When pile and mat foundations are chosen

Pile foundation is chosen when

• The top soil is weak, soft or loose to a large depth, and a hard layer lies deeper (end-bearing piles); or the soil is uniformly soft and friction piles are used.
• Building loads are heavy (high-rise buildings, bridges, towers) and settlement is critical.
• The site has a high water table, is submerged, or is near a river where scour is expected.
• The foundation must resist uplift or lateral load (towers, retaining structures, jetties).
• Expansive, liquefiable or filled ground is present.
• Shallow excavation near existing structures is not possible.

Mat (raft) foundation is chosen when

• The soil has low bearing capacity, so that individual footings would cover more than 50 % of the plan area.
• Loads are heavy and columns are closely spaced, so footings would overlap.
• Differential settlement must be reduced, such as on compressible or non-uniform soils.
• The basement is below the water table, so the raft acts as a waterproof floor and resists uplift.
• The structure is a silo, chimney, tank, or multi-storey building with a basement.
• The site is in an earthquake area, where a single rigid slab gives better behaviour.

Piles transfer the load to depth, while a mat spreads it over a large area at shallow depth; a mat is therefore cheaper when the soil near the surface is moderately firm.