Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 285

boring. When the superstructure load is applied to the piles they compress, but the soil follows
the pile movement, and any soil pressures developed on the underside of the basement slab
are relatively small. Hydrostatic pressure occurs in a water-bearing soil.
In all cases when designing piled basements the full design working load should be
considered as acting on the piles and, in the case of piles bearing on rock or granular
soils of low compressibility, the load on the underside of the basement slab can be limited
to that caused by the soil pressure (i.e. the overburden pressure measured from the ground
surface around the basement) and hydrostatic pressure. Sometimes a tall building is
constructed close to a low-rise podium (Figure 5.40) and both structures are provided with
a piled basement. Piling beneath the podium is required to reduce differential movement
between the heavily loaded tower block and the podium. Uplift of the latter may occur if the
weight of the superstructure is less that that of the soil removed in excavating for the base-
ment. In such a case the piles must be anchored below the zone of soil swelling and designed
to take or eliminate tension. The pressure on the underside of the podium basement slab
will be equal to the swelling pressure exerted by the soil unless a void former is used to
eliminate the pressure. A vertical movement joint passing completely through the basement
and superstructure should be provided between the tower and podium to allow freedom
of movement.

Tower block

Podium

Piles carring

net uplift

loads

Piles carring net

compressive loads

Movement joint

Basement

Figure 5.40Tower block and podium supported by piled basement.