Design of single piles - Piling Engineering - page 171

Figure 4.54 Typical variations with depth of negative skin friction.
is similar to that described by Fellenius (1989). For end-bearing piles, the distribution
of negative friction will generally increase steadily with depth, until a point close to
the pile base, where the soil movement is insufficient to mobilize full shaft friction
(see Figure 4.54(a)). For a long friction pile, however, there will be a neutral point,
generally some way above the mid-point of the pile, where pile and soil settlement are
equal, and below which positive friction is mobilized to carry the load applied to the
pile (see Figure 4.54(b)).
For single piles, the load transfer method may be used to assess pile response
under combinations of loads applied at the pile head and external soil movements
(for example, Randolph (2003)). A graphical method that illustrates the principles of
how to establish the level of the neutral plane is shown in Figure 4.55. The external
applied load, P 0 , and the load induced by negative friction, P n , is balanced by the end-
bearing resistance, Q b , and shaft capacity in the lower part of the pile, Q s . The neutral
plane occurs where pile and soil settlement are equal, and the depth of the plane will
rise as the external load increases. The ultimate pile capacity, P u or Q u , is unaffected