Qbu= Abq= Ab Nc cu
where
Ab =end-bearing area of pile
q= bearing capacity of soil
Nt =bearing-capacity factor
cu= undrained shear strength of soil within zone
1 pile diameter above and 2 diameters below pile tip
Although theoretical conditions suggest that Nc may vary between about 8 and 12, Nc is usually taken as 9.
For cohesionless soils, the toe resistance stress, q, is conventionally expressed in terms of a bearing capacity factor Nq and the effective overburden pressure at the pile tip
q=NNq (sigma)’vo<=qt
Some research indicates that, for piles in sands, q, like fs,
reaches a quasi-constant value, ql , after penetrations of the bearing stratum in the range of 10 to 20 pile diameters. Approximately:
ql=0.5Nq tan(phi)
where
phi= friction angle of the bearing soils below the critical depth. Empirical correlations of soil test data with q and ql have also been applied to predict successfully end bearing capacity of piles in sand.
