Shallow foundation are those foundations in which the depth at which the foundation is placed is less than the width of the foundation (D < B). Shallow foundations are generally termed as spread footing as they transmit the load of the super structure laterally into the ground.
Classification of Shallow Foundation:
On the basis of design, the shallow foundation are classified as:
- Wall Footing
- Isolated column or Column Footing
- Combined Footing
- Cantilever (Strap) Footing
- Mat (Raft) Foundation
This type of foundation runs continuous along the direction of the wall and helps to transmit the load of the wall into the ground. Wall footing are suitable where loads to be transmitted are small and are economical in dense sands and gravels. In this type of foundation the width is 2-3 times the width of the wall at ground level. Wall footing may be constructed through stone, brick, plain or reinforced cement concrete.
Column footing are suitable and economical for the depth greater than 1.5m. In this type of foundation the base of the column is enlarged. Column footing is in the form of flat slab and may be constructed through plain or reinforced concrete.
Combined footings are those foundations that are made common for two or more columns in a row. It is used when the footing for a column may extend beyond the property line. It is also suitable when the two columns are closely spaced and the soil on which the structure resist is of low bearing capacity. It may be rectangular or trapezoidal in shape.
When an edge footing cannot be extended beyond the property line the edge footing is linked up with the other interior footing by means of a strap beam. Such footings are called as strap footing. It is also know as cantilever footing.
A mat foundation is a combined footing which covers the entire area beneath of a structure and supports all the walls and columns. It is also known as raft foundation. Mat foundation is applicable when:
- Allowable bearing pressure is low.
- The structure is heavy.
- The site is with highly compressible layer.
The mat foundation can be further classified into following types:
- Flat slab type.
- Flat Slab thickened under column.
- Two way beam and slab type.
- Flat slab with pedestals.
- Rigid frame mat.
- Piled mat.