HOW TO CALCULATE PILE LOAD CAPACITY IN WEATHERED ROCK OR SOFT ROCK?

Pile embeded in weathered rock
Pile embeded in weathered rock

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.

Cole & Stroud Formula

Where,

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

Fig-1-(Relation between shear strength or rock and spt values)
Fig-1-(Relation between shear strength of rock and SPT (N) values)

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.

Reference

IS: 2911-Part-1-Sec-2

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