Pile Design and Construction Practice, Fifth edition

structural actions and A2 to geotechnical actions. The R3 factors in the DA3 approach are
all unity requiring the ground resistances to be calculated directly from the characteristic
ground properties in a single calculation to verify that Rcdis equal to or greater than Fd.
It will be noted from Tables 4.3 to 4.5 that higher factors are used for uplift loading. The
application of these factors is discussed in Chapter 6.
Whichever of the design approaches or combinations of partial factors are used the
factors for structural actions and geotechnical actions must be selected separately. EC7 gives
no guidance on the factors to be used to obtain the design value of Fdwhere this is caused
by geotechnical actions. Frank et al.(1.5)recommend that the material and resistance factors
as shown in Tables 4.2 to 4.5 should be applied to the characteristic values of the geotechnical
actions to obtain the design values. Because they are equal to unity or greater than unity they
should be applied as multipliersto ensure that Fd(geotechnical) is equal to or less than the
factored ground resistance.
An alternative to the model pile procedure is permitted. Characteristic values of the soil
parameters over the penetration depth of the pile, as determined by field or laboratory
testing, are used to obtain the components Rbkand Rskcharacteristic of the whole site or
homogeneous area of the site. Correlation factors are not applied to these components. It is
likely that their sum, i.e. the characteristic total pile resistance, will be higher than that
calculated using the model pile procedure. Clause 7.6.2.3(8) of EC7 states, ‘If this alternative
procedure is applied the values of the partial factorsbandsrecommended in Annex A
may need to be corrected by a model factor larger than 1.0. The value of the model factor
may be set by the National Annex.’The purpose of the model factor is to make the results
of the alternative method compatible with the model pile method. The partial factor values
shown in Tables 4.1 to 4.17 in this and other chapters of this book are as set out in Annex A
of EC7 but correction factors to modifybandsare not shown in Annex A and the British
National Annex has not been published at the time of preparing this edition.
One further step may need to be taken. If the superstructure or substructure supported by
the piles is stiff enough to redistribute loads from the weaker to the stronger piles, Clause
7.6.2.3(7) allows the correlation factors and to be divided by 1.1 provided that is
never less than 1.0.
Experimental modelsare not used in the day-to-day design of piled foundations. Scale
models (including centrifugal models) have their uses as a general research tool, provided
that they reproduce the pile installation method, and the findings are verified by full-scale
tests and by experience.

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Resistance of piles to compressive loads 149

Table 4.6Correlation factors ( ) to derive characteristic values from

ground test results

n

12345710

1.40 1.35 1.33 1.31 1.29 1.27 1.25

1.40 1.27 1.23 1.20 1.15 1.12 1.06

Notes

n denotes the number of test profiles.

denotes on the mean values of the calculated resistances from ground test

results.

 4 denotes on the minimum values of ground test results.

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