Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 420

Table 10.10 Comparison of recommended partial factor values in Eurocode 7 for bored piles with
those in UK National Annex

Partial resistance
factor

Recommended
values Parameter UK National Annex values

Bored pile
foundations

R1 R4 R1 R4 without explicit

SLS verification

R4 with explicit
SLS verification

γ R

Base resistance 1.25 1.6 1.0 2.0 1.7 γ b

Shaft (compression) 1.0 1.3 1.0 1.6 1.4

γ s

Total (compression) 1.15 1.5 1.0 2.0 1.7 γ t

Shaft in tension 1.25 1.6 1.0 2.0 1.7 γ t;s

may be used to justify lower partial factor values and hence, a more economical pile design.
The UK National Annex states that the lower R4 values may be adopted if

a. Load tests carried out on more than 1% of the constructed piles to loads not less than
1.5 times the representative load for which they are designed.

b. Settlement is predicted by a means no less reliable than in (a).

c. Settlement at the serviceability limit state is of no concern.

It should be noted that, while the R4 values are increased in the UK National Annex,
Table 10.10 shows that the R1 values are all equal to 1.0, that is, some are reduced. The rea-
son for this is so that when designing piles to DA1 using the UK National Annex, the DA1.
C2 partial factors always control the design of the length of the pile.

10.5.3 reliability differentiation
EN 1990 offers the following two methods for differentiating the reliability of ULS designs:

1. The introduction of Consequences Classes (CC) and Reliability Classes (RC) estab-
lished by considering the consequences of failure or malfunction of the structure and
the exposure of the construction works to hazards. The bases of these classes are
similar to the bases of the Eurocode 7 Geotechnical Categories, referred to in Section
10.2.2.

2. The introduction of different quality levels for the design supervision and inspection
during execution related to the Reliability Class of the structure.

The Consequences Classes relate to the consequences of failure or malfunction of the
structure with regard to the consequences for loss of human life as well as the economic,
social, and environmental consequences. These range from low for CC1, through medium
for CC2, to high for CC3, as shown in Table 10.11 , adapted from Orr (2013), which
includes the examples given in EN 1990. Three Reliability Classes are defined with the
minimum 50-year target reliability index β values for each shown in Table 10.11 . The three
Reliability Classes may be associated with the three Consequences Classes as is also shown
in Table 10.11 . The Consequences Classes, as defined in EN 1990, differ from the Eurocode
7 Geotechnical Categories in that they only take into account the consequences of failure,
that is, the vulnerability factors in Table 10.1 , and do not take into account the hazards that
contribute to the complexity and risk in geotechnical designs.
Eurocode 7 introduces a further method for reliability differentiation related to the dura-
tion of the action and based on the Design Situations, which are defined in §1.5.2.2 of EN