SHALLOW FOUNDATION - Mat Foundation (Raft Foundation).

A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the walls and columns. When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one half the area and it may prove more economical to use mat or raft foundation. They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be diflicuht to control. The mat or raft tends to bridge over the erratic deposits and eliminates the difĂ­erential setlements. Raft foundation is also used to reduce settlement above highly compressible soils. by making the weight of structure and raft approximately equal to the weight of the soil excavated.

Rafts may be divided into three types, based on their design and construction:

(i)   Solid slab system
(i)   Beam slab system
(iii) Celular system

All the three types are basically the same, consisting of a large, generally unbroken area of slab covering the whole or the large part of the structure. The thickness of the slab and the size of beams (if any) will be governed by the spacing and loading of the column and degree of rigidity required in the raft.

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