When and why the mat or raft foundations are used?

Q. When and why the mat or raft foundations are used?

A.
Mat or raft foundations are type of the shallow foundations, which covers the entire area beneath a structure and provides support to all the columns and walls. It is also a type of the combined footing where all the footings are combined in the following cases:

1. When the columns are so close such that their footings nearly touch each other.
2. When the soil is weak and having the low bearing capacity.
3. Where chances of differential settlement exist due to either the existence of the different soils or variation of the moisture content.
4. or where there is a large variation in the loading in the adjacent columns.

reference: 'Analysis and design of substructures' by Swami Saran.

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