Types of Foundation
Two types of foundation
1. Shallow foundation
2.Deep foundation
Before the types of foundation first known about
What is footing?
The term footing is commonly used in conjunction with shallow. A footing is a foundation unit constructed in brick work, masonry or concrete under the base of wall or column for the purpose of distributing the load over a large area.
shallow Foundation
A foundation is shallow if its depth is equal to or less than its width. A shallow foundation is also known as open foundation. Since such foundation is constructed by open excavation.
Hence those foundation, Which have depth even greater than its width but are constructed by a way of open excavation also come under "shallow foundation".
A shallow foundation is placed immediately below the lower part of the super-structure supported by it.
Types of shallow foundation
1. Spread footing
2. Combined footing
3. Strap footing
4. Mat footing
Spread footing
Spread footing are those which spread the super imposed load of wall or column over a large area. Spread footing support either a column or wall. Spread footing may be following kind. It is also known as isolated footing.
a. single footing for a column
single footing for a column in which the loaded area (b×b) of the column has been spread to the size B×B through a single spread. The base of column generally made of concrete.
b. stepped footing for a columnFig shows the stepped footing for a heavily loaded column, which require greater spread.The base of a column is made of concrete.
c. sloped footing for a columnFig shows the case in which the concrete base does not have uniform thickness, but is made sloped, with greater thickness at its junction with the column and smaller thickness at the ends.
d. Wall footing without stepspread footing for a wall, consisting of concrete base without any step.
e. Stepped footing for a wallstepped footing for a wall with a concrete base.
A spread footing which supports two or more columns is termed as combined footing. Combined footing may be of following kinds:
a. Rectangular combined footing for column
The combined footing for a column will be rectangular in shape if they carry equal loads. The design of rigid rectangular combined footing should be done in such a way that centre of gravity of column loads coincide with the centroid with the footing area.
b. Trapezoidal combined footing for column
If the column carry unequal loads, the footing is termed as trapezoidal in shape.
c. Rectangular and Trapezoidal combined footing with column and wall
sometime it may required to provide combined footing for column and wall.When column carry equal loads rectangular combined footing for column and wall provided.
When column carry unequal loads than trapezoidal combined footing for a column and wall provided.
If the independent footings of two columns are connected by beam it is termed as strap footing.
A strap footing may be used where the distance between the column is so great that a combined trapezoidal footing becomes quite narrow with high bending movement. In that case each column is provided with its independent footing and a beam is used to connect these footings.
The strap footing does not remain in contact with soil and thus does not transfer any pressure to the soil.
It is also known as raft foundation.A raft or mat is a combined footing that a covers the entire area beneath a structure and supports all the walls and column.
Uses
1. When the allowable soil pressure is low, or the building loads are heavy, it may prove more economical to use mat or raft foundation.
2. They are also uses where the soil mass contain compressible lenses or the soil is sufficiently erratic.
3. Raft foundation is also used to reduce settlement above highly compressible soil.
4.The raft tends to bridge over the erratic deposits and eliminates the differential settlements.
Raft foundation may be divided into three types
a. solid slab system
b. beam slab system
c. cellular system
Deep foundation
Types of deep foundation
1. pile foundation
2. pier foundation
3. well foundation
Pile foundation
Pile foundation is the type of deep foundation in which the loads are taken to a low level by means of vertical member. These vertical members may be of steel, timber or concrete.
There are 4 types of pile foundation:
a. End bearing pile
end bearing piles are used to transfer load through water or soft soil to a suitable bearing stratum.
USES: such piles are used to carry heavy loads safety to hard strata. Multi-stored building are invariably founded on end bearing pile so that the settlement are minimised.
b. Friction pile
These piles are used to transfer load to a depth of a friction load carrying material by means of skin friction along the length of the pile.
Uses:Such pile are generally used in granular soil where the depth of hard stratum is very great. c. Combined end bearing and friction pile
These piles which transfer the super imposed load both through side friction and end bearing. such piles are more common when the end bearing pile pass through granular soil.
d. Compaction pile: These piles are used to compact loose granular soil, thus increasing there bearing capacity. The compaction pile do not carry a load themselves. hence they may be of weak material such as timber, bamboo stick etc. A pier foundation consists of a cylindrical column of a large diameter to support and transfer large super-imposed loads to the firm strata below. Pier foundation may be of the following types:
a. Masonry or concrete pier
when a good bearing stratum exists upto 5 m
below ground level, brick, masonry or concrete foundation piers in excavated pits may be used.The size and spacing of piers depends upon the depth of hard bed, nature of overlying soil and super-imposed loads.
The term drilled caissons, foundation pier or sub-pier are in terchangeably used by enginnering to donate a cylindrical foundation.A drilled caissons is a largly a compressed member subjected to an axial load at the top reaction at the bottom.
drilled caissons may be of 3 types:
1. concrete caisson with enlarged bottom.
2. caisson of steel pipe with concrete filled in the pipe.
3. caisson with concrete and steel core in steel pipe.c. Well foundation well foundation or ciassons are box like structure
circular or rectangular which are sunk from the surface of either land or water to the desired depth.
well foundation are hollow from inside which may be filled with sand, and are plugged at the bottom. well foundation are not used for building.
The load is transferred through the permanent wall called steining.
1. Bridge pier and abutments in rivers, lakes etc
2. Quay wall, docks
3. Large water front structure such as pump houses.
4. Break water and other structure for shore protection.
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