Types of Foundations
Foundation:
Foundation is one of the essential parts of the structure. The purpose of providing a foundation to a structure is to distribute the load to the soil in such a way that the maximum pressure on the soil does not exceed its permissible bearing value and at the same time settlement is within the permissible limit.
Foundation is the part of a structure on which the building stands. The solid ground on which it rests is known as foundation bed.
Foundation is provided and fulfils the following objectives:
· Distribute the weight of the structure over a large area of soil.
· Avoid unequal settlement.
· Prevent the lateral movement of the structure.
· Increase structural stability.
Footing:
The footing is the portion of the foundation which ultimate load delivers to the soil.
Types of Foundations
Mainly there are two types of foundation:
1. Shallow Foundation
2. Deep Foundation
According to Terzaghi (Father of Geotech Engineering), a foundation is shallow if its depth is equal or less than its width. Following are shallow foundation types:
I. Spread or Isolated Footing
II. Strip Footing
III. Combined Footing
a. Rectangular Combined Footing
b. Trapezoidal Combined Footing
c. Strap footing
d. Raft Footing
IV. Strap or Cantilever Footing
V. Mat or Raft
VI. Grillage Foundation
a. Steel Grillage Foundation
b. Timber Grillage Foundation
In the case of deep Foundation, the depth is greater than width. Apart from deep strip, rectangular or square foundations other common forms of the deep foundation are:
1) Pier Foundation
2) Pile Foundation
3) Well Foundation
Shallow Foundation
Spread or Isolated footing foundation
Isolated footings (also known as Pad or Spread footings) are commonly used for shallow foundations in order to carry and spread concentrated loads, caused for example by columns or pillars. Isolated footings can consist either of reinforced or non-reinforced material. . This kind of footing is used to support single-columns and when the columns are arranged relatively at long distance.
Strip Footing
Strip footings, otherwise known as strip foundations, a wall footing or strip footing is a continuous strip of concrete that serves to spread the weight of a load-bearing wall across an area of soil. It is the component of a shallow foundation. Usually boasting a founding level that is no greater than 3m from the ground surface. As the name suggests, the formation is in a strip of linear structure that ultimately serves to spread the weight across the total area of the soil.
Combined Footing
When individual footing has to combine two or in exceptional case more than two columns is known as the combined footing. Or when two or three columns are located very near to each other, we provide a single substructure for them and the footing is known as a combined footing. Combined footings are also usually made of reinforced concrete. A combined footing may be rectangle or trapezoidal in plan. The object is to get uniform pressure distribution under the footing.
A combined footing is used in the following conditions:
1. When two columns are very close enough to each other such that their footing overlap.
2. When the load-bearing capacity of the soil is low, the requirement of more area under the individual footing.
3. When the column is close to the property line, So that footing cannot be spread in that direction and centre of gravity of column will not coincide with the centre of gravity of footing.
Strap or cantilever footing Foundation
Strap footing consists of two or more individual footings connected by a beam called a strap. It is also sometimes called a cantilever footing. This type of footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow with high bending moments.
Mat or Raft Foundation
Raft foundation is very useful when the load coming on the soil is practically uniform while the soil is soft clay or reclaimed soil.
The method consists of providing RCC slab of suitable thickness and with necessary reinforcement. The raft is designed in this way that the allowable bearing power of the soil is not exceeded. If required slab and beam construction in RCC. Can also be carried out. The raft is designed as an inverted RCC roof with a uniformly distributed load of the soil pressure and supported by wall, beam and columns.
If more than 50% of the plan area of a building gets covered by the combined plan areas of all the individual or combined footing, which needed to be provided. It is usually more economical to make a mat or raft. A raft is structurally rigid and when it settles, settles uniformly as a monolithic entity.
Grillage Foundation
Grillage Foundation definition – The grillage foundation consists of two or more tiers of beams laid at right angles to distribute the load over a large area. This type of foundation is generally used for heavy structure columns piers and stanchions.
This type of foundation consists of steel or wooden joints arranged in a stepped manner. It distributes the load over a large area.
A trench 90cm to 150 cm deep is excavated and levelled. Then a layer of 23cm to 30 cm of cement concrete 1:2:4 or 1:1.5:3 is spread and compacted.
Over this concrete steel, I-beams are laid and are placed at s suitable intervals of 45 to 90 cm. The length of these I-beam is equal to the width of the foundation.
The space between the I beams is filled with cement concrete. Over these, the second layer of I-beams is placed at a right angle to the first layer.
Again the internal space is filled with cement concrete. The wall is built directly over the grillage bed.
This type of foundation is used for heavy building such as factories, town halls and towers.
Deep Foundation
Pile foundation
Pile foundation is a deep foundation used where the topsoil is relatively weak. Piles transfer the load to a lower stratum of greater bearing capacity by way of end bearing or to the intermediate soil through skin friction. This is the most common type of deep foundation generally used for buildings where a group of piles transfer the load of the superstructure to the sub-soil.
Well foundation
This type of foundation is usually adopted in the construction of the bridge and long span culverts or where the soil or quite soft and sandy generally in water. Well foundations are made to masonry or concrete.
Pier Foundation
Pier foundation is a type of deep foundation which consists of a cylindrical column of large diameter to support and transfer large superimposed loads to firm strata below. Pier is inserted down to the bedrock. Pier has a footing. Pier is typically dug out and cast in place using formwork. There are two types of pier foundation:
· Masonry or Concrete Pier
· Drilled Cassion
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