Isolated footing: Isolated footings are provided under each column. These may be square, rectangular or circular in plan. This type of footing is preferred when only single column is appearing in the foundation plan of a particular project.
Combined footing: Combined footing supports two or more column loads. These may be continuous with rectangular or trapezoidal in plan. Need:
1) When the isolated footing overlaps.
2) When the exterior column is close to the property line such that isolated footing cannot be provided.
1) This type of footing is also known as Mat footing.
2) When the soil is having very low bearing capacity or column loads are heavy, the required footing area becomes very large and uneconomical.
3) In such case, raft or mat foundation consisting of a solid reinforcement concrete slab covering entire area beneath the structure and supporting all columns is provided.
4) It minimizes differential settlement.