When the soil at and near the ground surface is not capable of supporting a structure, deep foundation are required to transfer the load to deeper strata. Pile foundation is such type deep foundation mostly used all over the world. A pile is either driven into soil or formed in-situ by excavating a hole and filling it with concrete.
When a pile is fully embedded in soil with undrained shear strength greater than 10 KN/m2 , the axial carrying capacity of it should not be limited by its strength as long column. If the soil is weak ( undrained shear strength less than 10 KN/m2), it should be examine whether the shaft would behave as a long column.
Where buckling may occur, its structural strength should be decreased to account for reduction in capacity. In such case, in determining effective length of a pile which is not safe against buckling by adequate bracing, the fixity condition imposed on it by the structure it supports and by the nature of soil in which it is installed, is used.