Pile Design and Construction Practice, Fifth edition

174 Resistance of piles to compressive loads
End-bearing capacity (MN/m 2 )/Embedded length (m)
2.0
1.5
1.0
0.5
500
Pile diameter (mm)
1000 1500
Figure 4.16 Reduction in end-bearing capacity of open-end piles driven into sand due to
increase in diameter (after Hight et al.(4.24)).
the arching capacity is related principally to the pile diameter. Clearly it is not related to the
soil density because the soil forming the plug is compacted by the pile driving. The estimated
ultimate bearing resistances of sand-plugged piles obtained from published and unpublished
sources have been plotted against the pile diameters by Hight et al. (4.24) Approximate upper
and lower limits of the plotted points are shown in Figure 4.16. In most cases the piles were
driven into dense or very dense soils and the test evidence pointed clearly to failure within
the plug and not to yielding of the soil beneath the pile toe.
4.3.4 Driven and cast-in-place piles in coarse-grained soils
Both the base resistance and shaft friction of driven and cast in-situ piles can be calculated
in the same way as described for driven piles in the preceding section. The installation of
driven and cast in-situ types does not loosen the soil beneath the base in any way, and if there
is some loosening of the soil around the shaft as the driving tube is pulled out the original
state of density is restored, if not exceeded, as the concrete is rammed or vibrated into place
while pulling out the tube. Loosening around the shaft must be allowed for if no positive
means are provided for this operation. The provision of an enlarged base adds considerably
to the end-bearing resistance of these piles in loose to medium-dense sands and gravels. The
gain is not so marked where the base is formed in dense soils, since the enlargement will not
greatly exceed the shaft diameter and, in any case, full utilization of the end-bearing resistance
may not be possible because of the need to keep the compressive stress on the pile shaft
within allowable limits (see Table 2.4).