# HOW TO CALCULATE PILE LOAD CAPACITY? (STATIC ANALYSIS)

|The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground.

The bearing capacity of a pile depends primarily on 3 factors as given below,

- Type of soil through which pile is embedded
- Method of pile installation
- Pile dimension (cross section & length of pile)

While calculating pile load capacity for cast in situ concrete piles, using static analysis, we need to use soil shear strength parameter and dimension of pile.

**Load Carrying Capacity of Pile Using Static Analysis**

The pile transfers the load into the soil in two ways. Firstly, through the tip-in compression, termed as “*end-bearing*” or “*point-bearing*”; secondly, by shear along the surface termed as “*skin friction*”.

**Load carrying capacity of cast in-situ piles in cohesive soil**

The ultimate load carrying capacity (Q_{u}) of pile in cohesive soils is given by the formula given below, where the first term represents the *end bearing resistance* (Q_{b}) and the second term gives the *skin friction resistance* (Q_{s}).

Where,

Q_{u} = Ultimate load capacity, kN

A_{p} = Cross-sectional area of pile tip, in m^{2}

N_{c} = Bearing capacity factor, may be taken as 9

α_{i} = 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.

c_{i} = Average cohesion for the ith layer, in kN/m^{2}

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

A minimum factor of safety of 2.5 is used to arrive at the safe pile load capacity (Q_{safe}) from ultimate load capacity (Q_{u}).

Q_{safe} = Q_{u}/2.5

**Load carrying capacity of cast in-situ piles in cohesion less soil**

The ultimate load carrying capacity of pile, “Q_{u}”, consists of two parts. One part is due to friction, called *skin friction* or *shaft friction* or *side shear* denoted as “Q_{s}” and the other is due to *end bearing* at the base or tip of the pile toe, “Q_{b}”.

The equation given below is used to calculate the ultimate load carrying capacity of pile.

Where,

A_{p} = 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}

δ_{i} = Angle of wall friction between pile and soil for the ith layer

A_{si} = 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 (**Q _{b}**) and the second term is the expression for the skin friction capacity of pile (

**Q**).

_{s}A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity (Q_{safe}) from ultimate load capacity (Q_{u}).

**Q _{safe}**

**= Q**

_{u }/ 2.5**Important Notes to remember**

- The value of bearing capacity factor N
_{q}is obtained from the figure given below.

- The value of bearing capacity factor N
_{γ}is computed using the equation given below.

- For driven piles in loose to dense sand 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 stem.
- 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.

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Please send me how we determine pile capacity using test!