282 Pile groups under compressive loading
the centre of a group, a coarse-grained soil will ‘tighten-up’so much due to ground
vibrations that it will be found impossible to drive the interior piles.
It is desirable to adopt systematic monitoring of the behaviour of all piles installed in groups
by taking check levels on the pile heads, by carrying out re-driving tests, and by making
loading tests on working piles selected at random from within the groups. Loading tests
undertaken on isolated piles before the main pile driving commences give no indication of
the possible detrimental effects of heave. Lateral movements should also be monitored
as necessary.
5.9 Pile groups beneath basements
Basements may be required beneath a building for their functional purpose, for example, as
an underground car park or for storage. The provision of a basement can be advantageous in
reducing the loading which is applied to the soil by the building. For example, if a basement
is constructed in an excavation 7 m deep the soil at foundation level is relieved of a pressure
equivalent to 7 m of overburden, and the gross loading imposed by the building is reduced
by this amount of pressure relief. It is thus possible to relieve completely the net loading
on the soil. An approximate guide to the required depth of excavation is the fact that a
multistorey dwelling block in reinforced concrete with brick and concrete external walls,
lightweight concrete partition walls, and plastered finishes weighs about 12.5 kN/m^2 per
storey. This loading is inclusive of 100% of the dead load and 60% of the design imposed
load. Thus a 20-storey building would weigh 250 kN/m^2 at ground level, requiring a
basement to be excavated to a depth of about 20 m to balance the loading (assuming the
groundwater level to be 3 m below ground level and taking the submerged density of the soil
below water level).
Deep basement excavations in soft compressible soils can cause considerable constructional
problems due to heave, instability and the settlement of the surrounding ground surface.
Because of this it may be desirable to adopt only a partial relief of loading by excavating a
basement to a moderate depth and then carrying the net loading on piles taken down to soil
having a lesser compressibility.
In all cases where piles are installed to support structures it is necessary to consider the
effects of soil swelling and heave on the transfer of load from the basement floor slab to
the piles. Four cases can be considered as described below and shown in Figure 5.39.
Piles wholly in compressible clay (Figure 5.39a)
In this case the soil initially heaves due to swelling consequent on excavating the foundation,
and further heave results from pile driving. The heaved soil is then trimmed off to the correct
level and the basement slab concreted. If the concreting is undertaken within a few days or
a week after the pile driving there is a tendency for the heaved soil to slump down, particu-
larly in a soft clay which developed high pore pressures. A space may tend to open between
the underside of the concrete and the soil surface. When the superstructure is erected the
piles will carry their working load and if correctly designed they will settle to an acceptable
degree. This will in turn cause the basement slab to settle but pressure will not develop on
its underside because the soil within and beneath the settling piles will move down with
them. Thus the maximum pressure on the underside of the basement slab is due to the soil
swelling at an early stage before partial slumping of the heaved soil takes place and before