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INTRODUCTION
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.
Pile foundations are often required to resist lateral loading. Lateral loads come from a
varietyof sources including wind, earthquakes, waves, and ship impacts. The lateral
capacity of a pile isusually much smaller than the axial capacity and as a result groups of
piles are often installed toincrease the lateral capacity of the entire foundation system.
When vertical or plumb pile groups donot provide sufficient lateral resistance the piles
can be battered in order to mobilize some of thehigher axial capacity to resist the lateral
load.
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.
HISTORY OF PILE FOUNDATION:
Pile foundation have been used for many years, for carrying and transferring the loads to soil
considered to be weak in structure due to the soil conditions. In the early stages of development,
villages and towns were located in the close vicinity of lakes and rivers due to the availability of
water , and, also, to ensure proper protection of the area. Therefore, the weak bearing ground was
reinforced by the use of timber piles that were manually forced into the ground, or fixed into the
holes that were filled with stones and sand. The primitive methods of pile installation were
modified after the industrial revolution, and the techniques of installation by steam or diesel
driven machines were introduced. With the advancement in the technologies of soil mechanics
and other related disciplines, superior piles and pile installation system have been developed.
NECESSITY OF PILE FOUNDATION:
When the strata at or just below the ground surface is highly compressible and very weak to support the load transmitted by the structure.
When the plan of the structure is irregular relative to its outline and load
distribution. It would cause non-uniform settlement if a shallow foundation
is constructed. A pile foundation is required to reduce differential settlement.
Pile foundations are required for the transmission of structure loads through
deep water to a firm stratum.
Pile foundations are used to resist horizontal forces in addition to support the
vertical loads in earth-retaining structures and tall structures that are
subjected to horizontal forces due to wind and earthquake.
Piles are required when the soil conditions are such that a washout, erosion
or scour of soil may occur from underneath a shallow foundation.
In case of expansive soil, such as black cotton soil, which swell or shrink as
the water content changes, piles are used to transfer the load below the
active zone.
Collapsible soils, such as loess, have a breakdown of structure accompanied
by a sudden decrease in void ratio, when there is increase in water content.
Piles are used to transfer the load beyond the zone of possible moisture
changes in such soils.
Pile foundation vswell foundation:
Well foundations provide a solid and massive foundation for heavy
loads as against a cluster of piles which are slender and weak
individually and are liable to get damaged when hit by floating trees or boulder rolling in river bed.
Wells provide a large section modulus with the minimum cross
sectional area and hence efficient in taking large vertical and
horizontal loads even when the unsupported length is large.
Concreting of well steining is done under dry and controlled
conditions and hence quality of work is assured, however same
cannot hold good in case of cast-in-situ bored piles where concreting
is to be done under water or below ground level. Even in case of
precast piles, the concrete is subjected of heavy stresses during
driving operation and consequent damages cannot be ruled out.
When scour takes place, the piles act as long struts and have to be
designed for buckling stresses, which are quite heavy due to the
bending moments contributed by the longitudinal forces on the bridge
deck due to tractive effort and braking forces.
It is difficult to drive the piles through the strata having boulders and
tree logs which are frequently encountered in alluvial soil, whereas in
the case of a well foundation there is sufficiently access to remove
the obstruction. Quite often the skin friction developed is of much
magnitude as to prevent further driving of a pile although a firm
stratum has not been reached.
The adoption of pile foundations is advantageous over well
foundations where the soil characteristics and conditions of water
table are such that the phenomenon of blow occurs during dewatering of the well.
Increased mechanization and advent of new machinery have brought
down the cost of foundation with piles considerably low in comparison
to well. New testing techniques for checking the integrity of piles and
information about strata through piles have passed or resting have
removed the uncertainty of load carrying capacity of piles to large
extent.
Pile foundations have a clear advantage over well foundations in
terms of speedy construction. Wherever time is the criterion, the pile
foundation is the natural choice.
CLASSIFICATION OF PILE FOUNDATION:
Pile foundations can be classified according to
the type of pile
(different structures to be supported, and different ground conditions, require
different types of resistance) and
the type of construction
(different materials, structures and processes can be used)
the type of material used
TYPES OF PILE:
End Bearing piles.
Friction piles.
Settlement reducing piles.
Tension piles.
Laterally loaded piles.
Piles in fill.
Piles are often used because adequate bearing capacity cannot be found at
shallow enough depths to support the structural loads. It is important to
understand that piles get support from both end bearing and skin friction. The
proportion of carrying capacity generated by either end bearing or skin friction
depends on the soil conditions. Piles can be used to support various different
types of structural loads.
Settlement reducing piles 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
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.
PILES IN FILL:
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.
TYPES OF PILE COSTRUCTION:
Precast Driven Piles –These are usually of RCC or pre-stressed
concrete and generally small in size for ease in handling. The main
advantage of this type of pile is that its quality, in terms of dimension,
use of reinforcement and concrete, can be ensured as the piles are
cast in a yard under controlled conditions. However care is needed
while handling, transporting and driving the pile to avoid damages.
More to it, the limitation of length depending upon the capacity of the
driving equipment is a disadvantage as these cannot be taken very
deep except by joining. Generally, the depth over which these are
used is restricted to 36 mt.
Driven Cast-in-Situ Piles- A steel casing pile with a shoe at the
bottom is driven first to the required depth. The reinforcement cage
for the pile is then lowered inside the casing and the pile is concreted.
As the concreting of the pile proceeds upwards, the casing is
withdrawn keeping a suitable overlapping length. When such piles
are driven in soft soil and the tube is withdrawn while concreting, it
affects resistance and changes the property of the soil and this also
affects the capacity of individual piles. These are not suitable for use
in soft soils, in greater depths or where keying with the rock is
required.