1) Deep Foundations
1) Deep Foundations
1) Deep Foundations
Super structure
Ground Level
Basement
Sub structure
Foundation
Components of a building
Purpose of foundations:
It is often misunderstood that the foundation is provided
to support the load of the structure. In fact it is the
device to transmit the load of the structure to the soil
below. Foundation is provided for the following main
purposes:
To distribute the weight of the structure over large area
so as to avoid over-loading of the soil beneath.
To load the substratum evenly and thus to prevent
unequal settlement.
To provide a level surface for building operations.
To take the structure deep into the ground and thus to
increase its stability, preventing overturning.
Foundation loads:
A foundation supports a number of different kinds of
loads:
The dead load of the building, which is the sum of the
weights of the frame, the floors, the roofs, and walls, the
electrical and mechanical equipment and the foundation
itself.
The live load, which is the sum of the weights of the
people in the building, the furnishing and the equipments
they use, and snow, ice and water on the roof.
Wind loads, which can apply lateral, downward, and
uplift loads to a foundation
Foundation loads:
Horizontal pressures of earth and water against
basement walls.
In some buildings, horizontal or inclined thrusts from
arches, rigid frames, domes, vaults or tensile structures.
On some building sites, buoyant uplift forces from
underground water, identical to the forces that cause a
boat to float.
During earthquakes, horizontal forces caused by the
motion of the ground relative to the building.
Requirements of foundations:
A satisfactory foundation for a building must meet
three general requirements:
The foundation, including the underlying soil and rock,
must be safe against a structural failure that could
result in collapse.
During the life of the building, the foundation must not
settle in such a way as to damage the structure or
impair its function.
The foundation must be feasible both technically and
economically, and practical to build without adverse
effects to surrounding property.
Types Of Foundations:
There are two basic types of foundations:
Shallow foundations are those that transfer the load
the load to the earth at the base of the column or wall of
the substructure. Shallow foundations are generally
less expensive than deep ones and can be used where
suitable soil is found at the level at the bottom of the
substructure, whether this be a few feet or several
stories below the surface.
Deep foundations transfer the load at a point some
distance below the substructure. Deep foundations,
either piles or caissons, penetrate through upper layers
of incompetent soil in order to reach competent bearing
soil or rock deeper within the earth.
DEEP FOUNDATIONS
In case, the stratum of good bearing capacity is not
available near the ground, the foundation of the
structure has to be taken deep with the purpose of
attaining a bearing stratum which is suitable in all
respects.
In addition, there may be many other conditions which
may require deep foundations for ensuring stability
and durability of a structure. For example, the
foundation for a bridge pier must be placed below the
scour depth, although suitable bearing stratum may exist
at a higher level.
DEEP FOUNDATIONS
The most common forms of construction pertaining to
deep foundations are:
Piles
Cofferdams
Caissons
Raft foundations
Out of these, pile foundation is more commonly used in
building construction.
Pile foundation
The word pile is used to describe columns, usually of
reinforced concrete, driven or cast in situ in the ground in
order to carry foundation loads to some deep
underlying firm stratum or to transmit loads to the
subsoil by the friction of their surfaces in contact
with the subsoil.
The main function of the pile is to transmit loads to lower
level of ground by a combination of friction along their
sides and end bearing at the pile point or base.
Classification of Piles
Piles can be classified on following criteria:
On the basis of mode of load transfer:
End bearing
Friction
Composite
On the basis of function or use:
Sheet pile
Anchor Pile
Batter Pile
Fender Pile
Compaction pile
Classification of Piles
On the basis of cross sectional area:
circular
square
tubular
octagonal
H-piles
On the basis of size of the piles:
Micro(mini) Piles (< 150 mm)
Small diameter Piles (>150 and < 600 mm)
Large diameter Piles (>600 mm )
Classification of Piles
On the basis of material:
Timber
steel
PCC
RCC
composite
On the basis of method of construction/ execution
Driven/displacement precast piles
Driven/displacement cast-in-situ pile
Bored/replacement precast pile
Bored/replacement cast-in-situ pile
Types of piles
Friction piles:
Piles that transfer loads
mainly by friction (skin
friction) to clays and silts
are termed as friction piles.
The friction piles are driven at
a site where the soil is weak
or soft to a considerable
depth i.e. in the type of soil
whose strength does not
increase with depth or, where
rate of increase in strength
with depth is very slow.
Types of piles
Sheet piles
Sheet pile walls are constructed
by driving prefabricated sections
into the ground.
Soil conditions may allow for the
sections to be vibrated into ground
instead of it being hammer driven.
The full wall is formed by
connecting the joints of adjacent
sheet pile sections in sequential
installation.
Sheet pile walls provide structural
resistance by utilizing the full
section.
Steel sheet piles are most
commonly
used
in
deep
excavations, although reinforced
concrete sheet piles have also being
used successfully
Steel sheet piling is the most common because of several advantages over other
materials:
1. Provides high resistance to driving stresses.
2. Light weight
3. Can be reused on several projects.
4. Long service life above or below water with modest protection.
5. Easy to adapt the pile length by either welding or bolting
6. Joints are less apt to deform during driving.
Anchor piles:
When piles are used to
provide
anchorage
against horizontal pull
from sheet piling walls or
other pulling forces, they
are termed as anchor
piles.
Batter piles:
When piles are driven at
an inclination to resist
large
horizontal
or
inclined forces, the piles
are termed as batter
piles.
Fender piles:
When the piles are
used
to
protect
concrete deck or other
water front structures
from the abrasion or
impact that may be
caused from the ships
or barges (when they
are tied up at the deck)
they are called fender
piles.
The fender piles are
ordinarily made up of
timber.
Compaction piles:
When piles are driven in
granular soil with the aim of
increasing
the
bearing
capacity of the soil, the piles
are termed as compaction
piles.