What is raft foundation?

A raft or mat can be considered as a combined footing which covers the entire building area and supports all columns and walls. Such a foundation may consist of a slab of uniform thickness or a slab stiffened by beams either above or below the slab as demonstrated in figure as:

RAFT FOUNDATION

When the allowable soil pressure is low or the building loads are heavy, the use of individual spread footings would cover a major portion of the plan area. When the total area of the footings covers more than fifty percent of the area, it is more economical to use a raft for the entire structure.