The following are the
different types of foundations, which are generally used for different
structures :
1. Spread Footing Foundation.
2. Benching or Stepped Foundation.
3. Pile Foundation.
4. Raft Foundation.
5. Well Foundation.
6. Caisson's Foundation.
7. Cantilever Foundation.
8. Combined Footing Foundation.
9. Inverted Arch Foundation.
10. Grillage Foundation.
1. Spread Footing Foundation. This is the simplest type of foundation and is generally used for
ordinary buildings on alluvial soils. This type of foundation can normally be
used for three to four-storied buildings on common type of alluvial soils.
The spread footing
foundation consists of a concrete base, generally lime concrete and a series of
footings below the ground level. The depth and width of foundation depends on
the bearing capacity of the soil and the intensity of load. The depth of foundation
can be calculated by the Rankine's formula which is
d = P/W K²
K = 1-sinø
1+sinø
where P=safe bearing capacity of
soil
w = wt. of soil per cu. metre
ø= angle of repose of
soil.
The width of foundation is given by
W = P / L
where W =Width of foundation.
L = Load of structure
per running metre.
P = Safe Bearing Capacity of soil.
The width of foundation
should in no case be less than 2T+2J, where T is the thickness of
wall and J is the concrete offset to be provided. If the width of
foundation is taken as 2T + 2J, then the number of footings in this
foundation will be equal to number of half bricks in the thickness of wall,
excluding the concrete offsets. For example, for the foundation of 1½ brick
thick wall, three offsets excluding concrete offset, will be provided.
2. Benching on Stepped
Foundation. This type of foundation is provided
on hilly places or in those situations where the ground is slopy. In this
foundation the excavation trenches are made in the form of steps. All the steps
should be preferable of equal length and depth. The function of providing steps
is to avoid unnecessary cutting and filling. The plinth of the structure should
start after the highest point of the ground. Sometimes R.C.C. pile is driven
along the lowest base of the footing to avoid any slipping of the structure
along with the foundation.
3. Pile Foundation. It is one of the important types of foundation
which is used in the following situations :
1. When it is not economical to provide spread foundation and
hard soil is at a greater depth.
2. When it is very expensive to provide raft or grillage
foundation.
3. When heavy concentrated loads are to be taken up by the
foundation.
4. When the top soil is of made up type and of compressible
nature.
5. When there are chances construction of irrigation canals in
the nearby area.
6. In case of bridges when the scouring is more in the river
bed.
7. In marshy places.
Piles are vertical
columns driven into the ground on which wooden or concrete platforms are
supported. The piles are driven at regular distances. The size and distance
apart, of the piles depends
upon the bearing capacity and type of soil and the load of the structure.
Classification of Piles. The piles can be classified according to
(i) Material and (ii) Working.
1. Material classification. The piles are classified as
(a) Wooden Piles.
(b) Concrete Piles.
(c) R.C.C. Piles
(d) Sheet Piles.
(a) Wooden Piles. These are made from trunks of trees, such as Teak, Sal,
Babul, Deodar etc. The wooden or timber piles are generally circular in shape,
the diameter varying from 20 cm to 50 cm. The length of the pile is generally
20 times the diameter. The top of the pile is provided with an iron ring or cap
and the bottom is sharpened and provided with iron shoe. Jf the soil is soft,
blunt piles may be used, but if the ground Contains boulders, metal point
should be Used. Timber piles should be driven below the permanent water table,
otherwise they decay to fungi and insects. These piles are economical and can
be driven rapidly without heavy machinery and much technical supervision.
(b) Concrete Pile. Concrete piles are made cast-in-situ. Holes of the
specified diameter are made into the ground and filled with cement cencrete. Sometimes, the shell driven for making the
hole is left inside and the concrete is filled. The advantage of this is that
there is the shell to protect the cement concrete of the pile from getting
disturbed or eroded by the action of acidic water encountered in the
sub-stratum. These piles are used when they are to be driven to a hard stratum
passing through plastic soils. These are sound in construction as they have not
to bear hammer blows. These are cast in exact lengths and there is no wastage
like in precast piles. The main drawback of these piles is that they cannot
constructed under water.
(c) Reinforced Cement Concrete
Piles. R.C.C. piles are generally precast
and their feet are bevelled like wooden piles. The R.C.C. piles can be
octagonal, square or circular in shape with steel helmets on their top. After
the piles are cured and seasoned, they are driven into the ground. These piles
are 15 cm to 60 cm in diameter and can be 3 m to 30 m in length R.CC. piles
should not contain more than 4% steel. These piles
can be cast early before starting the foundation work and the execution of the
work can be done very quickly. Unlike timber piles these can be used above the
ground water table. But these piles are very heavy and cause difficulty in
transpiration and there are changes of their being damaged m transit.
(d) Sheet Piles. This class of pile is essentially used during the
construction of foundation and not as foundation member of £ structure. Their
main function is to enclose a certain area of the ground within which the
foundation work can be carried and also to confine loose soil and prevent it
from spreading. Sheet piles can be wooden, steel, concrete or R.C.C.
2. Working
classification. This classification is based on the
mode of working of the piles. According to this classification piles are
divided into two groups, (i) Bearing pills, and (ft) Friction piles.
(i) Bearing Piles. These
piles are used to bear vertical loads on their ends. Bearing piles are used in
those places where the depth of hard stratum is not much.
When piles are driven
upto the hard stratum, they transfer the load of the structure to the hard
stratum below, those piles virtually act as columns.
(ii) Friction Piles. When the soil is very loose or soft to a
considerable depth, friction piles are used. These piles balance the load of
the structure by the friction offered by the surrounding soil on the
sides of the piles. They are generally short in length and are not driven to
the hard bed. The surface of the friction piles is made rough so as to increase
skin friction.
The problem of friction
piles is controversial. In some of the soils, the so is become loose due to
some reason or the other and reduce the friction, which may result in the
failure of the structure.
4. Raft Foundation. Raft or mat foundation is used in those places where spread
footing or pile foundation cannot be used advantageously. This type of
foundation is also recommended in such situations where the bearing capacity of
the soil is very poor, the load of the structure is distributed over the whole
floor area, or where a structure is subjected to constant shocks or jerks.
The raft foundation
consists of a reinforced cement concrete Slab or R.C.C. T-beam slab placed over
the entire area. The T-beam slab may consist of primary and secondary beams.
The T-beam may be
inverted also. The inverted T-beam raft foundation is most suited to columned
structures, such as in factories •or
work-shops. The beams and the slab should be constructed all at a time so as to
act as monolithic. The R.C.C. work is laid at the required depth of foundation
and then upto the plinth, the inside spaces are filled with dry sand and
gravel. The R.C.C. slab and beams can be laid directly over the rammed ground
surface or over a bed of lime concrete.
5. Wall Foundation. Wells are a convenient method of securing a trustworthy
foundation in deep sandy and soft solid Well foundation is generally provided
.for in the construction of bridge piers, ghats etc., where the depth of water
is moderate and the foundations are to be carried out in deep sandy soils of
soft soils.
For the construction of
well foundation in running water as for the construction of a bridge pier, a
temporary dam is constructed to exclude the water from the place of
construction. This temporary structure is known as cofferdam. The water from
the inside of the cofferdam is pumped out. Now a well curb made of steel,
concrete or wood with steel cutting edges, is placed over the desired position
where the well is to be sunk. A masonry or concrete steining wall is constructed
upto a height of 1 m. It is then allowed to dry. 1 he earth from the inside of
the well is scooped out either by manual labour or by draggers, and then the
well is allowed to sink. Another height of steining is constructed and the
material from the inside is dragged out. The well sinks due to its own weight.
The process is repeated till the well sinks to the acquired depth or reaches
some hard stratum as the case may be. Before descending the outer surfaces of
the steining is plastered smooth so as to minimize the frictional resistances.
The sinking is tested by putting the desired loads on the top of the well.
When the sinking in all
respects is completed, the lower portion upto a depth of nearly 3 m is plugged
with cement concrete, the middle portion with sand and gravel and the top
portion with cement concrete.
Now an R.C.C. well cup
is constructed over the well. The top of the well cup should be below the bed
level of the river. Over the well cap is now constructed the super structure of
the bridge pier.
6. Caisson's Foundation.
When the depth of water is considerable
and the flow of water is such that cofferdam cannot be constructed easily and
economically, then another method of well foundation is used which is called
Caisson Foundation.
A caisson is a box made
of steel, double walled and water-tight, Laving a well curb with cutting edges
attached to its bottom. The drum is carried to the site, i.e. the
position where it is to be placed. The drum is made to sink with the help of
steel rails or sand bags-and is kept in position upright by means of steel
ropes. The double walled steel caisson is filled with cement concrete, and the
water from the inside in purred out. Now the soil from the inside is scooped
out with the help of draggers and the caisson is allowed to sink slowly. The
length of the caisson is increased by attaching another length of the caisson,
and filled with cement concrete (with some reinforcement it required), soil is
dragged out and it is allowed So sink to the required depth or when it reaches
the hard stratum.The sinking is tested by putting the designed loads over it.
After the sinking is
completed the bottom portion is plugged with cement concrete, middle portion
with sand and gravel and •again the top portion with cement
concrete. The steel caisson above She bed of the river is removed if possible
and the remaining is allowed with the steining. In this case also R.C.C. well
cup is constructed over the top of the caisson which the masonry pier is
constructed.
7. Cantilever Foundation.
This is a typical type of foundation,
which is provided in such pleases where eccentric footings are to be provided
for the external walls or columns due to restrictions of space or some other
reasons. In this type of foundation separate footings are provided for the
external and internal walls and they are simply connected with each other by a
cantilever beam. The tendency of the exterior load to overturn, is balanced by
whole or part of the downward pressure, acting at the other end of it.
8. Combined Footing Foundation. When two or more
columns are supported by a single base area, the foundations to be provided in
such cases are called combined footing foundations. The combined footings are
also provided to establish the exterior columns along the boundry line, for
white symmetrical footings are not possible. The exterior and the interior
columns are constructed on the same base in such a way that the base area of
the combined footing should be equal to the total load of the two columns,
divided by the safe bearing capacity of the soil. The base area should be so
shaped as to be symmetrical along the centre line of the columns.
9. Inverted Arch
Foundation. This is not a common type of
foundation. This type of foundation is used in such places where the bearing
capacity of the soil is very poor and load of the structure is concentrated
over the pillars. The other conditions of the soil are such that deep
excavation are also not possible. For this foundation an inverted arch is
constructed below the foot of pires etc. Generally segmental arches with a rise
of l/5th to l/10th of the span are used. The span of arches will of course
depend upon the arrangement of the pillars. The thickness of the arch ring,
should not be less than 30 cm.
10. Grillage Foundation. This is also a very important type of
foundation and is suitable for those situations where the load of the structure
is pretty heavy and the hearing capacity of the soil is very poor. This
foundation is specially suited where deep excavations are not possible.
Grillage foundations are usually provided for the construction of stanchions.
It consists of a
concrete base over which are placed one or two tiers of I-sections at right
angle to each other. The area of the concrete base is calculated by dividing
the total load of the structure by the bearing capacity of soil. A trench of
the required dimensions, is excavated. Over this a cement concrete block
generally 30 cm to 45 cm in thickness is spread and properly consolidated. When
the contrite is partially dry, I-sections, i.e. Rolled Mild steel joists
are placed at regular distances. (The size and the distance apart of the
I-sections depends upon the load of the structure and the bearing; capacity of
the soil). The lower flanges of the I-sections are connected to the concrete
block by rich cement mortar. The I-sections are themselves connected to each
other by pipes and bolts so as to form a rigid mass. Another tier of I-sections
is placed at right angles to the previous one and connected by means of nuts
and bolts. The whole unit is now embedded cement concrete so as to protect the
steel from corrosion. Over this the structure is, constructed.
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