1. Mahatma Gandhi Institute of Technical Education & Research Centre
BASICS OF FOUNDATION
Group No.6
Prepaid by:
Bhoya Minal
Mahla Hiral
Guided by : Asst. Prof.Priyanka Patel

2. Introduction : Purpose of foundation:
It is the lowest part of a structure below the ground level which is in direct contact with the ground and transmits all the loads to the ground.
Majority of structure fails due to failure of their foundation.
Purpose of foundation:
To distribute the weight(load) of the structure over large area so as bring down the intensity of load at it base below the Safe Bearing Capacity of sub-soil.
To support the structures.
To distribute the non-uniform load of the superstructure, uniformly to the sub-soil.
To provide a level and hard surface for concreting and masonry work.
To take the structure deep into the ground and thus increase its stability preventing overturning.
To load the sub-stratum evenly and thus prevent unequal settlement.

3. TYPES OF FOUNDATION A) Shallow foundation 1) Spread footing
2) Combined footing
3) Strap footing
4) Raft or mat foundation
5) Grillage foundation
B) Deep foundation
1) Pile foundation
2) Caissons well foundation
3) Coffer dams

4. 1) Spread footing:
In case of spread footing is made wider so as to distribute the load over wider area.
1. Wall footings
2. Reinforced footings
3. Inverted arch footings
4. Column footings

5. 1) Wall Footings : a) Simple wall footing without masonry offsets.
A spread footing for a continuous wall is called wall footing or strip footing:
a) Simple wall footing without masonry offsets.
b) Wall footing with masonry offsets.

6. 2) Reinforced concrete footing:
In places where the wall are subjected to heavy loads and the bearing capacity of the soil is low , the wall footing described above will work out to be massive in size.
In such cases is desirable to provide reinforced concrete footing below the wall. This appreciably reduces the volume of masonry work in footing and depth of bed concrete and as such proves to be economical. A 7 to 8cm thick bed of lean concrete is usually provided below the reinforced concrete footing.

7. When the wall carries light loads or when the safe bearing capacity of soil is very high, simple strip footing as shown in figure is provided. A bed of clean concrete (B.B.C.C) of uniform thickness not less than 15cm is provided below wall.
When the wall carries heavy loads, or when the safe bearing capacity of soil is less , the base width required will be more than (2t+2p).In that case , it is essential to provide masonry offsets, to achieve larger spread ,before the load is transferred to concrete base as shown in figure.

8. 3) Inverted arch footing:
This type of construction is used on soft solids to reduce the depth of foundation.
One of the draw backs in this type of construction is that the end columns must be strong enough to resist the outward pressure caused by the earth action.
However, with the advent of reinforced cement concrete, inverted arch construction is rarely done these days.

9. 4) Column footing (Isolated footings):
They are used to support individuals columns. They can be either of stepped type or projection in the concrete base. In case of heavy loaded columns, steel reinforcement is provided in both the direction is concrete bed.Generally,15cm offset is provided on all sides of concrete bed. In case of brick masonry columns an offset of 5cm is provided on all the four sides in regular layers shown in figure.

10. 2) Combined footing:
A combined footing supports two columns. It may be rectangular or trapezoidal in shape.
A combined footing is provided:
a) When the two columns are so close to each other that their individual footings would overlap.
b) When the property line is close to one column that a spread footing would eccentrically loaded when kept entirely within the property line.

11. 3) Strap footing:
A strap footing consists of two isolated footing connected with a structural strap or lever as shown in figure. The strap simply acts as a connecting beam and does not take any soil reaction. A strap is more economical than footing when the allowable soil pressure is relatively high and the distance between the columns is large.

12. 4) Raft foundation:
A raft or mat foundation is a large slab supporting a number of columns and walls under the entire structure or a large part of the structure.
A raft foundation is suitable under the following situations:
a) When the allowable soil pressure is low.
b) When the structure loads are heavy.
c) When the columns and walls are so close that their individual footing would overlap.
d) where there is a large variation in the loads on individual columns.
e) When foundation soil is non-homogeneous and there are changes of differential settlements.

13. 5) Grillage foundation:
This type of foundation is used to transmit heavy loads from steel columns to the soil having low bearing power. This type of arrangement avoids deep excavation and provides necessary area at the base to residue the intensity of pressure.
Grillage foundation is made up of Rolled Steel Joists(RSJ),known as grillage beam provided in single one double tiers. The grillage beams are held in position by 20mm dia.spacer bars or 25mm dia.
Pipe separators. In order to protect the beams against corrosion, a minimum cover of 10cm is kept on the outer sides of external beams as well as above the upper flange of top tier.Incase of lower beams the concrete cover under the beam should be more than 15 to 20 cm.

14. Thank you…..