Foundation

The foundations of the building transfer the weight of the building to the ground.  While 'foundation' is a general word, normally, every building has a number of individual foundations, commonly called footings.  Usually each column of the building will have its own footing.
Since the weight of the building rests on the soil (or rock), engineers have to study the properties of the soil very carefully to ensure that it can carry the loads imposed by the building.


Shallow Foundation
A shallow foundation is a type of foundation which transfers building loads to the earth very near  the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earth bag foundations.

Spread footing foundation
A spread footing foundation, which is typical in residential building, has a wider bottom portion than the load-bearing foundation walls it supports. This wider part "spreads" the weight of the structure over more area for greater stability.


 Mat-slab foundations
Mat-slab foundations, also called on-grade mat foundations for expansive soils, are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.


 Slab-on-Grade foundation
Slab-on-grade or floating slab foundations are a structural engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then placed into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor.


 Deep Foundation:
Deep Foundations are those in which the depth of the foundation is very large in comparison to its width.
Which are not constructed by ordinary methods of open pit   excavations.

Uses:
In cases where the strata of good bearing capacity is not available near the ground
The space is restricted to allow  for spread  footings.
In these cases the foundation of the structure has to be taken deep with the purpose of attaining a bearing stratum which is suitable and which ensures stability and durability of a structure.
The bearing stratum is not the only case. There may be many other cases. For example, the foundation for a bridge pier must be placed below the scour depth, although suitable bearing stratum may exist at a higher level.

Types of Deep Found
Most common forms of construction pertaining to deep foundations are:
  • Pile Foundation (more commonly used in building construction)
  • Cofferdams
  • Caisson or Well Foundation