Why design a mat foundation

A mat (raft) foundation is a large slab covering the whole building area and supporting all columns. It is designed (chosen) in the following situations.

• Low bearing capacity: if spread footings would cover more than about 50% of the plan area, or would overlap, a single mat is cheaper and simpler.
• Reduce settlement: a mat spreads the load over a wide area and reduces the net pressure, and its stiffness reduces differential settlement on erratic or compressible soil.
• Basement and water table: a mat acts as a floor of the basement, resists uplift and water pressure, and is waterproofed easily.
• Compensated foundation: removing the soil weight reduces net pressure on soft clay.
• Heavy, unequal column loads or loads from tall buildings and silos, chimneys, towers.
• Earthquake areas: a rigid mat ties the structure into one unit.