Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 418

Table 10.9 Target reliability index, β values in EN 1990

Target reliability index, β
1 year Limit state 50 years

Ultimate limit state 4.7 3.8

Serviceability limit state (irreversible) 2.9 1.5

reliability levels for representative structures are as close as possible to the target reliability
index, β values, which are given in Table 10.9 .
The values of the partial factors and combination factors determine the degrees of reliabil-
ity and hence the levels of safety of structures designed using the Eurocode limit state design
method. The recommended partial factor and combination factor values in the Eurocodes,
including the recommended partial factor values in Eurocode 7 shown in Tables 10.7 and
10.8 and the values in the national annexes, have been chosen for the most part on the basis
of a long experience of structural and geotechnical design and building tradition.
The recommended values of partial material factors for soil parameters γ M and partial
resistance factors γ R given in Eurocode 7 for structural and geotechnical designs are for
persistent and transient design situations for different types of ultimate limit states. For acci-
dental design situations, Eurocode 7 states in §2.4.7.1(3) that the partial factors on actions
and the effects of actions should normally be equal to 1.0, and that all partial factors on
resistances should be selected according to the particular circumstances of the accidental
situation. For serviceability limit states, Eurocode 7 states in §2.4.8(2) that the values of the
partial factors should normally be equal to 1.0.

10.5.2 Partial factors in spread and pile foundations designs
The recommended partial factors given in Eurocode 7 have been chosen specifically to verify
that the occurrence of just ultimate limit states is sufficiently unlikely. On account of this,
the combination of partial factors in designs to Eurocode 7 can result in overall factors of
safety for some design situations that are less than those used in previous practice, which
were chosen to limit deformations as well as to avoid failure. An example of this is the use of
the partial factor for the undrained shear strength of soil γ cu = 1.4 in the design using DA1.
C2 of a spread foundation loaded, for example, by a characteristic central vertical load V k
consisting of 70% permanent load G k and 30% variable load Q k . If the foundation rests on
the ground surface, the characteristic resistance is R k = A c u;k N c , where A is the base area of
the foundation and N c = (π +2). The overall factor of safety OFS is defined as

R
V

k
k

(10.27)

OFS =

For designing to Eurocode 7 using DA1.C2, the design resistance, R d = A c u;d N c = A (c u;k /γ cu ) N c
= R k /γ cu . Hence, using this equation for R d in Equation 10.1 and substituting the partial factors
from Table 10.7 gives

GQ R

R

( ) ≤ →××+××≤

k
cu

k

ER ≤→ + γ γ (. 10 07 . V 13 . 03 . V )

d d Gk Qk

γ

k k

14 .

R

k
≤ 14 → 109V .

k

.