Peck, Hanson and Thornburn (1974) – Peck, Hanson and Thornburn (1974) have modified the original Terzaghi and Peck (1948) recommendations and presented charts as shown in Fig.1 to obtain allowable bearing pressure to give a settlement of 25 mm for a footing of known width, B with its base at a depth, Df resting on a granular deposit in which N (corrected SPT) values are measured.
- N (corrected SPT) value should be corrected with respect to dilitancy and overburden.
- Average of the corrected value is considered.
- qa is obtained from the graph of (Df/B) verses N (corrected SPT) value.
- If the water table is not deep the Correction is applied on the obtained allowable soil pressure (qa) by a correction factor of RW = 0.5 + 0.5 DW/(D+B
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 to l.
qa(corrected) = qa(graph) Ã— RW
The value of bearing capacity factors can be found out from the approximate correlation between the N value and the angle of shearing resistance proposed by Peck, Hanson and Thornburn (1974) as shown in Fig.2 below.
Peck, Hanson and Thornburn (1974 have also given an equation for net safe bearing capacity of spread footing to give a permissible settlement s and a differential settlement of 3/4th of s. The equation is:
qns = 0.042 RWNs ———- (1)
Where, N = Average SPT value corrected for dilatancy and overburden pressure
s = Permissible settlement, mm
RW = Water table correction factor