Pile Design and Construction Practice, Fifth edition

piles showed that the distribution of interface shear stress could be represented by the
relationship f= 0.009(h/d)–0.5qfin compression and 0.003(h/d)–0.5qcin tension.
The ICP method uses CPT data to calculate the base resistance. For closed-end piles the
equation is:

qb= qc[1 – 0.5 log(D/DCPT)] (4.34)

where qcis the cone resistance averaged over 1.5 pile diameters above and below the toe, D
is the pile diameter, and DCPTis the cone diameter. The equation is valid provided that the
variations in qcare not extreme and the depth intervals between peaks and troughs of the qc
values are not greater than D/2. If these conditions are not met a qcvalue below the mean
should be adopted. A lower limit for qbof 0.3qcis suggested for piles having diameters
greater than 0.9 m.
A rigid basal plug within an open-end pile is assumed to develop if the inner diameter in
metres is less than 0.02 (Dr 30) where the relative density Dris expressed as a percentage.
Also Dinner/DCPTshould be less than 0.83qc/Paand the absolute atmospheric pressure Pais
taken as 100 kN/m^2.
If the above criteria are satisfied the fully plugged pile is stated to develop a base resistance
of 50% of that of a closed-end pile after the head has settled by one-tenth of the diameter.
A lower limit of qbis that it should not be less than that of the unplugged pile and should
not be less than 0.15qcfor piles having diameters greater than 0.9 m.
The base resistance of unplugged piles is taken as 0.5qcmultiplied by the net cross-
sectional area of the pile at the toe, where qcis the cone resistance at toe level. No
contribution is allowed from the inner wall shaft friction. For a solid end pile qbat the toe is
determined from equation 4.34.

186 Resistance of piles to compressive loads

25

0

10

20

30

Depth below ground level (m)

40

50

50

Interface shear stress (kN/m^2 )

75 100

ICP method

Pile toe 44.2 m

Measured on test pile

Test pile:

125 150

762 mm OD × 38 mm WT

Driven with open end into

medium-fine silty micaceous sand

Figure 4.22Comparison of measured and calculated interface shear stress on the shaft of a steel tube
pile driven into sand.