Design of single piles - Piling Engineering - page 96

Figure 4.1 Axially loaded pile.

where A b is area of the pile base.
q b is end-bearing pressure.

A s is area of the pile shaft.

τ s is average limiting shear stress down the pile shaft.

The relative magnitude of the shaft and base capacities will depend on the geometry
of the pile and the soil profile. Piles which penetrate a relatively soft layer of soil to
found on a firmer stratum are referred to as 'end-bearing' piles and will derive most
of their capacity from the base capacity, Q b . Where no particularly firm stratum is
available to found the piles on, the piles are known as 'friction' or 'floating' piles. In
cohesive soil, the shaft capacity is generally paramount, while in non-cohesive soil or
where the pile base is underreamed, the overall capacity will be more evenly divided
between shaft and base. In cohesive soil, it is generally assumed that the shaft capacity
in uplift is similar to that under downward loading, but it is now acknowledged that
the shaft capacity in uplift for piles in non-cohesive soil is significantly less than that
in compression (see section 4.1.1).
The shaft capacity of a pile is mobilized at much smaller displacements of the pile
(typically 0.5 to 2% of the pile diameter) than is the base capacity. The latter may
require displacements as large as 5 to 10% of the pile base diameter (even larger for
low-displacement piles in granular soil) in order to be fully mobilized. This difference
between the load deformation characteristics of the pile shaft and that of the pile
base is important in determining the settlement response of a pile, and the sharing of
load between shaft and base, under working conditions. To illustrate this, Figure 4.2
shows results from a load test on a bored pile, 0.8 m diameter by 20 m long. The
measured response has been simulated numerically using a load transfer approach