(a) Shallow foundations
(b) Deep foundations
According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width. A shallow foundation is also known as an open foundation, since such foundation is constructed by open excavation. Hence those foundations, which have depth even greater than its width, but are constructed by way of open excavation also come under ‘shallow foundations’. A shallow foundation is placed immediately below the lowest part of the super-structure supported by it.
The term footing is commonly used in conjunction with shallow foundations. A footing is a foundation unit constructed. in brick work, masonry or concrete under the base of a wall or column for the purpose of distributing the load over a large area.
From the point of view of design, footings are classified into four types:
1. Spread footing
2. Combined footing
3. Strap footing
and 4. Mat or raft foundation.
A Spread footing is the one which supports either one wall or one column. A spread footing may be of the followíng types:
1. Strip footing It is the spread footing for wall.
2. Pad footing. It is the isolated footing for a column.
When a spread footing supports the load of more than one column or wall, it is called a combined footing or strap footing. A mat foundation is a foundation unit continuous in two directions covering an area equal to or greater than the base area of the building.