The conditions at the interface can be simulated by determining in a ring shear apparatus
where the remoulded clay is sheared against an annular ring fabricated from the same material
and having the same roughness as the surface of the pile. Details of the apparatus and
the testing technique are given in the IC publication which should be consulted for further
information on the development and applications of the ICP method.
For calculating the shaft capacity of open-end piles in clay an equivalent radius R* is
substituted for Rin the h/R term where
R* = (R^2 outer– R^2 inner)0.5 (4.24)and h/R* is not less than 8.
Dealing with the base resistance of closed-end piles in clay, the ICP method does not accept
the widely used practice of calculating the ultimate resistance from Qb= NccuAbwhere the
bearing capacity factor Ncis assumed to be equal to 9. The data base of instrumented pile tests
used in the IC research showed a wide variation in Ncwhich was found to be higher than 9 in
all the tests analysed. However, the results did demonstrate a close correlation with the results
of static cone penetration tests and led to a recommendation to adopt the relationships:
qb= 0.8qcfor undrained loadingand (4.25)
qb= 1.3qcfor drained loading (4.26)The cone resistance qcis obtained from CPT’s by averaging the readings over a distance
of 1.5 pile diameters above and below the toe.
For open-end piles, plugging of the pile toe with clay is defined as the stage when the plug
is carried down by the pile during driving. This is deemed to occur when [Dinner/DCPT
0.45qc]/Pais less than 36. The cone diameter DCPTis 0.036 m and the atmospheric pressure
Pais 100 kN/m^2.
Fully plugged piles as defined above develop half the base resistance calculated by
equations 4.25 and 4.26 for undrained and drained loading respectively, after a pile head
displacement of D/10.
The base resistance of an unplugged open-end pile is calculated on the annular area of
steel only, when qbis taken as the average qcat founding depth it is stated that Qbmay be
increased by a factor of 1.6 for drained loading.
It is evident from the foregoing account of the application of the ICP method to piles in
clay that the reliability of the method depends in the first instance on obtaining good quality
undisturbed samples taken by piston samplers and using thin-wall tubes. Second, the labo-
ratory operations involving oedometer and ring shear testing require special apparatus
handled by skilled technicians. These facilities are not widely available to UK commercial
site investigation contractors. Nevertheless, the accuracy of the predictions by the ICP
method appears to justify its use for offshore construction where savings in estimated pile
lengths are more than offset by the corresponding reduced construction costs.
The ratio of calculated to measured pile resistance derived from the IC data base of 43
piles ranging in diameter from 100 to 570 mm and in length from 3.5 to 57 m showed a sta-
tistical mean of 1.03, a standard deviation coefficient of 0.21 and a coefficient of variation
of 0.20, compared with the corresponding figures of 0.99, 0.32 and 0.33 using the API
(1993) recommendations.
Resistance of piles to compressive loads 183