Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 400

situation can occur in the case of designs involving rock where the rock strength is a func-
tion of subjectively determined fuzzy parameters, such as the Geological Strength Index
(GSI). In such situations, it may be appropriate to assess the design value of the geotechni-
cal parameter directly. If this approach is adopted for an ultimate limit state design, then
it is necessary to select a suitably very cautious value such that a worse value is extremely
unlikely to occur. It is noted in §2.4.6.1(5) that the values of the recommended partial fac-
tors in Eurocode 7 should be used as a guide to the required level of safety when design
values are selected directly.

10.4.2.5 Design effects of actions and design resistances
A particular feature of geotechnical design, noted already in Section 10.4.2.1, is that some
actions are a function of the soil strength, for example, the active pressure on a retaining
structure. Also, since soil is a frictional material, the soil strength and resistance in drained
situations are functions of the normal stress and therefore the actions. Hence, the following
general equations, that are functions of the actions, material properties, and dimensions, are
given in Eurocode 7 for the design effect of actions and the design resistance:

E d = γ E E{γ F F rep , X k /γ M , a d } (10.6)

R d = R{γ F F rep , X k /γ M , a d }/γ R] (10.7)

where γ E is the partial action factor and γ R is the partial resistance factor. It should be noted
that if the partial factors γ F and γ M within the brackets are greater than unity, that is, if a
material factor approach is adopted, then the partial factors γ E and γ R are equal to unity; and
similarly, if γ E and γ R are greater than unity, that is, a resistance factor approach is adopted,
then the partial factors γ F and γ M are equal to unity. This is so as to avoid double factoring.

10.4.3 Characteristic parameter values
10.4.3.1 Definition and selection of characteristic values
EN 1990 was originally developed for manufactured materials prepared under controlled
conditions having properties, which are random variables that can be represented by par-
ticular probability distribution, generally assumed to be Gaussian. The characteristic value
of these properties is defined in EN 1990 §1.5.4.1 as the value of a material or product
property having a prescribed probability of not being attained in a hypothetical unlimited
test series. This value generally corresponds to a specified fractile of the assumed statisti-
cal distribution of the particular property of the material or product. When the low value
of the property is critical, EN 1990 states in §4.2(3) that the characteristic value should be
defined as the 5% fractile, unless stated otherwise stated (author's underline). Such a situa-
tion occurs in Eurocode 7 in the case of geotechnical design where an alternative definition
is given for the characteristic value of geotechnical parameters as explained below.
Due to the particular features of soil, the application of the design method in EN 1990 to
geotechnical design, including the definition of the characteristic value of a material prop-
erty, has caused some difficulties. The features of soil that cause geotechnical designs to
differ from structural designs, and hence, Eurocode 7 to differ from the other Eurocodes,
with regard to uncertainty and risk, have been identified by Orr (2012). These include the
fact that soil is a natural material, nonhomogeneous, and often very variable. The properties
of soil and rock vary over a very wide range compared to the properties of manufactured