7.8 The design of pile caps
A pile cap has the function of spreading the load from a compression or tension member
onto a group of piles so that, as far as possible, the load is shared equally between the piles.
The pile cap also accommodates deviations from the intended positions of piles, and by
rigidly connecting all the piles in one group by a massive block of concrete, the ill-effects
of one or more defective piles are overcome by redistributing the loads. The minimum
number of small diameter piles which is permitted in an isolatedpile cap is three. Caps for
single piles must be interconnected by ground beams in two directions, and for twin piles by
ground beams in a line transverse to the common axis of the pair.
A single large-diameter pile carrying a column does not necessarily require a cap. Any
weak concrete or laitance at the pile head can be cut away and the projecting reinforcing bars
bonded to the starter bars of the column reinforcement. Where a steel column is carried by
a single large-diameter pile, the concrete is cut down and roughened to key to the pedestal
beneath the column base. The heads of large-diameter piles are cast into the ground floor or
basement floor concrete in order to distribute the horizontal wind forces on the superstructure
to all the supporting piles.
To facilitate construction, ground beams should be arranged, where possible, to pass
across the tops of the pile caps and not to frame into the sides of the caps, the connection
between the cap and the ground beam being provided by column starter bars and by the
friction and bond between cap and beam. The concrete forming the caps may then be placed
in one operation and without the inconvenience and potential weakness that result from the
formation of pockets to receive the ground beams. If the beams must frame into the cap
sides, an alternative to providing pockets is to place the concrete in the caps in two
operations, a horizontal construction joint being formed in each cap at the level of the
underside of the ground beams.
Site setting out is also simplified by locating the ground beams on top of the pile caps.
The caps (and column starter bars, if required) can be constructed with reference to the min-
imum number of drawings, and fixed points on the site are then available for setting out the
formwork for the ground beams. Provision often has to be made for services to pass through
a foundation. If the ground beams are all situated on top of the pile caps, the routes of the
services are not obstructed by any pile caps, since the services may pass over the cap
through holes or sleeves left in the ground beams. The apparent economy in materials and
excavation gained by framing ground beams into the sides of pile caps can easily be lost by
the inconvenience it causes to other operations.
A deep cap is suitable for four piles, as shown in Figure 7.8. By adopting this arrange-
ment the column load is transferred directly into the pile heads in compression. The bend-
ing and shearing forces are negligible, requiring only the minimum proportion of steel in
two directions at the bottom of the cap. EC2 (Clause 9.8.1) requires the distance from the
outer edge of the pile to the edge of the pile cap to be sufficient to allow the tie forces in the
cap to be properly anchored. The form of the distribution of compressive force from the top
of the pile to the body of the cap is shown in Figure 7.9. The extent of the compressive zone
can be allowed for when determining the anchorage length of the main reinforcement. This
is most efficiently concentrated in the stressed zone between the tops of the piles (Figure
7.10). The minimum diameter of this reinforcement is required to be 8 mm. If the area of
main reinforcement distributed over the pile heads is equal to the area required by consid-
erations of control of cracking (Clause 7.3 of EC2) then evenly distributed bars along the
388 Structural design of piles and pile groups