Piled foundations - The Mechanics of Soils and Foundations - page 373

Figure 23.5 Negative shaft friction due to ground settlement.

φ r ≤ δ ≤ φ p . For clays, pile and the soil and for a rough pile this will be in the range

Eq. (23.7) is often simplified to

τ s = βσ z

(23.8)

δ is an empirical parameter that depends on the nature of the soil
and on the method of pile installation.
Pile installation influences both

β =

where K tan

δ and K but differently. When a pile is driven into
the ground there will be very large shear displacements between the pile and the soil,
and in clays these displacements will probably be enough to reduce the soil strength
to its residual value. However, pile driving is likely to increase the horizontal effective
stresses which will tend to increase the shaft friction. On the other hand if a pile is
driven into cemented soil, the horizontal stress after driving and the available shaft
friction could be very small indeed. A cast in situ concrete pile is likely to have very
rough sides and so the available shearing resistance will lie between the peak and the
critical state strength of the soil. However, boring a hole in the ground to construct a
cast in situ pile will reduce the horizontal stresses which may be reduced still further
as the concrete shrinks during setting and curing. For both driven and cast in situ piles
there are compensating effects on

δ and on K .
Notice that in a soil that is settling, perhaps due to the weight of fill placed at the
surface or due to groundwater lowering, the shaft friction will act downwards on the
pile as shown in Fig. 23.5, causing negative shaft friction.

23.4 Pile testing and driving formulae
Because of the considerable uncertainties in the analysis of pile load capacity, both in
calculation of base resistance and shaft friction, some of the piles on a job will often
be subjected to load tests to demonstrate that their capacity is adequate. In typical