Pile Design and Construction Practice, Fifth edition

146 Resistance of piles to compressive loads
actions can also occur from transversely applied loads such as those on piles supporting bridge
abutments caused by surcharge from the adjacent approach embankments.
EC7 in Clause 7.3.2.1(3)P states that evaluation of geotechnical actions can be undertaken
in two ways:
(a) by pile–soil interaction analyses when the degree of relative pile–soil movement is
estimated and t–z curves are produced by computer to give the corresponding strains
and axial forces in the pile shaft (Section 4.6). In the case of transversely applied actions
a p–y analysis is performed (Section 6.3.5). Alternatively actions can be estimated from
other forms of analysis, such as finite element analysis.
(b) The upper-bound force exerted on the pile by the ground movement is calculated and
treated as an action.
Method (b) when applied to actions from negative skin friction can give over-conservative
designs if due consideration is not given to variations in frictional forces over the depth of
the pile shaft (Section 4.8).
Having determined the actions, treating structural and geotechnical actions separately, it is
then necessary to show that the design value of resistance of the ground against the pile (Rcd) at the ultimate limit state is equal to or greater than the design value of the action (Fd). Rcd for example, the resistance to axial compression can be calculated by the ‘model pile’ method
which assumes that a pile of the same penetration depth and cross-sectional dimensions as
proposed for the project is installed at the location of each borehole or in-situ test. The two
components of total pile resistance, that is, the shaft and base resistance, are calculated for
the mean and minimum soil parameters for each borehole or test profile. The two components
are then divided by a correlation factor, �,
which depends on the number of ground test
profiles on the project site or particular area of the site exhibiting homogeneous ground
properties. Clause 7.6.2.3.5(P) does not make it clear whether the profiles represent mean or
lower bound lines drawn through the plotted points of laboratory test results on samples from
boreholes, or whether they refer only to profiles from in-situ tests such as the cone or
pressuremeter test. The authors have assumed that profiles of laboratory test results can be
used to obtain the correlation factors as shown in Table A10 of Annex A in EC7 (Table 4.6 in
this book). The resulting characteristic pile resistances are given by the equations:
and
R ck mean � (R b cal mean � R s cal mean)/ (4.3a)
R ck min � (R b cal min � R s cal min)/ (4.3b)
� 4
� 3
where Rck mean and Rck min are the mean and minimum characteristic pile resistances respectively,
and Rb cal and Rs cal are the calculated base and shaft resistances.
Rck mean is calculated from the arithmetic average of the total resistance Rc cal mean obtained
from each borehole or in-situ test profile on the project site or part of the site, while Rc cal min
is selected from each borehole or test profile showing minimum values. Rs cal is calculated
from the average of the ground properties over the depth of the pile shaft, and Rb cal from the
properties in the region of the pile base.
The correlation factors �3 and �4 in the above equations refer specifically to calculations
of mean and minimum ground resistance based on the results of tests on borehole samples