The foundation of a building is the soil or rock on which it sits. The footing is that portion of its structure that serves to transfer the weight of the building into the ground itself.

Leaning tower of Pisa,
due to faulty foundation

Most foundations extend underground, and the foundations of large buildings often penetrate to the bedrock. One common type of foundation consists of walls that extend below the frost line and transfer the weight to wider footings. Other kinds of foundations include slab foundations , pier and beam foundations and piles.

The primary dangers to a foundation are movement and uneven support. Change in ground water table is a common cause of foundation failure. Also flowing water can remove supporting soil from underneath a bridge foundation and freezing water can heave the supporting soil in one direction and then in the other direction when it thaws.

Changes in soil moisture can cause 'reactive' clay soil to swell (or shrink). This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause a footing sitting on the reactive foundation soil to distort, cracking the structure over it. This is a particular problem for house footings in semi-arid climates such as South Australia, Southern USA, Israel, and South Africa where wet winters are followed by hot dry summers. Raft slabs with inherent stiffness have been developed in Australia with capabilities to resist this movement

When heavy structures are built in arctic areas where the ground is permanently frozen, special refrigeration units must be used to keep the pressure of the structure from melting the supporting soil.

See also

• Building construction
• Geotechnical engineering