Hydrostatic pressure will, of course, act on the basement slab in water-bearing soil. The
piles must be designed to carry the net full weight of the structure (i.e. the total weight less
the weight of soil and soil water excavated from the basement).
When installing piles for ‘top-down’construction the empty borehole from the pile head
below the lowest basement floor slab up to the ground surface is supported by temporary
casing. Steel columns are set on the pile heads and are surrounded with sand before the casing
is extracted. Particular care is necessary in establishing the position of the pile borehole and
maintaining verticality in drilling. If this is not done there could be considerable error in the
position of the pile head and difficulties in locating the column in the design position.
The tolerance in pile position permitted by the Institution of Civil Engineers’Specification
for Piling(2.5)is 75 mm and the permitted deviation from the vertical of a bored pile is 1 in 75.
(see also Section 3.4.12) Taking as an example a 3-storey basement with an overall depth
from ground surface to pile head level (beneath the lowest floor slab) of 15 m the centre of
the pile could be 275 mm from the design position. Consequently, either the column set in
its intended position in the superstructure would be off-plumb or the column would apply an
eccentric load to the pile head. The permitted tolerances should be kept in mind when con-
sidering the pile diameter and the design of reinforcement to provide for eccentric loading.
Piles driven through compressible clay to
bedrock (Figure 5.39b)
In this case soil swelling takes place at the base of the excavation followed by heave if driven
piles are employed. As before, the heaved soil tends to slump away from the underside of
the basement slab if the latter is concreted soon after pile driving. Any gap which might form
will be permanent since the piles will not settle except due to a very small elastic shortening
of the shaft. If bored piles are adopted, with a long delay between concreting the base slab
and applying the superstructure loading to the piles, the pressure of the underside of the
slab due to long-term soil swelling might be sufficient to cause the piles to lift from their
seating on the rock. The remedy then is to provide a void beneath the slab, and to anchor the
piles to rock or to sleeve them through the swelling zone.
Piles driven through soft clay into stiff clay
(Figure 5.39c)
This case is intermediate between the first two. There is a continuing tendency for the heaved
soft clay to settle away from the underside of the basement slab, because the settlement of the
piles taking their bearing in the stiff clay is less than that caused by the reconsolidation of the
heaved and disturbed soft clay. Uplift pressure occurs on the underside of the base slab if
bored piles are used, and a design value equal at least to one-half of the combined dead and
imposed load of the superstructure should be considered. Alternatively, the effects of heave
should be eliminated as described above.
Piles driven into loose sand (Figure 5.39d)
In this case it is presumed that the piles are driven through loose sand to an end bearing in
deeper and denser sand. The slight heave of the soil caused by excavating the basement is
an instantaneous elastic movement. No heave occurs because either pile driving causes some
settlement of the ground surface due to densification, or a loss of ground results due to pile
284 Pile groups under compressive loading