Pile Design and Construction Practice, Fifth edition

also significant. In old fill which has become fully consolidated under its own weight and
where it is not proposed to impose surcharge loading the negative skin friction may be
neglected, but shaft friction within the fill layer should not be allowed to help support the
pile. In the case of recently placed fill it may settle by a substantial amount over a long
period of years. The fill may also be causing consolidation and settlement of the natural soil,
within which the pile obtains its bearing. The case of recent fill placed over a compressible
soil which becomes stiffer and less compressible with depth is shown in Figure 4.41.
Modelling the load transfer by drag-down from fill and the underlying compressible soil
and the distribution of resistance in positive shaft friction can be undertaken by using a
pile–soil interaction analysis as described in Section 4.6. This procedure is particularly effec-
tive because the resulting t–zcurves give a more accurate estimate in separate or combined
form of the distribution of axial forces over the depth of the pile shaft from the compression
load applied to the pile head and the shear stress on the pile surface from the drag-down loading,
than is possible from semi-empirical diagrams such as shown in Figures 4.38, 4.40, and
4.41. In particular the t–zcurves indicate the depth Hin Figure 4.41, that is, the depth to the
‘neutral point’at which the shear stress changes from negative, caused by drag-down, to
positive, acting in support of the pile.
It is good practice to ignore the contribution to the support provided by friction over
the length of a pile in soft clay, where the pile is driven through a soft layer to less
compressible soil. This is because of the drag-down force on the pile shaft caused by heave
and reconsolidation of the soft clay. The same effect occurs, of course, if a pile is driven into
a stiff clay but the stiff clay continues to act in support of the pile if yielding at the toe is
permitted.
Very large drag-down forces can occur on long piles. In some circumstances they may
exceed the working load applied to the head of the pile. Fellenius(4.61)measured the

Resistance of piles to compressive loads 217

Original surface of fill

Yielding

W

H

0.1H

Distribution of negative

skin friction on pile

shaft

Depth depends on

relative compressibility

of fill and clay

Depth depends on

settlement of clay under

loading from clay relative

to permissible yielding of

pile toe

Figure 4.41Distribution of negative skin friction on pile driven through recent fill into

compressible clay stratum.