Civil Engineering Reference
In-Depth Information
into, or through, a layer of soil that is consolidating, usually due to the recent placement
of surface loading. Where the downward movement of the surrounding soil exceeds
the settlement of the pile, the pile resists the movement and load in excess of the exter-
nal applied load will be transferred into the pile. The shear stresses on the pile shaft
will act in the reverse sense by comparison with the normal situation for a pile loaded
in compression; accordingly, the term 'negative friction', or 'downdrag' is used (see
Figure 4.53(a)).
Piles may also be loaded laterally by soil movement, for example on a steeply sloping
site, where creep of the upper layers of soil may occur. Eccentric (vertical) loading of
the ground surface adjacent to a piled foundation may also lead to significant lateral
pressures on the piles, as the soil moves away from the loaded area (see Figure 4.53(b)).
4.5.1 Negative friction
Negative friction or downdrag of piles may occur wherever piles are driven through,
or adjacent to, recently placed fill (Johanessen and Bjerrum, 1965). Such fill may
merely be to raise the existing ground level, or may be part of an embankment, for
Figure 4.53
Piles in consolidating soil.
