Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 02 : Shallow Foundations

Raft Foundation (Mat Foundation)

• A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the walls and columns.
• It may prove more economical to use mat or raft foundation, when the allowable soil pressure is low, or the building loads are heavy or the use of spread footings would cover more than one half the area.
• They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.
• They tend to bridge over the erratic deposits and eliminates the differential settlements.
• 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.
• Raft foundations may be divided into three types, based on their design and construction.
  • Solid slab system.
  • Beam slab system.
  • Cellular 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 will be governed by the spacing and loading of the column and degree of rigidity required in the raft.