Piles are structural members made of steel, concrete or timber that are used to build deep foundations called pile foundations. Pile foundations are necessary when the soil is too weak, compressible or expansive to support the load of a structure using a shallow foundation. They are also used when lateral forces from wind, earthquakes or soil erosion must be resisted. Piles transmit structural loads deeply into the ground to stronger soil or bedrock layers, ensuring structural safety even in poor soil conditions. Common types of piles include precast concrete, cast-in-place concrete, steel, and timber piles.

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Deep Foundations-Piles and
Drilled shafts
Piles are structural members made of steel,
concrete and timber.
Pile foundations are deep and more costly than
shallow foundations but necessary to ensure
structural safety.

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Pile Foundations
Need for Pile Foundations
• When the upper soil layer is highly compressible and too
weak to support the load transmitted by the
superstructure
• When the bedrock is not encountered at a reasonable
depth below the ground surface, piles are used to
transmit the structural load to the soil gradually.
• When the tall buildings, and earth-retaining structures
are subjected to horizontal forces such as strong wind
and/or earthquake forces.
• When the soils at the proposed site are expansive and
collapsible. Expansive soils swell and shrink as the
moisture content increases and decreases.
• The swelling pressure of such soils can be considerable.

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piles

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piles

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The need for piles
• Soils such as loess are collapsible. When the moisture
content of these soils increases, their structures may
break down. A sudden decrease in the void ratio of the
soil induces larger settlements of structures supported
by shallow foundations.
• The foundations of some structures, such as
transmission towers, offshore platforms, and basement
mates below the water table, are subjected uplifting
forces.
• Bridge abutments and piers are usually constructed
over pile foundations because of soil erosion at the
ground surface.

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•Piles are structural members made of steel,
concrete, and or timber.
•Piles are used to build pile foundations,
which are deep and more costly than
shallow foundations.
•The use of piles ensure structural safety.
•Drilled shafts are cast-in-place piles that
generally have diameter greater than 750
mm

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•When the upper soil layers are highly compressible and too weak
or expansive and collapsible, to support the load transmitted by
the superstructure, piles are used to transmit the load to the
underlying bedrock or a stronger soil layer.
•Pile foundations resist lateral forces in earth retaining structures
and tall buildings where there are subjected to strong wind
andor earthquake forces.
• Oil platforms Foundations and basements mats below the water
table which are subjected to uplifting forces need pile
foundations.
•Bridge abutments and piers are constructed over pile
foundations to avoid the possible loss of bearing capacity that a
shallow foundation might suffer due to soil erosion and scouring.

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Types of piles and their structural
characteristics
• Different types of piles are used in the
construction work, depending on the type of
load to be carried, the subsoil conditions, and
the water table.
• Piles can be divided into the following:
• Steel piles
• Concrete piles
• Wooden (timber) piles

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Types of piles and their structural
characteristics

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Types of piles and their structural
characteristics

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Types of piles and their structural
characteristics

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Steel piles
• Steel piles generally are either pipe piles or rolled steel H-section piles.
• Pipe piles can be driven into the ground with their ends open or closed.
• H section piles are usually preferred because their web and flange
thicknesses are equal.
• When necessary, steel piles are spliced by welding or by riveting or bolts.
• When hard driving conditions are expected, such as driving through dense
gravel, shale and soft rock, steel piles can be fitted with driving points or
shoes.
• usual length: 15-60 m; usual load 300 kn -1200kN
• advantages a) easy to handle with respect to cutoff and extension to the
desired length.
• Can stand high driving stresses
• High load-carrying capacity

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Steel piles
• Disadvantages
• Relatively costly material
• High level of noise during pile driving
• Subject to corrosion
• H-piles may be damaged or deflected from the
vertical during driving through hard layers or
past major obstructions

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Concrete piles
• Concrete piles may be divided into two basic
types: precast piles and cast-in place piles.
• Precast piles can be prepared using ordinary
reinforcement and they can be square or
octogonal in cross section.
• Reinforcement is provided to enable the pile to
resist the bending moment developed during
pickup and transportation, the vertical load, and
the bending moment caused by lateral load.
• The piles are cast to desired lengths and cured
before being transported to the work sites.

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Precast piles
• Precast piles can also be prestressed by using high-
strength steel prestressing cables. The ultimate
strength of these cables is about 1800 MN/m2 .
• The general of the precast concrete piles are:
• Usual lenth:10 – 15 m
• Usual load : 300 kN-3000 kN
• Advantages: can be subjected to hard driving
• Corrosion resistant and can be easily combined with
concrete superstructure
• Disadvantages: difficult to achieve proper cutoff and
transported

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Installation of piles
• Most piles are driven into the ground by hammers or vibratory
drivers.
• The types of hammer used for pile driving include the a) drop
hammer b)single acting air or steam hammer or double –acting and
differential air or steam hammer c) diesel hammer.
• In the driving operation, a cap is attached to the top of the pile. A
cushion is sometimes used between the cap and the pile to reduce
the impact force.
• Based on the nature of their placement, driven piles such as
concrete piles and closed ended pipe piles are displacement piles
because they move some soils laterally; hence, there is a tendency
for the densification of soil surrounding them
• Steel H-piles displace less soils and also bored piles(piers) are non
displacement piles.
• Driven piles are costly and noisy as compared to cast-in-place piles

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Cast-in-situ piles
• Piles are built by making a hole in the ground and then
filling it with concrete.
• In firm soils, contractors use the dry method (the open-
hole method) to build the shaft. These holes usually
advance quickly using conventional flight augers and
remain open without any special support.
• After checking the open hole for cleanliness and alignment,
it is simple matter to insert the steel reinforcement cage
and dump concrete in from the top using tremie or a
concrete pump to deliver the concrete.
• For excavation below the water table, the contractor simply
pumps the water out as the hole advances and places the
concrete in the dewatered shaft.

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Drilling technigues in caving or
squeezing soils
• A hole is said to be caving when the sides collapse
before or during concrete placement. This is especially
likely in clean sands below the water table.
• Squeezing refers to the sides of the hole bulging
inward, either during or after drilling, and is most likely
in soft clays and silts or highly organic soils.
• Either of these conditions could produce necking in the
shaft (a local reduction in its diameter) or soil
inclusions in the concrete, both of which could have
disastrous consequences.
• To prevent caving or soils, casing or drilling fluid are
used.

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Casing method
• Drill the hole using conventional methods until
encountering the caving strata.
• Insert a steel pipe (the casing) into the hole and
advance it past the caving strata. The diameter of
the casing is usually 50 -150 mm less than the
diameter of the upper part of the shaft.
• Drill through the casing and into the non-caving
soils below using a smaller auger,
• Place the reinforcing steel cage and the concrete
through the casing.

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DRILLING FLUID METHOD OR SLURRY
METHOD
• Drill a starter hole, perhaps 3 meter deep
• Fill the starter hole with a mixture of water and betonite clay to
form a drilling mud or slurry
• This material will keep the hole open because of the hydrostatic
pressure it applies to bthe soil.
• Advance the hole by passing the drilling tools through the slurry.
Continue to add water and bentonite as necessaey.
• Insert the reinforcing steel cage directly into the slurry
• Fill the hole with concrete using a pipe that extends to the bottom.
The concrete pushes the slurry to the ground surface, where it is
captured.
• This method is quite effective but slow and messy.
•

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Underreamed shafts
• An underreamed shaft also known as belled shafts is
one with enlarged base
• The larger base area of underreamed shafts increases
their end bearing capacity.
• Concrete
• Concrete for drilled shafts must have sufficient slump
(100 -200) mm to flow properly and completely fill the
hole
• Using concrete that is too stiff creats voids that
weakens the shafts

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