Foundation Engineering
by Omkar Kr
FOUNDATION
Foundation is the lowest portion of a structure which transmits the load
into the supporting soil. The main purpose of the foundation is to distribute
the total weight of the superstructure over a large area of soil. Various types
of foundation are described below which are used in construction.
TYPES OF FOUNDATION:
Foundation can be classified into two general categories:
1. Shallow Foundation.
2. Deep Foundation.
1. SHALLOW FOUNDATION:
A Shallow foundation is a type of foundation in which the foundation is
situated instantly below the lowest part of a structure. The depth of
foundation is equal or less than its width.
In this foundation, the total loads of the structure are distributed
over a horizontal area at shallow depth below the ground level.
CLASSIFICATION OF SHALLOW FOUNDATION:
1. Spread Footings.
2. Combined Footings.
3. Mat Or Raft
Foundation.
SPREAD FOOTINGS:
Generally, spread footing consists of a wide base of foundation for
transmitting the load to the soil over a wider area.
COMBINED FOOTINGS:
Combined footing consists of a common footing provided to two
columns which may be either rectangular or trapezoidal.
§ Strap Footing.
§ Strip Footing.
RAFT FOUNDATION:
Raft foundation consists of dense reinforced concrete slab which covers
the total bottom area of the structure. It is provided in the soil with low
bearing capacity where structural loads are heavy.
2. DEEP FOUNDATION:
A deep foundation is a type of foundation in which the foundation is
placed at a deeper depth below the ground level. The depth of foundation is
much greater than its width.
Deep foundation can be further classified into three categories:
1. Pile Foundation.
2. Cofferdams.
3. Caisson Foundation.
PILE FOUNDATION:
Pile foundation is a type of
foundation where a slender member of wood or concrete or steel is inserted into the
ground for transferring the load of a structure. The load is transferred to
a stronger stratum by friction or by bearing.
Classification Of Pile Foundation:
Classification Based On function:
1. Bearing Piles.
2. Friction Piles.
3. Sheet piles.
4. Anchor Piles.
5. Batter Piles.
6. Fender Piles.
7. Compaction Piles.
Classification based On Material:
1. Timber piles.
2. Concrete Piles.
3. Steel Piles.
COFFERDAMS:
A Cofferdam is a temporary structure which excludes the water from a
given site to enable the construction on a dry surface.
Classification Of Cofferdams Based On Material:
1. Earthen Cofferdam.
2. Rock-fill
Cofferdam.
3. Single-walled
cofferdam.
4. Double-walled
cofferdam.
5. Crib Cofferdam.
6. Cellular Cofferdam.
CAISSON FOUNDATION:
Caisson is a watertight structure made of wood, steel or reinforced
concrete which excavates for the foundation of bridges, piers
etc.
Types Of Caissons:
1. Open Caisson.
2. Box Caisson.
3. Pneumatic Caisson.
PILE FOUNDATION:
In
pile foundation, a thin member made of steel, concrete or wood is inserted into
the poor ground (Soil having low bearing capacity) for transferring the load of
a superstructure. The load can be transmitted to a strong soil layer by
friction or by bearing.
PURPOSE OF PILE FOUNDATION:
1.
The main purpose of a pile foundation is to transfer the loads
into a strong stratum.
2.
Generally, pile foundations are used when the bearing capacity
of soil is very low and the structural load is heavy.
3.
Compressible soil and waterlogged soil is ideal for this type of
foundations.
4.
Piles are mostly suitable for the foundation of high-rise buildings,
bridges, piers, docks etc.
CLASSIFICATION OF PILE FOUNDATION:
Depending
upon the function, material and method of installation pile foundation
can be further classified into following categories;
CLASSIFICATION DEPENDING UPON THE FUNCTION OF PILES:
1.
Bearing Piles:
Bearing
piles are the pile that is driven through a soft soil until it reaches to a
stronger base. These piles are used as piers to support the superstructure and
to transmit the load into a safe stratum.
2.
Friction Piles:
When
piles are compelled through a soft and weak soil by developing friction between
surrounding earth and outside of the pile, it is known as friction pile.
3.
Sheet piles:
These
piles are used on a rare occasion like retaining wall construction. It helps to
minimize the lateral subsidence of retaining soil.
4.
Anchor Piles:
This
pile opposes the horizontal pull by providing anchorage slip.
5.
Batter Piles:
Batter
piles are used to prevent horizontal and inclined forces.
6.
Fender Piles:
When
ships are bounded at the deck, the concrete deck being damaged. This injury of
concrete deck is protected by fender piles.
7.
Compaction Piles:
Compaction
piles are used to increase the bearing capacity of granular soil.
CLASSIFICATION DEPENDING UPON THE MATERIALS OF PILES:
1.
Timber piles.
2.
Concrete Piles.
3.
Steel Piles.
GRILLAGE
FOUNDATION DESIGN:
Figure
1 demonstrates a simple design of a one tier grillage foundation for a steel
stanchion. The bending moment and shear force required for the grillage beam
design can easily be arrived by the following simple method.
Let,
W = Load supported by one beam in N.
L
= The length of the beam in m.
l
= The length of the base plate in m.
Figure
1 also shows the load distribution. It is apparent the highest bending moment
in the beam
=
(W/2 * L/4) – (W/2 * L/4)
=
W/8(L-l) in Nm
In
the beam maximum shear force develops at the edge of the base plate. The upward
pressure of soil on the beam = W/L N/m
Cantilever
projection of the beam beyond the edge of the base plate
=
(L-l/2)
Maximum
shear force in the beam = W/L (L-l/2)
RAFT
FOUNDATION:
Raft foundation is a type of shallow foundation that is made with dense
reinforced concrete slab covering the total area of the bottom of a structure.
Sometimes the slabs are used at the bottom and the beams at the top. This
foundation is mostly suitable for constructing heavy structures on soft made
ground or marshy sites having a low bearing capacity.
Raft foundation is also provided in the mining area where the structure
is liable to fail due to uncertain behaviour. It gives an economical solution
to difficult site conditions when pile foundation can not be used. This
foundation sometimes is also known as mat foundation because the RCC slab
covers the whole area of a structure like a mat.
METHOD OF CONSTRUCTION:
The raft slab is projected at a distance of 30 cm to 40 cm on every side
of the outer walls of a structure. Thus the area of excavation becomes slightly
greater than the area of the structure. The entire area is then excavated to
its required depth and consolidated. When the surface becomes dry it gives a
strong base on which the raft slab is established.
All the necessary precautions should be taken during the construction of
RCC structures. Further construction should be started after the curing of the
raft.
ADVANTAGES OF RAFT FOUNDATION:
The advantages of raft foundation are as follows:
1. Raft foundation can
be suitably used in difficult site conditions. It is mostly preferred for the
erratic, soft and marshy sites. It bridges the erratic deposits and reduces the
differential settlement.
2. The intensity of
pressure on the foundation soil is considerably decreased due to the
distribution of total load over a large area.
3. The load is
uniformly distributed on the entire foundation area.
PIER
FOUNDATION:
When
a heavy structure or building is to be constructed in soft or sandy soil and
hard bed stratum for the foundation is situated at the reasonable depth pier
foundation is suitably provided for transmitting the load into strong stratum.
In
this method, vertical shafts are made upto hard bed and then filled with freshly
mixed concrete. Thus concrete piers rest on the hard stratum and transfer the
load of the structure into it,
The
diameter and center to center to spacing of shafts mainly depends upon the
character of soil, depth at which the hard stratum is located, condition of
loading etc,
The
diameter of the shaft (In case of square or rectangular pier) should be less
than 1/12 th of its height. Sometimes the shaft may be lined with timber to
prevent thee side earth sliding. The timber lining is eliminated after filling
the shaft with concrete. Generally, reinforce concrete, or grillage cap or
concrete arch is used to connect the shafts with each other. The arrangement of
pier foundation is shown in the following figure.