Very rarely are structures founded on single piles. Normally, there will be a minimum of three piles
under a column or a foundation element because of alignment problems and inadvertent
eccentricities. The spacing of piles in a group depends upon many factors such as
1. overlapping of stresses of adjacent piles,
2. cost of foundation,
3. efficiency of the pile group.
The pressure isobars of a single pile with load Q acting on the top are shown in Fig. 1(a).
When piles are placed in a group, there is a possibility the pressure isobars of adjacent piles will overlap each other as shown in Fig. 1(b). The soil is highly stressed in the zones of overlapping of pressures. With sufficient overlap, either the soil will fail or the pile group will settle excessively since the combined pressure bulb extends to a considerable depth below the base of the piles. It is possible to avoid overlap by installing the piles further apart as shown in Fig. 1(c). Large spacings are not recommended sometimes, since this would result in a larger pile cap which would increase the cost of the foundation.
The spacing of piles depends upon the method of installing the piles and the type of soil. The piles can be driven piles or cast-in-situ piles. When the piles are driven there will be greater overlapping of stresses due to the displacement of soil. If the displacement of soil compacts the soil in between the piles as in the case of loose sandy soils, the piles may be placed at closer intervals.
Figure 1 Pressure isobars of (a) single pile, (b) group of piles, closely spaced,
and (c) group of piles with piles far apart.
But if the piles are driven into saturated clay or silty soils, the displaced soil will not compact the soil between the piles. As a result the soil between the piles may move upwards and in this process lift the pile cap. Greater spacing between piles is required in soils of this type to avoid lifting of piles. When piles are cast-in-situ, the soils adjacent to the piles are not stressed to that extent and as such smaller spacings are permitted.
Generally, the spacing for point bearing piles, such as piles founded on rock, can be much less than for friction piles since the high-point-bearing stresses and the superposition effect of overlap of the point stresses will most likely not overstress the underlying material nor cause excessive settlements.
The minimum allowable spacing of piles is usually stipulated in building codes. The spacings for straight uniform diameter piles may vary from 2 to 6 times the diameter of the shaft. For friction piles, the minimum spacing recommended is 3d where d is the diameter of the pile. For end bearing piles passing through relatively compressible strata, the spacing of piles shall not be less than 2.5d.
For end bearing piles passing through compressible strata and resting in stiff clay, the spacing may be increased to 3.5d. For compaction piles, the spacing may be Id. Typical arrangements of piles in groups are shown in Fig. 2.
Figure 2 Typical arrangements of piles in groups