Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 177

Table 4.12Relationships between pile shaft friction and cone resistance

(after Meigh(4.25))

Pile type Ultimate unit

shaft friction

Timber 0.012 qc

Precast concrete 0.012 qc

Precast concrete enlarged basea 0.018 qc

Steel displacement 0.012 qc

Open-ended steel tubeb 0.008 qc

Open-ended steel tube driven into fine to medium sand 0.0033

Notes

a Applicable only to piles driven in dense groups otherwise use 0.003 where shaft

size is less than enlarged base.

b Also applicable to H-section piles.

Although engineers in the Netherlands and others elsewhere base shaft friction values on
the measured local sleeve friction (fs), the authors prefer to use established empirical correlations
between unit friction and cone resistance (qc). This is because the cone resistance values are
more sensitive to variations in soil density than the sleeve friction and identification of the soil
type from the ratio of qcto fsis not always clear-cut. Empirical relationships of pile friction
to cone resistance are shown in Table 4.12.
A limiting value of 0.12 MN/m^2 is used for the ultimate shaft friction. The values shown
in Table 4.12 are applicable to piles under static compression loading and a safety factor of
2.5 is used for qcvalues obtained from the electrical cone and 3.0 for the mechanical cone
(see Chapter 11). A somewhat higher safety factor would be used for piles subjected to
cyclic compression loading to allow for degradation of the assumed siliceous sand (see
Section 6.2.2 for piles carrying uplift loading).
Cone-resistance values should not be used to determine the shaft friction of bored piles.
This is because of the loosening of the soil caused by drilling as described in the preceding
section.
The end-bearing resistance of piles is calculated from the relationship:

(4.18)

where is the average cone resistance within the zone influenced by stresses imposed by
the toe of the pile. This average value can be obtained by plotting the variation of qcagainst
depth for all tests made within a given area. An average curve is then drawn through the
plots either visually or using a statistical method. The allowablebase pressure is then
determined from the value of the average curve at pile toe level divided by the appropriate
safety factor (Figure 4.18). The value of the safety factor will depend on the scatter of
results. It is normally 2.5 but it is a good practice to draw a lower bound line through the
lower cone-resistance values, ignoring sharp peak depressions provided that these are not
clay bands in a sand deposit. The allowable base pressure selected from the average curve
should have a small safety factor when calculated from the lower bound qcat the toe level
(Figure 4.18a).

qc

qub qc