The most commoly used formula to determine the allowable static load on a pile is the Engineering News formula.This is given below:

P_a=(2WH)/(p+1)

For piles driven by a steam hammer, the allowable load is given as:

P_a=(2WH)/(p+0.1)

where
P_a=allowable pile load, tons (kg)
W= weight of hammer, tons (kg)
H= height of drop, ft (m)

p= penetration of pile per blow, in (mm)

The two equations include a factor of safety of 6. For a group of piles penetrating a soil stratum of good bearing characteristics and transferring their loads to the soil by point bearing on the ends of the piles, the total allowable load would be the sum of the individual allowable loads for each pile. For piles transferring their loads to the soil by skin friction on the sides of the piles, the total allowable load would be less than the sum on the individual allowable loads for each pile, because of the interaction of the shearing stresses and strains caused in the soil by each pile.