WORLD FOR CIVIL

Toe Capacity Load

When the piles are inserted in cohesive soils then the ultimate load can calculated by:

Q_bu= A_bq= A_b N_c c_u

where
A_b =end-bearing area of pile

q= bearing capacity of soil
N_t =bearing-capacity factor

c_u= undrained shear strength of soil within zone
1 pile diameter above and 2 diameters below pile tip

Although theoretical conditions suggest that N_c 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 N_q and the effective overburden pressure at the pile tip

q=NN_q ?’_vo<=q_t

Some research indicates that, for piles in sands, q, like f_s,
reaches a quasi-constant value, q_l , after penetrations of the bearing stratum in the range of 10 to 20 pile diameters. Approximately:

q_l=0.5N_q tan?

where
?= friction angle of the bearing soils below the critical depth. Empirical correlations of soil test data with q and q_l have also been applied to predict successfully end bearing capacity of piles in sand.