Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.
Deep foundations are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers.
Deep foundations derive their support from deeper soils or bedrock
Common Types of Deep Foundations are :
1. Pile foundations
are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
Thick slabs used to tie a group of piles together to support and transmit column loads to the piles.
These are foundations for carrying a heavy structural load which is constructed in situ in a deep excavation.
are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.
4. Compensated foundations
are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement.