As per IS-2911-Part1 / Sec-2, the ultimate load carrying capacity (Qu) of pile in cohesive soils is given by the given below, where the first term represents the end bearing resistance(Qb) and the second term gives the skin friction resistance(Qs).
Qu = Utimate load capacity, kN
Ap = Cross-sectional area of pile tip, in m2
Nc = Bearing capacity factor, may be taken as 9
αi = Adhesion factor for the ith layer depending on the consistency of soil. It depends upon the undrained shear strength of soil and may be obtained from the figure given below.
ci = Average cohesion for the ith layer, in kN/m2
Asi = Surface area of pile shaft in the ith layer, in m2
A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity(Qsafe) from ultimate load capacity (Qu).