Deep foundations are used for structures or heavy loads when shallow foundations cannot provide adequate capacity, due to size and structural limitations. They may also be used to transfer building loads past weak, compressible or expansive soil layers. While shallow foundations rely solely on the bearing capacity of the soil beneath them, deep foundations can rely on end bearing resistance, frictional resistance along their length, or both in developing the required capacity. Geotechnical engineers use specialized tools, such as the cone penetration test, to estimate the amount of skin and end bearing resistance available in the subsurface.
There are many types of deep foundations including piles, drilled shafts, caissons, helical piers, and earth stabilized columns. Large buildings such as skyscrapers typically require deep foundations.In buildings that are constructed and found to undergo settlement and need foundation repair, underpinning piles such as helical piers can be used to stabilize the existing building.