As per IS-2911-Part1/sec2, the ultimate load carrying capacity of pile, Qu, consists of two parts. One part is due to friction, called skin friction or shaft friction or side shear, Qs and the other is due to end bearing at the base or tip of the pile toe, Qb.
The equation given below is used to calculate the ultimate load carrying capacity of pile.
Ap = cross-sectional area of pile base, m2
D = diameter of pile shaft, m
γ = effective unit weight of the soil at pile tip, kN/m3
Nγ= bearing capacity factor
Nq = bearing capacity factor
Φ = Angle of internal friction at pile tip
PD = Effective overburden pressure at pile tip, in kN/m2
Ki = Coefficient of earth pressure applicable for the ith layer
PDi = Effective overburden pressure for the ith layer, in kN/m2
δi = Aangle of wall friction between pile and soil for the ith layer
Asi = Surface area of pile shaft in the ith layer, in m2
The first term is the expression for the end bearing capacity of pile (Qb) and the second term is the expression for the skin friction capacity of pile (Qs).
A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity(Qsafe) from ultimate load capacity (Qu).
Qsafe = Qu/2.5
Important Notes to remember
The value of bearing capacity factor Nq is obtained from the figure given below.
The value of bearing capacity factor Nγ is computed using the equation given below.
For driven piles in loose to dense sand with with φ varying between 300 to 400 , ki values in the range of 1 to 1.5 may be used.
δ, the angle of wall friction may be taken equal to the friction angle of the soil around the pile sem.
The maximum effective overburden at the pile base should correspond to the critical depth, which may be taken as 15 times the diameter of the pile shaft for φ ≤ 300 and increasing to 20 times for φ ≥ 400
For piles passing through cohesive strata and terminating in a granular stratum, a penetration of at least twice the diameter of the pile shaft should be given into the granular stratum.