Pile groups under compressive loading 283the piles carry any of their designed loading. The uplift pressure on the basement slab will
be greater if bored piles are used since no heaving of the soil is caused by installing the piles,
and if the basement slab is completed and attached to the piles soon after completing
the excavation, the swelling pressures on the underside of the slab will cause tension to be
developed in the piles. This is particularly liable to happen where bored piles are installed
from the ground surface before the excavation for the basement commences. Concreting
of the pile shaft is terminated at the level of the underside of the basement slab and the
construction of the basement slab usually takes place immediately after the completion of
excavation and before any heave of the excavation can take place to relieve the swelling pres-
sure. Generally, in any piled basement where bored pilesare installed wholly in compressible
clay, the basement slab should be designed to withstand an uplift pressure equal at least to
one-half of the dead and sustained imposed load of the superstructure. Alternatively, a void
can be provided beneath the basement slab by means of collapsible cardboard or plastics
formers. The piles can be designed to be anchored against uplift or they can be sleeved over
the zone of swelling. Anchoring the piles against uplift by increasing the shaft length to
increase shaft friction below the swelling zone is often the most economical solution to the
problem. Where void formers made of cardboard or plastics are used to eliminate swelling
pressure beneath the basement slab, there is a risk of bio-degradation of the organic materials
causing an accumulation of methane gas in the void. Venting the underside of the slab can
be difficult and costly.
Providing an increased shaft length can be made more economical than sleeving the pile
shaft within the swelling zone. Fleming and Powderham(5.34)recommended that where piles
are reinforced to restrain uplift the friction forces should not be underestimated and they
suggest that if the forces are estimated conservatively it would be appropriate to reduce the
load factors on the steel, perhaps to about 1.1.
(a) (b) (c) (d)Figure 5.39Piled basements in various ground conditions (a) Wholly in compressible clay
(b) Compressible clay over bedrock (c) Soft clay over stiff clay (d) Loose sand becoming
denser with depth.