Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 281
Detrimental effects from heave are not usually experienced when driving piles in
groups in coarse soils. A loose soil is densified, thus requiring imported filling to make
up the subsided ground surface within and around the group. Adjacent structures may be
damaged if they are within the area of subsidence. A problem can arise when the first piles
to be installed drive easily through a loose sand but, as more piles are driven, the sand
becomes denser thus preventing the full penetration of all the remaining piles. This problem
can be avoided by paying attention to the order of driving, as described in Section 5.8.
Subsidence due to the loss of ground within and around a group in a coarse soil can be
quite severe when bored and cast-in-place piles are installed, particularly when ‘shelling’ is
used as the boring method (see Section 3.3.7). The subsidence can be very much reduced, if
not entirely eliminated, by the use of rotary drilling with the assistance of a bentonite slurry
(see Section 3.3.8).
5.8 Precautions against heave effects in pile groups
It will have been noted from Section 5.7 that the principal problems with soil heave and the
uplift of piles occur when large displacement piles are driven into clay. In coarse soils
the problems can be overcome to a great extent by using small displacement piles such as
H-sections or open-ended steel tubes. To adopt a spacing between piles of 10 or more diameters
is not usually practical if pile group dimensions are to be kept within economical
limits. Pre-boring the pile shaft is not always effective unless the pre-bored hole is taken
down to the pile base, in which case the shaft friction will be substantially reduced if not
entirely eliminated. Jetting piles is only effective in a coarse soil and the problems associated
with this method are described in Section 3.1.9. The most effective method is to re-drive any
risen piles, after driving all the piles in a cluster that are separated from adjacent piles by at
least 12 diameters has been completed.
In the case of driven and cast-in-place piles, a permanent casing should be used and the
re-driving of the risen casing and pile base should be effected by tapping the permanent
casing with a 3-tonne hammer, as described by Brzezinski et al. (5.32) Alternatively, the
‘Multitube’ method described by Cole (5.33) can be used. This consists of providing sufficient
lengths of withdrawable casing to enable all the piling tubes to be driven to their full depth
and all the pile bases to be formed before the pile shafts in any given cluster are concreted.
An individual cluster dealt with in this way must be separated from a neighbouring cluster
by a sufficient distance to prevent the uplift of neighbouring piles or to reduce this to an
acceptable amount. On the three sites described by Cole it was found possible to drive piles
to within 6.5 diameters of adjacent clusters without causing an uplift of more than 3 mm to
the latter. This movement was not regarded as detrimental to the load/settlement behaviour.
Cole stated that, although the ‘Multitube’ system required eight driving tubes to each piling
rig, the cost did not exceed that of an additional 2 m on each pile.
Curtis (3.26) states that it is possible to re-drive risen driven and cast in-situ piles using a
3-to 4-tonne hammer with a drop not exceeding 1.5 m. The head of the pile should be
protected by casting on a 0.6 m capping cube in rapid-hardening cement concrete.
Cole (5.33) stated that the order of driving piles did not affect the incidence of risen piles
but it did change the degree of uplift on any given pile in a group. Generally, the aim should
be to work progressively outwards or across a group, and in the case of an elongated group
from end to end or from the middle outwards in both directions. This procedure is particularly
important when driving piles in coarse soils. If piles are driven from the perimeter towards