Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 215

(a) (b)

Residual

negative

skin friction

operating

Peak negative

skin friction

Movement between

pile and soil

insufficient to

mobilize peak

value

Compression of pile

due to load W causes

pile to move downwards

relative to fill, thus

reducing negative skin

friction

Negative skin friction

eliminated by elastic

compression of pile

Profile for soils

or fills which do

not show reduction

in negative skin

friction for

‘long strain’

condition

Fill or

compressible

soil settling

under own

weight

Zero settlement

of fill above rockhead

Surface W Residual

W

H

0.IH

0.IH

0.8H

Rockhead

Original ground

No load on

pile head

Peak negative

skin friction

Residual

value of

negative

friction

operating

Profile of unit

negative skin

friction

(c)

Peak

Figure 4.38Distribution of negative skin friction on piles terminated on relatively incompressible
stratum (a) No load on pile head (b) Compressive load on pile head (c) Design curve for
loaded pile.

Values of the negative skin friction factor allowing for reduction of the effective angle of

friction with increasing depth to the residual value (^) rare shown in Figure 4.39.
Taking the case of a pile bearing on a compressible stratum, where yielding of the pile toe
occurs under the drag-down forces and the subsequently applied working load, the down-
ward movement of the pile relative to the lower part of the fill may then be quite large, and
such that negative skin friction is not developed over quite an appreciable proportion of the
length of the shaft within the fill. Over the upper part of the shaft the fill moves downwards
relative to the pile shaft to an extent such that the negative skin friction operates, whereas in
the middle portion of the pile shaft the small relative movement between the fill and the pile
may be insufficient to mobilize the peak skin friction as a drag-down force. The distribution
for the unloaded pile is shown in Figure 4.40a.
When the working load is applied to the head of the pile, elastic shortening of the pile
occurs, but since the load is limited by the bearing characteristics of the soil at the pile toe
the movement may not be large enough to eliminate the drag-down force. The distribution
of negative friction is then shown in Figure 4.40b. The diagram in Figure 4.40c can be used
for design purposes, with the peak value calculated as described in Section 4.3 for coarse
soils and fill and by using equation 4.50 and Figure 4.39 for soft to firm clays.
It may be seen from Figure 4.40a to c that at no time does the maximum skin friction
operate as a drag-down force over the full length of the pile shaft. It is not suggested that
these simplified profiles of distribution of negative skin friction represent the actual condi-
tions in all cases where it occurs, since so much depends on the stage reached in the
consolidation of the fill, and the compression of the natural soil beneath the fill. The time
interval between the installation of the pile and the application of the working load is