280 Pile groups under compressive loading
taken by providing a permanent casing to the piles and the concrete was not placed in the
shafts until the pile bases had been re-driven by tapping with a drop hammer to the extent
necessary to overcome the effects of uplift. The measured heave of a cross-section of the
piled area is shown in Figure 5.38. It was found that the soil heave caused the permanent
casing to become detached from the bases, as much as 300 mm of separation being
observed. Heave effects were not observed if the piles were driven at a spacing wider than
12 diameters. This agrees with the curves established by Chow and Teh(5.29)which show a
pile head heave of only about 1 mm for a spacing of 12 diameters.
Similar effects were observed by Cole(5.33). At three sites the heave was negligible at pile
spacings wider than 8 to 10 diameters. Cole observed that uplift was more a function of
the pile diameter and spacing than of the soil type or pile length. Where piles carry their load
mainly in end bearing, the effect of uplift is most damaging to their performance and on all
sites where soil displacement is liable to cause uplift, precautions must be taken as described
in Section 5.8. Heave is not necessarily detrimental where piles are carried by shaft friction
in firm to stiff clays in which there will be no appreciable subsidence of the heaved soil to
cause negative skin friction to develop on the pile shaft. On a site where a 12-storey block
of flats was supported by driven and cast in-situ piles installed in 5 m of firm London Clay
to terminate at the base of a 4 m layer of stiff London clay, about 0.5 m of heave was
observed in the ground surface after 70 piles had been driven within the 2420 m area of
the block. A pile was tested in an area where 220 mm of heave had occurred. The settlement
at 1300 kN (i.e. twice the working load) was 23 mm, while the settlement at the working
load was only 2.5 mm.
Heaving and the development of high pore pressures do not occur when bored and cast-
in-place piles are installed in groups. However, general subsidence around the piled area can
be caused by the ‘draw’or relaxation of the ground during boring. In soft sensitive clays the
bottom of a pile borehole can heave up due to ‘piping’, with a considerable loss of ground.
These effects can be minimized by keeping the pile borehole full of water or bentonite slurry
during drilling and by placing the concrete within a casing which is only withdrawn after all
concrete placing is completed.
400Number of piles(^3) in group 13 12
Glacial
till
Clay
12 13 3
300
200
100
0
500
250
0
0
2.5
5.0
7.5
Penetration in m10.0
Estimatedground heavein mm
Movement of top of piling tubein mm
Net heave after retapping
Maximum heave
after pile driving
Figure 5.38Observations of heave due to pile driving in clay (after Brzezinski et al.(5.32)).