Geoscience Reference
In-Depth Information
capacity is derived partly from end bearing and partly from skin friction between
the embedded surface of the soil and the surrounding soil.
•
Settlement reducing piles
: these are usually incorporated beneath the central part
of a raft foundation in order to reduce differential settlement to an acceptable level.
Such piles act to reinforce the soil beneath the raft and help to prevent dishing of
the raft in the centre.
•
Tension piles
: structures such as tall chimneys, transmission towers and jetties
can be subject to large overturning moments, so piles are often used to resist the
resulting uplift forces at the foundations. In such cases the resulting forces are
transmitted to the soil along the embedded length of the pile. The resisting force
can be increased in the case of bored piles by under-reaming. In the design of
tension piles the effect of radial contraction of the pile must be taken into account,
as this can cause about a 10-20% reduction in shaft resistance.
•
Laterally loaded piles: almost all piled foundations are subjected to at least some
degree of horizontal loading. The magnitude of the loads in relation to the applied
vertical axial loading will generally be small and no additional design calculations
will normally be necessary. However, in the case of wharves and jetties carrying
the impact forces of berthing ships, piled foundations to bridge piers, trestles to
overhead cranes, tall chimneys and retaining walls, the horizontal component is
relatively large and may prove critical in design. Traditionally piles have been
installed at an angle to the vertical in such cases, providing sufficient horizontal
resistance by virtue of the component of axial capacity of the pile which acts hor-
izontally. However, the capacity of a vertical pile to resist loads applied normally
to the axis, although significantly smaller than the axial capacity of that pile,
may be sufficient to avoid the need for such 'raking' or 'battered' piles which are
more expensive to install. When designing piles to take lateral forces it is therefore
important to take this into account.
•
Piles in fills
: piles that pass through layers of moderately to poorly compacted fill
will be affected by negative skin friction, which produces a downward drag along
the pile shaft and therefore an additional load on the pile. This occurs as the fill
consolidates under its own weight.
6.5.2 Pile behaviour
Pile behaviour during construction/installation portrays a significant threat to founda-
tion design which needs to be considered with a view to providing adequate solutions
prior to construction. Such behaviours may include the following.
6.5.2.1 Geological behaviour
It is natural to assume that soil layers are horizontal in nature, including the under-
lying layer where foundations are situated. Poulos (2005) stated that the effects of
construction processes on the piles are also frequently ignored or simplified. Such
ideal conditions are rarely, if ever, encountered in real life. Both natural geological cir-
cumstances and the processes involved in construction will generally lead to the above
assumptions being invalid, at least to some degree.
