Abstract :
Pile
foundations are often required to resist lateral loading.
Lateral loads come from a variety of sources including wind, earthquakes, waves, and ship
impacts. The lateral capacity of a pile is usually much smaller than the axial
capacity and as a result groups of piles are often installed to increase the lateral
capacity of the entire foundation system.
When
vertical or plumb pile groups do not provide sufficient lateral resistance the
piles can be battered in order to mobilize some of the higher axial capacity
to resist the lateral load. Pile foundations consist of piles that are dug into
soil till a layer of stable soil is reached. Pile foundations transfer building load to
the bearing ground with the greater bearing capacity. Pile foundations are
useful in regions with unstable upper soil that may erode, or for large
structures.
Piles are
relatively long, slender members that transmit foundation loads through soil
strata of low bearing capacity to deeper soil or rock strata having a high
bearing capacity. They are used when for economic, constructional or soil
condition considerations it is desirable to transmit loads to strata beyond the
practical reach of shallow foundations. In addition to supporting structures, piles are also
used to anchor structures
against uplift forces and to assist structures in resisting lateral and overturning
forces.
Tension piles
Structures such as tall
chimneys, transmission towers and jetties can be subjected to large overturning moments and 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 horizontally. 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.
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