Correlation between Corrected SPT(N) and pressure by Teraghi and Peak

Engineeering Projects

Terzaghi and Peck (1948) were the first to propose a correlation between the N (corrected SPT) Value and allowed pressure, by presenting a relationship between the size of a footing, the N (corrected SPT) value, and the applied pressure to give a settlement of 25 mm for a deep ground water table, This correlation is shown in Fig.1 below.

correlation of allowable bearing pressure to give 25 mm settlement to spt n value after terzaghi and peak

Procedure:

  • N should not be corrected with respect to overburden.
  • N should be averaged from the given values.
  • The graph has been developed with the condition that the water table is deep.
  • If the Water table is not deep the correction is applied on the obtained allowable soil pressure (qa) by a correction factor of RW.

RW = ½ {1 + Dw/(D + B)}   ———- (1)

water table correction1

Where, DW = Depth of water table measured from the ground surface. DW is equal to 1 if the water table is at great depth. If the water table is in between the ground surface and sum of depth and width of foundation then DW lies between 0.5 t0 1.

q_a(corrected) = q_a(graph) \times R_W           ……..(2)

This standard Terzaghi and Peck Procedure is generally recognized as being conservative as experience was gained.

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