Types of shallow foundation are constructed where the soil layer at shallow depth ( up to 1.5m) is able to support the structural loads. The depth of the shallow foundation is less than its width.
It is used where the bearing capacity of soil on which the structure is to be constructed is maximum then a shallow foundation is used. The minimum depth of the shallow foundation is 800 mm and the maximum depth is not taken more than 4 m.
Types of Shallow Foundation
- Strip footing
- Spread or isolated footing
- Combined footing
- Strap or cantilever footing
- Mat or raft foundation
It is provided for a row of columns that are closely spaced that their spread footing overlap or nearly touch each other.
It is also provided for a load-bearing wall. In this case, it is more economical to provide a strip footing than to provide a number of spread footing in one line. It is also known as continuous footing.
Spread or isolated footing
A spread footing is also called isolated footing. The spread footing is structural members used to support columns and walls and to transmit and distribute their loads to the soil.
The shape of spread footing is circular, square, or rectangular. IT is used to provide support for an individual column.
A combined footing supports two columns. It is used when the two columns are very close to each other. It is also provided when the property line is Very close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line.
It may be rectangular or trapezoidal in plan. It is used where soil baring capacity is low.
Strap or cantilever footing
A strap footing consists of two isolated footings connected with a structural strap. A strap footing connects two footings in such a way that they behave as one unit. It is designed as a rigid beam.
A strap footing is economical more than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is high. The individual footing is designed such that their combined line of action passes through the results of the total load.
Mat or Raft foundations
A mat or raft foundation is a large slab that supports a number of columns and walls under the entire structure or a large part of the structure. A mat is used when the allowable soil pressure is low and where the columns and walls are very close that individual footing would overlap or nearly touch each other.
Mat foundations are useful in reducing the differential settlement s on non-homogeneous soils or where there is a large variation in the loads on individual columns.