Pile Design and Construction Practice, Fifth edition

214 Resistance of piles to compressive loads
or rock. No support for compressive loads from shaft friction can be assumed over the length
of the pile shaft through the fill. This is because of the downward movement of the fill as it
compresses under its own weight or under the weight of further soil or surcharge placed over
the fill area. The downward movement results in drag-down forces, generally known as
negative skin friction, on the pile shaft. Where fill is placed over a compressible natural soil
the latter consolidates and moves downwards relative to the pile. Thus the negative skin
friction occurs over the length of the shaft within the natural soil as well as within the fill.
Calculation of the magnitude of the negative skin friction is a complex problem which
depends on the following factors:
(1) The relative movement between the fill and the pile shaft
(2) The relative movement between any underlying compressible soil and the pile shaft
(3) The elastic compression of the pile under the working load and
(4) The rate of consolidation of the compressible layers.
The simplest case is fill that is placed over a relatively incompressible rock with piles
driven to refusal in the rock. The toe of the pile does not yield under the combined working
load and drag-down forces. Thus the negative skin friction on the upper part of the pile
shaft is equal to the fully mobilized value. Near the base of the fill its downward movement
may be insufficiently large to mobilize the full skin friction, and immediately above rockhead
the fill will not settle at all relative to the pile shaft. Thus negative skin friction cannot
occur at this point. The distribution of negative skin friction on the shaft of the unloaded
pile is shown in Figure 4.38a. If a heavy working load is now applied to the pile shaft, the
shaft compresses elastically and the head of the pile moves downwards relative to the fill.
The upper part of the fill now acts in support of the pile although this contribution is
neglected in calculating the pile resistance. The distribution of negative skin friction on the
shaft of the loaded pile is shown in Figure 4.38b. Where the fill has been placed at a
relatively short period of time before installing the piles, continuing consolidation of the
material will again cause it to slip downwards relative to the pile shaft, thus re-activating
the drag-down force.
The simplified profile of negative skin friction for a loaded pile on an incompressible
stratum is shown in Figure 4.38c. This diagram can be used to calculate the magnitude of
the drag-down forces. The peak values for coarse soils and fill material are calculated by the
method described in Section 4.3.
In the case where negative skin friction is developed in clays, the rate of loading must be
considered. It was noted in Section 4.2.4 that the capacity of a clay to support a pile in skin
friction is substantially reduced if the load is applied to the pile at a very slow rate. The same
consideration applies to negative skin friction, but in this case it works advantageously in
reducing the magnitude of the drag-down force. In most cases of negative skin friction in
clays the relative movement between the soil which causes drag-down and the pile takes
place at a very slow rate. The movement is due to the consolidation of the clay under its own
weight or under imposed loading, and this process is very slow compared with the rate of
application of the working load to the pile.
Meyerhof (4.37) advises that the negative skin friction on piles driven into soft to firm clays
should be calculated in terms of effective stress from the equation:
� s neg � ��� vo
(4.50)