1.1 Pile foundationsPile foundations are the part of a structure used to
carry and transfer the load of the structure to the bearing ground located at
some depth below ground surface. The main components of the foundation are the
pile cap and the piles. Piles are long and slender members which transfer the
load to deeper soil or rock of high bearing capacity avoiding shallow soil of
low bearing capacity The main types of materials used for piles are Wood, steel
and concrete. Piles made from these materials are driven, drilled or jacked into
the ground and connected to pile caps. Depending upon type of soil, pile
material and load transmitting characteristic piles are classified accordingly.
In the following chapter we learn about, classifications, functions and pros and
cons of piles.1.2 Historical

Pile foundations have been used as load carrying and load transferring systems
for many years.

In the early days of civilisation[2], from the communication, defence or
strategic point of view villages and towns were situated near to rivers and
lakes. It was therefore important to strengthen the bearing ground with some
form of piling.

Timber piles were driven in to the ground by hand or holes were dug and filled
with sand and stones.

In 1740 Christoffoer Polhem invented pile driving equipment which resembled to
days pile driving mechanism. Steel piles have been used since 1800 and concrete
piles since about 1900.

The industrial revolution brought about important changes to pile driving system
through the invention of steam and diesel driven machines.

More recently, the growing need for housing and construction has forced
authorities and development agencies to exploit lands with poor soil
characteristics. This has led to the development and improved piles and pile
driving systems. Today there are many advanced techniques of pile installation.

1.3 Function of piles

As with other types of foundations, the purpose of a pile foundations is:

to transmit a foundation load to a solid ground

to resist vertical, lateral and uplift load

A structure can be founded on piles if the soil immediately beneath its base
does not have adequate bearing capacity. If the results of site investigation
show that the shallow soil is unstable and weak or if the magnitude of the
estimated settlement is not acceptable a pile foundation may become considered.
Further, a cost estimate may indicate that a pile foundation may be cheaper than
any other compared ground improvement costs.

In the cases of heavy constructions, it is likely that the bearing capacity of
the shallow soil will not be satisfactory, and the construction should be built
on

pile foundations. Piles can also be used in normal ground conditions to resist
horizontal loads. Piles are a convenient method of foundation for works over
water, such as jetties or bridge piers.

1.4 Classification of piles

1.4.1 Classification of pile with respect to load transmission and functional
behaviour

End bearing piles (point bearing piles)

Friction piles (cohesion piles )

Combination of friction and cohesion piles

1.4.2 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 (see
figure 1.1). The pile behaves as an ordinary column and should be designed as
such. 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. Negative skin friction is
caused by the drainage of the ground water and consolidation of the soil. The
founding depth of the pile is influenced by the results of the site investigate
on and soil test.

1.4.3 Friction or cohesion piles

Carrying capacity is derived mainly from the adhesion or friction of the soil in
contact with the shaft of the pile (see fig 1.2).


Figure 1-1 End bearing piles

Figure 1-2 Friction or cohesion pile***1.4.4 Cohesion piles***These piles
transmit most of their load to the soil through skin friction. This process of
driving such piles close to each other in groups greatly reduces the porosity
and compressibility of the soil within and around the groups. Therefore piles of
this category are some times called compaction piles. During the process of
driving the pile into the ground, the soil becomes moulded and, as a result
loses some of its strength. Therefore the pile is not able to transfer the exact
amount of load which it is intended to immediately after it has been driven.
Usually, the soil regains some of its strength three to five months after it has
been driven.

1.4.5 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.

1.4.6 Combination of friction piles and cohesion piles

An extension of the end bearing pile when the bearing stratum is not hard, such
as a firm clay. The pile is driven far enough into the lower material to develop
adequate frictional resistance. A farther variation of the end bearing pile is
piles with enlarged bearing areas. This is achieved by forcing a bulb of
concrete into the soft stratum immediately above the firm layer to give an
enlarged base. A similar effect is produced with bored piles by forming a large
cone or bell at the bottom with a special reaming tool. Bored piles which are
provided with a bell have a high tensile strength and can be used as tension
piles (see fig.1-3)


Figure 1-3 under-reamed base enlargement to a bore-and-cast-in-situ pile***1.4.7
Classification of pile with respect to type of material*** * Timber

  • Concrete
  • Steel
  • Composite piles

1.4.8 Timber piles

Used from earliest record time and still used for permanent works in regions
where timber is plentiful. Timber is most suitable for long cohesion piling and
piling beneath embankments. The timber should be in a good condition and should
not have been attacked by insects. For timber piles of length less than 14
meters, the diameter of the tip should be greater than 150 mm. If the length is
greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is
essential that the timber is driven in the right direction and should not be
driven into firm ground. As this can easily damage the pile. Keeping the timber
below the ground water level will protect the timber against decay and
putrefaction. To protect and strengthen the tip of the pile, timber piles can be
provided with toe cover. Pressure creosoting is the usual method of protecting
timber piles.

1.4.9 Concrete pile

Pre cast concrete Piles or Pre fabricated concrete piles* :* Usually of square
(see fig 1-4 b), triangle, circle or octagonal section, they are produced in
short length in one metre 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 (fig 1-4 a) . 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 .


Figure 1-4 a) concrete pile connecting detail. b) squared pre-cast concert pile
The Hercules type of pile joint (Figure 1-5) is easily and accurately cast
into the pile and is quickly and safely joined on site. They are made to
accurate dimensional tolerances from high grade steels. 

 

 

Figure 1-5 Hercules type of pile joint
* **1.4.10 Driven and cast in place
Concrete piles
Two of the main types used in the UK are: West�s shell pile : Pre cast,
reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel
and driven into the ground after a concrete shoe has been placed at the front of
the shells. Once the shells have been driven to specified depth the mandrel is
withdrawn and reinforced concrete inserted in the core. Diameters vary from 325
to 600 mm.

Franki Pile: A steel tube is erected vertically over the place where the pile is
to be driven, and about a metre depth of gravel is placed at the end of the
tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid
plug which then penetrates the soil and takes the steel tube down with it. When
the required depth has been achieved the tube is raised slightly and the
aggregate broken out. Dry concrete is now added and hammered until a bulb is
formed. Reinforcement is placed in position and more dry concrete is placed and
rammed until the pile top comes up to ground level.

1.4.11 Steel piles

Steel piles: (figure 1.4) steel/ Iron piles 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, but risk of corrosion is not as
great as one might think. Although tar coating or cathodic protection can be
employed in permanent works.

It is common to allow for an amount of corrosion in design by simply over
dimensioning the cross-sectional area of the steel pile. In this way the
corrosion process can be prolonged up to 50 years. Normally the speed of
corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year


 

 

a) X- cross-sectionb) H – cross-sectionc) steel pipeFigure 1-6 Steel piles
cross-sections***1.4.12 Composite piles***Combination of different materials in
the same of pile. As indicated earlier, part of a timber pile which is installed
above ground water could be vulnerable to insect attack and decay. To avoid
this, concrete or steel pile is used above the ground water level, whilst wood
pile is installed under the ground water level (see figure 1.7).


Figure 1-7 Protecting timber piles from decay:a) by pre-cast concrete upper
section above water level.
b) by extending pile cap below water level***1.4.13 Classification of pile with
respect to effect on the soil***A simplified division into **driven **or bored piles is often employed.

1.4.14 Driven piles

Driven piles are considered to be displacement piles. In the process of driving
the pile into the ground, soil is moved radially as the pile shaft enters the
ground. There may also be a component of movement of the soil in the vertical
direction*.*


Figure 1-8 driven piles***1.4.15 Bored piles*Bored piles (Replacement piles)
are generally considered to be non-displacement piles a void is formed by boring
or excavation before piles is produced. Piles can be produced by casting
concrete in the void. Some soils such as stiff clays are particularly amenable
to the formation of piles in this way, since the bore hole walls do not requires
temporary support except cloth to the ground surface. In unstable ground, such
as gravel the ground requires temporary support from casing or bentonite slurry.
Alternatively the casing may be permanent, but driven into a hole which is bored
as casing is advanced. A different technique, which is still essentially
non-displacement, is to intrude, a grout or a concrete from an auger which is
rotated into the granular soil, and hence produced a grouted column of soil.

There are three non-displacement methods: bored cast- in – place piles,
particularly pre-formed piles and grout or concrete intruded piles.

The following are replacement piles:

Augered

Cable percussion drilling

Large-diameter under-reamed

Types incorporating pre caste concrete unite

Drilled-in tubes

Mini piles