Piled foundations - The Mechanics of Soils and Foundations - page 371

23.2 Base resistance of single piles
The base resistance of a single pile is given by

Q b = q b A b (23.2)

where q b is the bearing capacity at the toe and A b is the area of the pile base. The
general principles for calculation of the bearing capacity of piles are similar to those
for shallow foundations described in Chapter 22. The mechanism of slip surfaces at
the tip of a pile appropriate for an upper bound or limit equilibrium calculation will
be similar to that shown in Fig. 23.3 and we would expect the bearing capacity factors
for piles to be larger than those for shallow foundations. For undrained loading the
bearing capacity is given by

q b = s u N c (23.3)

and, for square or circular piles, N c ≈ 9 (Skempton, 1951). For drained loading the

bearing capacity is given by

q b = σ z N q (23.4)

σ z is the vertical effective stress at the level of the toe of the pile. Values for the
bearing capacity factor N q depend principally on

where

φ and there are a number of published
relationships based on theory and experiment. The values shown in Fig. 23.3(b) are
those given by Berezantzev, Khristoforov and Golubkov (1961).
The choice of the appropriate value of

φ is problematical. Soil below the toe of a
driven pile will be highly strained during driving while there is the possibility of stress
relief and softening at the base of a bored and cast in situ pile during construction.

Figure 23.3 Base resistance of piles.