piles embedded in weathered rock
piles embedded in weathered rock

Calculation of Pile Load Capacity in Weathering or Soft Rock

This method of calculating pile load capacity was suggested by Cole & Stroud. Cole & Stroud used the ‘n’ value, or the results of standard penetration tests, to determine the allowable load capacity of piles in weathered rocks.

The formula suggested by Cole & Stroud is given below. in formula, 1scheduled tribe Term end gives bearing resistance and 2Ra The term gives the socketed bond strength between the rock and the pile.

Cole and Stroud Formula


Whya = Allowable load capacity of the pile, kN. In

CU1 = shear strength of the rock below the base of the pile, kN/m. In2, (This is shown in Figure-1 below)

CU 2 = Average shear strength of the rock over the socketed length of the pile, kN/m. In2, (This is shown in Figure-1 below)

nC = bearing capacity factor 9. taken as

Fs = factor of safety usually 3 . is taken as

α = adhesion factor, (recommended value is 0.9)

B = Minimum width of pile shaft (or diameter in case of circular pile) in metres.

L = socket length of the pile, m . In

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

notes to remember

  • A rock is considered a weathering rock or soft rock when the SPT value or N value is greater than 60. If less then it is considered clay.
  • If N values ​​are greater than 200, the pile load capacity should be calculated on the basis of shear strength or rock.


IS: 2911-Part-1-Sec-2

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.