Why foundations; types of foundation

Why foundations are used

• To spread the structural load over a large soil area so the pressure is less than the safe bearing capacity.
• To keep total and differential settlement within permissible limits.
• To place the structure on a stable level below frost, scour and weak surface soil.
• To resist uplift, overturning, sliding and lateral/seismic forces.

Types of foundation

Shallow foundations ($D_f \le B$)

• Isolated (spread) footing: one footing under each column; square, rectangular or circular.
• Wall (strip) footing: continuous footing under a load-bearing wall.
• Combined footing: common footing for two or more columns.
• Strap footing: two footings joined by a strap beam.
• Raft (mat): one large slab under the whole structure; used on weak soil or heavy loads.

Deep foundations ($D_f > B$)

• Pile: slender driven or bored member; end-bearing or friction pile.
• Pier / drilled shaft: large-diameter column, bored and concreted.
• Well (caisson): large hollow masonry or concrete cylinder sunk by dredging, used for bridge piers in rivers.