INTRODUCTION:-
Foundation is
an important part of the structure which transfers the load of the
structure to the foundation soil. The foundation distributes the load over a
large area. So that pressure on the soil does not exceed its allowable bearing
capacity and restricts the settlement of the structure within the permissible
limits. Foundation increases the stability of the structure. The settlement of
the structure should be as uniform as possible and it should be within the
tolerable limits. (footings)
Why we
provide Footings?
In simple
words, Consider 1 cu.m of concrete weight 2400 Kgs to 2600 Kgs depending
on mix. Think for a Two storeyed building how much concrete needed? How much
quantity of bars needed? to construct a building. Foundation is to be strong
enough to bear that all loads without settlement, So for spreading
the vertical load to large area footings are constructed.
Main
Functions of foundation:-
- Distribution of loads
- Stability against sliding & overturning
- Minimize differential settlement
- Safe against undermining
- Provide level surface
- Minimize distress against soil movement
Depending on
Soil bearing capacity of a particular location. Different Types of Footing are
constructed.
Classification
of Foundations:-
Foundations
are mainly classified into two types:
- Shallow Foundations
- Deep foundations
If depth of
the footing is equal to or greater than its width, it is called deep footing,
otherwise it is called shallow footing.
Where We
Provide Shallow foundation and Deep foundation?
Foundations
may be shallow or deep foundations depending upon the load and type of
foundation soil. If the load to be supported is very high and soil is of low
bearing capacity, Deep foundations are provided. If the soil has adequate
bearing capacity at reasonable depth then shallow are provided.
Deep foundations
and Shallow foundations are further classified into following types:-
Shallow
Foundations or Spread Foundations:-
Spread
foundations are used when the soil has sufficient strength within a short depth
below the ground level. They need sufficient plan area to transfer the heavy
loads to the base soil. As mentioned above we chose shallow foundations when
the soil has adequate Soil bearing capacity. Footings in Shallow
foundations may be of Masonry, Plain concrete or reinforced
concrete. The depth of shallow foundations are generally less than
its width.
Different
types of Shallow foundations are further divided into two types depending on
the soil bearing capacity:-
I.
Isolated footing:-
Footings
which are provided under each column independently are called as isolated
footings. They are usually square, rectangular or circular in section. Footing
is laid on PCC. Before laying PCC termite control is done to restrict the
termites to damage the footing. These footings are most economical when
compared with the other kind of footings. These kind of footings are provided
where the soil bearing capacity is generally high. Isolated footings comprise
of a thick slab which may be flat or stepped or sloped.
Advantages:-
1. Economical when columns are placed at longer distances.
2. Workmen with little or no knowledge can easily construct.
3. Ease of Constructability:- Excavation, Form-work, Reinforcement placement
and placing of Concrete is at ease.
(i) Flat
or Pad or Plain footing:-
These kind of
footings are generally square or rectangular or circular in shape which are
provided under each column independently. Flat or Footing is one
of the Shallow Foundation. It is circular, square or rectangular slab of
uniform thickness.
(ii)
Stepped footing:-
As from the
name it’s resembling that footings are stacked upon one another as steps. In
these type of footing three concrete cross sections are stacked upon each
other. This type of footing is used generally in residential
buildings. These types of footings are constructed in olden days now they are
outdated.
(iii)
Sloped Footing:-
Sloped
footings are trapezoidal footings. They are done with great care to see that
the top slope of 45 degree is maintained from all sides. When compared
the trapezoidal footing with the flat footing the usage of concrete is less.
Thus, it reduces the cost of footing in concrete as well as reinforcement.
(iv) Shoe
or eccentric footing –
Shoe footing
is the half footing cut-out from the original footing and it has a shape of
shoe. It is provided in the corner of the plot . Where there is no provision of
setback area. They are constructed on property boundary. Column is provided or
loaded at the edges of footing. It is provided when the exterior column is
close to the boundary or property line and hence there is no scope to project
footing much beyond the column face. Shoe footings are constructed when the
soil bearing capacity is the normal 24KN/m3
(v)
Combined footing:-
A footing
which has more than one column is called as combined footing. This kind of
footing is adopted when there is a limited space. Due to lack of space we
cannot cast individual footing, Therefore footings are combined in one footing.
They are classified into two types based on their shape:
- Rectangular combined footing.
- Trapezoidal combined footing.
II. Raft
or Mat footing:-
When the
column loads are heavy or the safe bearing capacity of soil is very low,
The required footing area become very large. As mentioned this footing is
in shallow foundation. So in order to spread the load over large area with less
depth then we have to increase the footing area. If we increase footing area,
the footings are overlapped each other instead of providing each footing on
each column all columns are placed in common footing. A raft or mat
footing is a solid reinforced concrete slab covering entire area beneath
the structure and supporting all the columns. Such foundation due to its own
rigidity minimizes differential settlements.
It is
provided in a places like seashore area, coastal area area where the water
table is very high and soil bearing capacity is very weak.
When number
of column in more than one row, provided with a combined footing, the footing
is called mat or raft footings.
How to
decide which footing is to be adopted Isolated footing or Raft footing:-
- If the bearing capacity of the soil is very good and
the super structural load is very small. The use of isolated
foundation is recommended.
- If the bearing capacity of the soil is very low, like
less than 100 kpa ( this isn’t an exact number but it could be used as a
boundary)
- If the super structural load to be transferred to the
foundation is very high that the area of the isolated footing to
be used is more than half the area covered by the building( this is
recommended by Joseph Bowles)
- When we provide elevator in the building a separate
raft may be provided on the elevator shaft.
- When the soil contains lenses (or weak zones) that
should be bridged then raft might be used.
Raft footing
does not have Neck column, they start directly from the ground surface but
reinforcement of neck column starts from Raft.
Deep
foundations:-
If the depth
of a foundation is greater than its width, the foundation is known as deep
foundation. In deep foundation the depth to width ratio is usually greater than
4 to 5. Deep foundations as compare to Shallow foundations distribute the
load of the super structure vertically rather than laterally. Deep foundations
are provided when the expected loads from superstructure cannot be supported on
shallow foundations.
Pile
footings:-
A pile is a
long vertical load transferring member composed of either timber, steel or
concrete. In pile foundations, a number of piles are driven in the base
of the structure.
They are
constructed where excessive settlement is to be eliminated and where the load
is to be transferred through soft soil stratum, where the Soil bearing capacity
is sufficient.These types of footings are provided when the Soil bearing
capacity of soil is very weak and the Ground water table (level) is
high. These types of the footings are generally designed on sea
shore areas, bridges, etc.
The main
objective of providing piles under the footing is to prevent structure from
settlement. If we don’t provide pile under the footing, then the building will
have settlement. Piles are hammered in to the ground till hard strata (in
compressible) layer of earth is found.
Basically
they are divided into two types they are:-
1. Pre cast
Piles.
2.
Cast-in-situ piles.
1. Precast
Piles:
The piles
which are casted already and transported to the site. These kind of piles are
used where there is less place to cast pile. These are readymade piles and
requires more money to transport piles to the site.
The
pre-cast concrete piles are usually reinforced concrete or pre-stressed
concrete piles. These piles require space for casting and storage, more time to
set and cure before installations and heavy equipment handling and driving. The
precast concrete piles are generally used for a maximum design load of about
800 kN except for large pre-stressed piles. The length of precast concrete
piles varies from 4. 5 m to 30m The pre-stressed concrete piles as
compared to pre-cast and reinforced concrete piles are lesser m weight easy to
handle, have high load carrying capacity and are extremely durable.
Advantages:-
- Reinforcement provided in Precast piles are not
liable to change it place and disturbed.
- The cost of Precast piles manufacturing is less
because large number of precast piles are manufactured at a time.
- The defects in pile can be easily identified after
the removal of formwork, and these defects (such as presence of cavity or
hole) can be repaired before driving the pile in site.
Disadvantages:
- These pile are heavy so it requires special equipemnt
to carry and transport the pile to site.
- Sufficient care must be taken at the time of
transportation, otherwise piles may break.
- For embedding these piles in field heavy pile driving
equipment is required.
- These piles are costly as extra reinforcement is
required to bear handling and driving stresses
- Post insertion of pile in ground it is not possible
to increase the depth of pile. As the length of the pile is restricted.
2.
Cast-in-situ piles.
the piles
which are casted on site. And dont require any transportation is called cast in
situ piles. The cast-In-situ concrgte piles are casted in position inside
the ground and need not to be reinforced in ordinary cases. These piles
are not subjected to handling or driving stresses. The cast- in-situ concrete
piles are generally used for a maximum design load of 750 kN.
Piles are
more classified into different types. Basically there are classified as above.
