The shear modulus Gin equation 4.30 can be measured in the field using a pressuremeter
(Section 11.1.4) or a seismic cone penetrometer, or obtained by correlation with CPT data
using the relationship established by Chow(4.32):
(4.31)and
(4.32)The term (^) fin equation 4.30 is equal to twice the average roughness Rclaof the pile
surface which is the average height of the peaks and troughs above and below the centre line.
For lightly rusted steel ris 0.02 mm.
rfis inversely proportional to the pile radius and
tends to zero for large-diameter piles.
In equation 4.31:
A= 0.0203
B= 0.00125
C= 1.216 10 ^6
Piles driven with open ends develop a lower shaft resistance than closed-end piles because
of their smaller volume displacement when a solid plug is not carried down during driving.
The open unplugged end is allowed for by adopting an equivalent pile radius R (see
equation 4.24). Equation 4.29 becomes
= 0.029qc(/Pa)0.13(h/R)–0.38 (4.33)
To use the ICP method the embedded shaft length is divided into a number of short
sections of thickness hdepending on the layering of the soil and the variation with depth of
the CPT readings. A mean line is drawn through the plotted qcvalues over the depths of the
identified soil layers. A line somewhat higher than the mean is drawn when the ICP method
is used to estimate pile driveability when the shaft resistance must not be underestimated.
From a data base of pile tests in calcareous sands, Jardine et al.(4.30)stated that the ICP
method was viable in these materials and recommended that the density should be taken as
7.5 kN/m^3 for calculating and the interface angle (^) fas 25. The third term in equation
4.28 is omitted ( = ) and equation 4.29 for open-end piles is modified to become
= 72( /Pa)0.84(h/R)–0.35. For closed-end piles Ris substituted for R.
The ICP method was used to compare the calculated distribution of interface shear stress
at failure with stresses measured over the shaft depth of a well-instrumented 762 mm ODpile
driven with an open end to a depth of 44 m into medium-fine silty micaceous sand in
Bangladesh. The test was made as part of the trial piling for the foundations of the Jamuna
River bridge at Sirajgang(4.33, 4.34)as described in Section 9.6.2. The observed and calculated
distributions of stress are compared in Figure 4.22. It will be noted that the ICP method
considerably over-estimated the measured stresses. This was commented on by Jardine
et al.(4.30)with no conclusions as to the reasons for the over-estimate. However, he pointed
out that the Jamuna piles developed very marked increases in bearing capacity with time as
noted in Section 4.3.8. A study of the shaft friction measurements made on two 762 mm trial
rc vo
rf rc
vo
rc vo
qcPa voG qc(ABC^2 )Resistance of piles to compressive loads 185