PILED FOUNDATIONWHAT IS A PILED FOUNDATION?
A pile can be loosely defined as a column
inserted in the ground to transmit the structural
loads to a lower level of subsoil.
WHERE ARE PILED FOUNDATIONS USED?
Piled foundations are used when there is
unsuitability of the upper regions of the subsoil
caused by:
Low bearing capacity of the subsoil.
Heavy point loads of the structure exceeding
the soil bearing capacity.
High water table giving r ise to high
permanent dewatering costs.
Presence of layers of highly compressible
subsoils such as peat and recently placed
filling materials which have not sufficiently
consolidated.
Subsoils which may be subject to moisture
movement or plastic failure.
CLASSIFICATION
These foundations can be broadly classified as:
End Bearing
Friction
END BEARING PILES
These piles transfer their load on to a firm
stratum located at a considerable depth below the
base of the structure and they derive most of their
carrying capacity from the penetration resistance
of the soil at the toe of the pile. The pile behaves as
an ordinary column. Even in weak soil a pile will
not fail by buckling and this effect need only be
considered if part of the pile is unsupported, i.e. if
it is in either air or water. Load is transmitted to the
soil through friction or cohesion. But sometimes,
the soil surrounding the pile may adhere to the
surface of the pile and causes "Negative Skin
Friction" on the pile. This, sometimes have
considerable effect on the capacity of the pile.
FRICTION PILES
These piles also transfer their load to the
ground through skin friction. The process of
driving such piles does not compact the soil
appreciably. These types of pile foundations are
commonly known as floating pile foundations.
DISPLACEMENT PILES
These are often called driven piles since
they are usually driven into the ground displacing
the earth around the pile shaft. These piles can be
either preformed or partially preformed if they
are not cast in-situ and are available in a wide
variety of types and materials. The pile or forming
tube is driven into the required position to a
predetermined depth or to the required `set'
which is a measure of the subsoils resistance to the
penetration of the pile and hence its bearing
capacity by noting the amount of penetration
obtained by a fixed number of hammer blows.
REPLACEMENT PILES
These are often called bored piles since the
removal of the spoil to form the hole for the pile is
always carried out by a boring technique. They
are used primarily in cohesive subsoils for the
formation of friction piles and when forming pile
foundations close to existing buildings where the
allowable amount of noise and/or vibration is
limited.
DISPLACEMENT PILING METHODS
Pre cast concrete driven piles
Thick wall driven steel tubes
Thin wall bottom driven piles
Timber piles
Screw piles
Helical displacement piles
Vibro concrete columns (drive cast in-situ)
ADVANTAGES OF DISPLACEMENT PILING
Self testing as driven to refusal or "set
No pile arising to dispose of
Little disturbance
Limited access
High production
DISADVANTAGES OF DISPLACEMENT
PILING
Cannot penetrate obstructions
Cannot always penetrate desiccated clay
Vibration and noise may be an issue
Replacement Piles
Displacement Piles
REPLACEMENT PILING METHODS
Open hole auger piles
Continuous flight auger (CFA)
Large diameter rotary piles
Odex piles
Tripod piles
ADVANTAGES OF REPLACEMENT PILING
Effectively vibration free
Installed into non cohesive and water bearing
soils
High production
Restricted access
D I S A DVA N TAG E S O F R E P L AC E M E N T
PILING
It produces excavated material which requires
removal off site.
DISPLACEMENT PILES
TIMBER PILES Timber Piles are usually square sawn and
can be used for small contracts on sites with
shallow alluvial deposits overlying a suitable
bearing strata (e.g. river banks and estuaries.)
Timber piles are percussion driven. Timber piles
are not easy to splice and are liable to attack by
marine borers when set in water, therefore such
pi les are always treated wi th sui table
preservatives before being driven.
STEEL SECTION PILES Standard steel sheet pile sections can
be
used to form box section piles whereas the `H'
section piles are cut from standard rolled
sections. These piles are percussion driven and
are used mainly in connection with marine
structures. They are suitable for handling and
driving in long lengths. Their relatively small
cross-sectional area combined with their high
strength makes penetration easier in firm soil.
They can be easily cut off or joined by welding. If
the pile is driven into a soil with low pH value,
then there is a risk of corrosion.
PREFORMED CONCRETE PILES Preformed Concrete Piles are available
in variety of types
which are generally used on medium to large contracts of not
less
than one hundred piles where soft soil deposits overlie a
firmer
strata. It is formed and reinforced in a high-quality
controlled
concrete, Usually used of square, triangle, circle or
octagonal
section, they are produced in short length in one meter
intervals
between 3 and 13 meters. They are pre-caste so that they can
be
easily connected together in order to reach to the required
length. This will not decrease the design load capacity.
Reinforcement is necessary within the pile to help withstand
both handling and driving stresses. Pre stressed concrete
piles
are also used and are becoming more popular than the
ordinary
pre cast as less reinforcement is required. These piles are
percussion driven using a drop or single acting hammer
Preformed Concrete Piles are joint with a peripheral steel
splicing collar is adequate for most concentrically or
directly
loaded situations. Where very long piles are to be used
and/or
high stresses due to compression, tension and bending from
the
superstructure or the ground conditions are anticipated, the 4
or
8 lock pile joint [AARSLEFF PILING] may be considered.
STEEL SCREW PILES Steel Screw Piles are rotary driven and
are
used for dock and jetty works where support at
shallow depths in soft silts and sands is required.
STEEL TUBE PILES Steel Tube Piles are used on small to
medium size contracts for marine structures and
foundations in soft subsoils over a suitable
bearing strata. Tube piles are usually bottom
driven with an internal drop hammer. The loading
can be carried by the tube alone but it is usual to
fill the tube with mass concrete to form a
composite pile. Reinforcement, except for pile
cap bonding bars, is not normally required.
STEEL TUBE PILES Partially Preformed Piles are composite
piles of precast concrete and in-situ concrete or
steel and in-situ concrete. These percussion
driven piles are used on medium to large
contracts where bored piles would not be suitable
owing to running water or very loose soils.
