Foundation is a substructure which transfer the load of superstructure safely to the Load bearing soil Stratum. The types of foundation are as shown in figure.
Shallow Foundation :-
When the depth of the foundation is less than or equal to its width, it is defined as a shallow foundation. Shallow foundation is applied in cases where the good load-bearing soil is at a relatively low depth. The foundation depth must meet the safety requirements of the breakdown, whereby after the load application, the complete structure settlement will be within acceptable limits.
Isolated Footing :-
In building structure where several columns are to be constructed, isolated footing can be adopted. The column involved can be provided with masonry or concrete footing. If masonry footing is provided, Steps are given and the foundation area is thus increased so
that the stresses developed at the base are within the limit. Concrete can be moulded to
any shape and hence a concrete footing may be a sloping one to provide sufficient spread.
In case of masonry footing, the projection of each step must be 1/2 brick thick and each
step is made of 1 or 2 bricks put together. Masonry footing and RCC footing are shown in
Fig. Isolated footing is provided under a column to transfer the load safely to the soil
bed. If the column is loaded lightly, a spread is given under the base of the column. This
spread is known as footing. For heavily loaded column, the total width of the footing may
be very high. This is attained in three or four steps and is called stepped footing. If the total
width of the footing is attained by gradually increasing the width towards the bottom,
then it is called sloped footing.
If the footing is provided throughout the length of the wall in the case of load-bearing
walls, then it is also called as wall footing. Wall footings can be either simple or stepped.
Depth of footing The minimum depth of footing is given by
the Rankine’s formula as,
D = P/W {(1–
sinØ2 )/(1 + sinØ)}
where,
D = minimum depth of footing
P = safe bearing capacity of soil in kg/m2
W = unit weight of soil in kg/m3
Ø= angle of repose of soil in degrees
The minimum depth of footing for the load bearing wall is limited to 90 cm for the stability. Width of footing It is obtained by dividing the load including dead load, live load and wind load etc. by the allowable bearing capacity of the soil.
B=T/P
Where,
B= Width of footing in metre
T= Total load per metre run in kg
P= Safe bearing capacity of the soil in kg/m2
Strap/Cantilever Footing :-
Cantilever footing consists of an
eccentric footing for the exterior
column and a concentric footing
for the interior column and they are
connected by a strap or a cantilever
beam. Such footings are used when
it is not possible to place a footing
directly below a column because of
limitations of boundary or eccentric
loading conditions. The load from
the exterior column is balanced by a load of the interior column acting about a fulcrum.
Combined Footing :-
This type of footing is adopted when the space between two columns is so small that the
foundation for individual columns will overlap. Combined footings are proportioned in
such a way that the centre of gravity of the loads coincides with the centre of gravity of the foundation. Hence these footings have either a trapezoidal or a rectangular shape. The
plan of a combined footing is shown in Figure-a
 |
| Figure-a |
If a footing is constructed for two or more columns, it is called combined footing. The
shape of the combined footing is proportional in such a way that the centre of gravity of
the resultant area is in the same vertical line as the centre of gravity of the loads. Generally,
the shape of the footing is rectangular or trapezoidal as shown in Figure-b
 |
| Figure-b |
When the load of the structure is very heavy and the bearing capacity of the soil is very
low then raft foundation is adopted. In this type of foundation, the load is transmitted
to the soil by means of a continuous slab that covers the entire area of the bottom of
a structure similar to a floor. Due to low bearing capacity, large isolated footings are necessary to support the structure. If the sum of the base areas of the footings required
to support a structure exceeds about half the total building area then it is preferable to
combine the footings into a single raft. In such case, raft foundation exerts the minimum
contact pressure and also counteracts the effect of hydrostatic uplift.
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| Raft Foundation |
Grillage Foundation :-
Grillage foundation is used to transfer the heavy structural loads from steel columns to
a soil having low bearing capacity. It is constructed by rolled steel joists which are
placed in single or double tier. In double-tier grillage, the top tier is placed at right angles
to the bottom tier. The distance between the flanges of rolled steel joists should be equal to 1.5 to 2.0
times the width of the flange or 30 cm whichever is smaller. The steel joists of the grillage
are kept in position by pipe separators and nuts. The whole arrangement of the rolled
steel joists is completely embedded in concrete. The concrete filling is not supposed to
take any load but it keeps the steel joists in position and prevents them from corrosion.
The bed of concrete should have a minimum depth of 15 cm. A grillage foundation may
be constructed for a single column or for more than one column.
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| Grillage Foundation |
Deep Foundation :-
When the depth of the foundation is greater than its width, it is defined as a deep foundation. Deep foundation consists of pile and pier foundations. Pier
foundations are rarely used for buildings. This consists in
carrying down through the soil a huge masonry cylinder
which may be supported on solid rock.
Pile Foundation :-
Pile is an element of construction used
as foundation. It may be driven in the ground vertically or
with some inclination to transfer the load safely. Loads are
supported in two ways, i.e., either by the effect of friction
between the soil and the pile skin or by resting the pile on a
very hard stratum. The former is called friction the pile and
the latter one is the load bearing pile.
Friction piles may be made of cast iron, cement, concrete,
timber, steel, wrought iron and composite materials. Load
bearing piles are steel sheet piles, concrete piles and timber
piles. Piles may be cast-in-situ or precast. They may be cased
or uncased.
Pier Foundation :-
A pier foundation is a collection of large diameter cylindrical columns to support the superstructure and transfer large super-imposed loads to the firm strata below. It stood several feet above the ground. It is also known as “post foundation”.
A pier is a vertical column of a relatively larger cross-section than a pile. A pier is installed in an a dry area by excavating a cylindrical hole of a large diameter to the desired depth and then backfilling it with concrete
Caisson Foundation or Well Foundation :-
This is generally used for making foundations
under water, such as ducks, bridges, etc.
It is not a solid structure like a pier but is
hollow inside, resembling a well. The load
is transferred through the surrounding wall
around called staining. The well is constructed
and brought to the site. Then it is gradually
driven down by digging the soil from inside.
The bottom is plugged with concrete and the
hollow portion is filled with sand. The whole
well is then covered with a well cap above
which the super structure is constructed.
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| Well Foundation |
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