they function as supporting members in a foundation structure. Stresses due to
installation occur only in the case of piles driven as preformed elements. Such piles must be
capable of withstanding bending stresses when they are lifted from their fabrication bed and
pitched in the piling rig. They are then subjected to compressive, and sometimes to tensile,
stresses as they are being driven into the ground, and may also suffer bending stresses if they
deviate from their true alignment. Piles of all types may be subjected to bending stresses
caused by eccentric loading, either as a designed loading condition or as a result of the pile
heads deviating from their intended positions. Differential settlement between adjacent piles
or pile groups can induce bending moments near the pile heads as a result of distortion of
the pile caps or connecting beams.
The working stresses adopted for piles should take into account the effects of unseen
breakage caused during driving, possible imperfections in concrete cast in-situ, and the
long-term effects of corrosion or biological decay.
Pile caps, capping beams, and ground beams are designed to transfer loading from the
superstructure to the heads of the piles, and to withstand pressures from the soil beneath and
on the sides of the capping members. These soil pressures can be caused by settlement of
the piles, by swelling of the soil, and by the passive resistances resulting from lateral loads
transmitted to the pile caps from the superstructure.
In addition to guidance on structural design and detailing, matters of relevance to the
design of piled foundations in EC2 (BS EN 1992-1-1: 2004) include the following:
(1) Dimensional tolerances of cast-in-place piles (see Table 4.9)
(2) Partial factors for the ultimate limit state of materials
(3) The influence of soil–structure interaction caused by differential settlement
(4) Strength classes of concrete and reinforcement cover for various exposure conditions
(5) Slenderness and effective lengths of isolated members
(6) Punching shear and reinforcement in pile caps
(7) Limits for crack widths and
(8) Minimum reinforcement for bored piles.
Many of the above items have been dealt with in the previous chapters. Structural analysis,
design and detailing of reinforced concrete and prestressed concrete members will not,