# Load carrying capacity of cast in-situ piles in cohesion less soil-(IS-2911-Part1-Sec2-2010)

|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.

Where,

Ap = cross-sectional area of pile base, m^{2}

D = diameter of pile shaft, m

γ = effective unit weight of the soil at pile tip, kN/m^{3}

N_{γ}= bearing capacity factor

N_{q} = bearing capacity factor

Φ = Angle of internal friction at pile tip

P_{D} = Effective overburden pressure at pile tip, in kN/m^{2}

K_{i} = Coefficient of earth pressure applicable for the ith layer

P_{Di} = Effective overburden pressure for the ith layer, in kN/m^{2}

δi = Aangle of wall friction between pile and soil for the ith layer

Asi = Surface area of pile shaft in the ith layer, in m^{2}

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(Q_{safe}) from ultimate load capacity (Qu).

Q_{safe} = Q_{u}/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 30
^{0}to 40^{0}, k_{i}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 ≤ 30
^{0}and increasing to 20 times for φ ≥ 40^{0} - 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.