Pile Load Capacity Calculation in Weathered or Soft Rock
This method of calculating pile load capacity was suggested by Cole & Stroud. Cole & Stroud used ‘N’ values or results of standard penetration tests to find allowable load capacity of the pile in weathered rocks.
The formula suggested by Cole & Stroud is given below. In the formula, the 1st term gives the end bearing resistance and the 2nd term gives the socketed bond strength between rock & pile.
Qa = Allowable load capacity of pile, in kN
Cu1 = Shear strength of rock below the base of the pile, in kN/m2. (This is found out from fig-1, given below)
Cu2 = Average shear strength of rock in the socketed length of pile, in kN/m2. (This is found out from fig-1, given below)
Nc = Bearing capacity factor taken as 9
Fs = Factor of safety usually taken as 3
α = Adhesion factor, (recommended value is 0.9)
B = Minimum width of pile shaft (or diameter in case of circular pile) in m.
L = Socketed length of pile, in m
Notes to Remember
The rock is considered to be weathered rock or soft rock if SPT values or N values are more 60. If less then it is considered as soil.
If N values are more than 200, then, pile load capacity should be calculated on the basis of shear strength or rock.