At the time when the soil in or near the ground surface is not capable of supporting a structure then deep foundation are utilized for the transfer of the loads to the deeper and stronger/tougher strata. Thus the deep foundation are employed when the surface soil is unfit for the construction of shallow foundation and the firm stratum is deep such that it cannot be achieved economically using shallow foundations. The highly used types of deep foundations are piles, piers and caissons. The mechanism for the transfer of loads into the soil strata is usually similar in all types of deep foundations.
The deep foundations are usually very expensive than the shallow foundations. It has to be employed only when the shallow foundation is unfit or unfeasible. Sometimes, a fully compensated floating raft comes out to be better and economical than deep foundation. Sometimes it becomes feasible to improve the soil strata through various methods in order to make it suitable for the lay of shallow foundations than to use deep foundations.
Types of deep foundations:
Pile: It is a slender structural member constructed from steel, concrete or wood. The piles are either directly embedded or driven into the soil strata or may be formed in-situ by excavating appropriate hole and then filling it with concreting.
Pier: It is vertical slender column having relatively bigger cross-section than a pit. The pier is installed in a dry area by excavating a big cylindrical hole of required depth and then backfilled with concrete. Generally the cast in-situ pile having diameter bigger than 0.6m is called as pier.
Caisson: It is a hollow, water tight, air tight, well built box or chamber that is embedded into the ground for laying foundation under water. The pier and caisson only differ in construction.