Pile groups under compressive loading 287The weight of the building (including imposed load but excluding wind load) was
calculated to be 228 MN. The weight of soil removed when excavating through gravel on to
the stiff London clay at raft level was 107 MN, giving a net load to be transferred by the raft
and piles to the London clay of 121 MN, or a net bearing pressure at raft level of 196 kN/m^2.
Load cells were installed in three of the piles to measure the load transferred from the raft
to the pile shaft, and three earth pressure cells were placed between the raft and the soil to
measure the contact pressures developed at this interface. Settlements of the raft at various
points were also measured by means of levelling points installed at ground level.
The observations of pile loadings and contact pressures were used to estimate the
proportion of the total load carried by the piles and the basement raft from the initial stages
of construction up to 3 years after completing the building. The results of these calculations
are shown in Figure 5.41b and are compared with the calculated total weight of the building
at the various stages of construction. Hooper(5.35)estimated that at the end of construction
60% of the building load was carried by the piles and 40% by the underside of the raft. In
the post-construction period there was a continuing trend towards the slow transfer of more
load to the piles, about 6% of the total downward structural load being transferred to the
piles in the three-year period.
5.10 The optimization of pile groups to reduce
differential settlements in clay
Cooke et al.(5.36)measured the proportion of load shared between the piles and raft and also
the distribution of load to selected piles in different parts of a 43.3 m by 19.2 m piled raft
supporting a 16-storey building in London Clay at Stonebridge Park. There were 351 piles
in the group with a diameter of 0.45 m and a length of 13 m. The piles were uniformly
spaced on a 1.6 m square grid. The overall loading on the pile group was about 200 kN/m^2
At the end of construction the piles carried 78% of the total building load, the remainder
being carried by the raft. The distribution of the load to selected piles near the centre, at the
edges, and at the corners of the group is shown in Figure 5.42. It will be seen that the loads
carried by the corner and edge piles were much higher than those on the centre piles. The
loading was distributed in the ratio 2.2:1.4:1 for the corner, edge, and centre respectively.
Advantages can be taken of the load sharing between raft and piles and between various
piles in a group to optimize the load sharing whereby differential settlement is minimized
and economies obtained in the design of the structural frame and in the penetration depth
and /or diameter of the piles (Section 5.3). The procedure in optimization is described by
Padfield and Sharrock(5.37). Central piles are influenced by a larger number of adjacent piles
than those at the edges. Hence, they settle to a greater extent and produce the characteristic
dished settlement. Therefore, if longer stiffer piles are provided at the centre they will attract
a higher proportion of the load. The outer piles are shorter and thus less stiff and will yield
and settle more, thus reducing the differential settlement across the group. The alternative
method of varying the settlement response to load is to vary the cross-sectional dimensions.
The centre piles are made long with straight shafts and mobilize the whole of their bearing
capacity in shaft friction at a settlement of between 10 and 15 mm. The shorter outer piles
can be provided with enlarged bases which require a greater settlement to mobilize the total
ultimate bearing capacity (see Section 4.6). An example of this is given by Burland
and Kalra(5.17).