Raft / Mat foundation | principles and design of mat foundation | types of mat foundation

A mat foundation is also called as raft foundation. A raft foundation is a thick reinforced concrete slab which supports all the load-bearing walls and column loads of a structure or a large portion of structure.

Principles and design of mat / raft foundation:

• When the loads are heavy and the soil is very weak or highly compressible in this condition mat foundation is required.
• A raft foundation is more economical than individual footings when the total base area required for the individual footings exceeds about one-half of the area covered by the surface.
• For individual footings when the soil mass has very erratic properties and contains lenses of compressible soil in this case a raft foundation is preferred.
• If individual footings are provided in such a case, it would be difficult to control the differential settlements.
• Raft foundations are generally at some depth below the ground surface, a large volume of soil is excavated and the net pressure on the soil is considerably reduced. 

Types of mat / raft foundation:

1. flat plate type
2. flat plate thickened
3. beam and slab construction
4. box structures
5. mats placed on piles

Design methods for raft / mat foundations:

1. rigid beam method
2. simplified elastic method
3. elastic method
4. non-linear elastic method.