Pile groups under compressive loading 279pressures developed within the centre of a large group of driven piles at Pickering Nuclear
Power Station, Ontario. The horizontal ground strains were also measured at various radial
distances from the centre. The group consisted of 750 piles driven within a circle about 46 m
in diameter. Steel H-section piles were selected to give a minimum of displacement of the
15 m of firm to very stiff and dense glacial till, through which the piles were driven to reach
bedrock. From measurements of the change in the distance between adjacent surface
markers it was calculated that the horizontal earth pressure at a point 1.5 m from the edge
of the group was 84 kN/m^2 while at 18.8 m from the edge the calculated pressure was only
1 kN/m^2. Earth pressure cells mounted behind a retaining wall 9 m from the group showed
no increase in earth pressure due to the pile driving. Very high pore pressures were developed
at the centre of the piled area, the increase being 138 kN/m^2 at a depth of 6 m, dissipating
to 41 kN/m^2 , 80 days after completing driving of the instrumented pile, when all pile driving
in the group had been completed.
The average ground heave of 114 mm measured over the piled area represented a volume
of soil displacement greater than the volume of steel piles which had been driven into the
soil, for which the theoretical ground heave was 108 mm.
Substantial heave accompanied by the lifting of piles already driven can occur with large
displacement piles. Brzezinski et al.(5.32)made measurements of the heave of 270 driven
and cast-in-place piles in a group supporting a 14-storey building in Quebec. The piles had
a shaft diameter of 406 mm and the bases were expanded by driving. The piles were driven
through 6.7 to 11 m of stiff clay to a very dense glacial till. Precautions against uplift were
Overall loading
100 kN/m^275 kN/m^275 kN/m^250 kN/m^250 kN/m^225 kN/m^225 kN/m^250 kN/m^275 kN/m^2Soft compressible clay 25 kN/m^2
becoming stiffer and less
compressible with
increasing depthContours of vertical stress(a) (b) (c)Figure 5.37Comparison of stress distribution beneath shallow raft foundation and beneath pile
groups (a) Shallow raft (b) Short friction piles (c) Long friction piles.