There are different types of foundation are used for every different situation in the construction of building, house, dam, skyscraper.
Types of Foundation
Foundations may be broadly classified in two heads:
Shallow foundation and Deep foundation.
According to Terzaghi, if the depth of the foundation is equal to or less than its width,then it is called shallow foundation.
Foundations,where the depth is more than the width, are known as deep foundations.
Shallow foundations may be of the following four types:
Spread footings are those, which are having the base enlarged or spread. These footings,therefore,spread the superimposed load to a larger area. Spread footings may be of the following three types.
(a)Spread footing for a column:
In case of brick masonry columns, an offset of 50 mm is provided on all the four sides in regular layers.
Column footings are used to support individual columns. They can either be stepped type or have projections in the concrete base.
- Generally 150 mm offset is provided on all sides of concrete bed.
- The footing of concrete columns may be slab,stepped or sloped type.
(b) Spread footing for a wall:
Wall footings can either be simple or stepped. The base course of these footings can be of concrete or even of the same material.
- Simple footing can be used for light structures.
- They have only one projection beyond the width of the wall on either side.
- Usually,masonry walls have stepped footing with a concrete base.
(c) Grillage foundation-
Grillage types of foundation is used to transmit heavy loads from steel columns to the soils having low bearing capacity.
The depth of such a foundation is limited to 1 m to i.5 m.
Grillage foundation is made up of rolled steel joists known as grillage beams provided in single or double tiers.
(2) Combined footing
A footing, which supports two or more columns, is called a combined footing.
Combined footing is invariably constructed of reinforced concrete.
It may be of the following two types :
(a) Rectangular combined footing
(b) Trapezoidal combined footing
The design of the combined footing should be done in such a way that centre of gravity of column loads should coincide with the centroid of the footing area. Because of this, combined footing for columns will be rectangular in shape if they cany equal loads. In case of columns canying unequal loads, the footing is of trapezoidal shape. In case of space limitations also, footing is of trapezoidal shape.
(3) Strap types of footing
Strap footing is also known as cantilever footing. This type of footing may be used where the distance between the columns is so great that a combined footing will prove uneconomical.
In strap footing, two columns are connected by a beam called strap beam. The strap beam does not main in contact with the soil and thus does not transfer any load to the soil.
(4) Rift Foundation/Mat foundation
A raft or mat foundation is a combined footing that covers the entire area beneath a structure and supports all the columns.
When the allowable soil pressure is low or the loads are heavy, the footings would cover move than half of the building area and there it may prove more economical to use raft or mat foundations.
They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.
The mat or raft foundations tend to bridge over the erratic deposits and eliminates the differential settlements.
A raft foundation may undergo large settlements without causing harmful differential settlements. For this reason, almost double
The types of foundation whose depth is more than its width is called a deep foundation.
There are different types of deep foundations like pile foundation, well foundation etc.
Pile foundation is that type of deep foundation in which loads are taken to a low level by means of vertical members, which can be of concrete, timber or steel.
Necessity of Deep Foundations and Pile Foundations
Deep foundations or pile foundations are used in the following conditions :
- When the soil at or near the ground surface is not capable of supporting a structure,pile foundations or deep foundations are required to transfer the loads to deeper strata.
- When the plan of the structure is irregular with respect to its outline and the load distribution, which would cause a non-uniform settlement if a shallow foundation is constructed, pile foundations are adopted. Pile foundations reduce the differential settlement,
- Pile foundations are adopted for the transmission of structural loads through deep water to a firm stratum. Pile foundations are used to resist horizontal forces, in addition to the vertical forces, in earth-retaining structures and tall structures that are subjected to horizontal forces due to wind and earthquake.
- Pile foundations are required when the soil conditions are such that a washout, erosion or scour of soil may occur from underneath a shallow foundation.
- Pile foundations are used for structures like offshore platforms, transmission towers, which are subjected to uplift pressures.
- In case of expansive soils, such as black cotton soil, which swell or shrink as the water-content changes, piles are used to transfer the load below the active zone.
- Collapsible soils, such as loss, have a breakdown of structure accompanied by a sudden decrease in void ratio when there is an increase in water content. Piles are used to transfer the load beyond the zone of possible moisture changes in such soils.
Different Types of Deep Foundations Piles used for building foundation may be of four types:
- End bearing piles,
Combined end bearing and friction piles.
(1) End bearing piles-
End bearing piles transfer load through 8011 soil to a suitable bearing stratum. Multi-storeyed buildings are invariable founded on end bearing piles.
(2) Friction piles
The places, where hard strata is available at a very great depth, the piles are designed to transfer the load by the phenomenon of skin friction along the length of the pile. Such piles are called friction piles.
(3) Combined end bearing and friction piles
Sometimes piles transfer the superimposed load both through side friction as well as end bearing and therefore are called ‘ combined end bearing and friction piles.
(4) Compaction piles
Compaction piles are used to compact loose granular soils. They, thereby, increase the bearing capacity of the respective soils. Since the compaction piles themselves do not carry any load, they may be made with weaker materials like timber, bamboo sticks or even sand.