Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 419

Hence, the overall factor of safety for this situation is

R
V

k
k

OFS = = 153 .

Similar calculations using the DA1.C1 partial factors give a lower OFS of 1.40 so that
DA1.C2 controls the DA1 design. The OFS value of 1.53 is much less than the OFS value
formerly used in traditional foundation designs, which was typically between 2 and 3 for
spread foundations on fine-grained soils. Since the recommended partial factors are chosen
to ensure that an ultimate limit state does not occur, Eurocode 7 has the following provi-
sions to ensure the foundation has sufficient reliability against the occurrence of excessive
settlements causing a serviceability limit state. It states in §6.6.2(16) that for conventional
foundations on clays, the ratio R k /V k should be calculated and if this ratio is less than 3,
then settlement calculations should always be undertaken, and if it is less than 2, then the
nonlinear stiffness effects in the ground should be taken into consideration.
In the case of pile foundations bearing in medium to dense soils and for tension piles,
Eurocode 7 states in a note to §7.6.4.1(2) that the safety requirements for the ultimate limit
state design are normally sufficient to prevent a serviceability limit state in the supported
structure . However, the overall factors of safety provided by the recommended partial fac-
tors for pile design in Eurocode 7 are less than the overall factors that have been used for
many years in the design of piles in most countries. For example, using the DA1.C2 partial
factors in Table 10.7 , including γ t = 1.5 for the total compressive resistance, R k for a bored
pile, and with a characteristic vertical load, V k consisting, as in the example above, of 70%
permanent load, G k and 30% variable load, Q k , substitution in Equation 10.1 gives

GQ R

R

k
t

k

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

d d Gk Qk k k

γ 15 .

R
≤ 15

→ 109 k . V

k

.

Hence, the overall factor of safety is

R
V

k
k

OFS = = 164 .

The overall factor of safety is lower than the value, generally between 2 and 3, that was for-
merly used for the design of piles. Hence, most European countries in their national annexes
have adopted a method to increase the overall factor of safety in the design of piles so that
it is similar to that used previously in the design. These methods include choosing partial
resistance factor values for piles that are greater than those recommended in Eurocode 7,
introducing a model factor to increase the resistance factor, or increasing the ξ values.
The partial resistance factors in the UK National Annex for the design of bored piles are
shown in Table 10.10 . These show that the R4 values, which are used with DA1.C2, are
increased compared to the recommended values and that two sets are provided: one set for
when the serviceability limit state is not explicitly verified and another set for when it is.
When the SLS is not explicitly verified, the recommended partial resistance factor on the
total pile resistance is increased by a factor of 1.54, whereas when the SLS is explicitly veri-
fied, the recommended partial resistance factor on the total pile resistance is increased by a
reduced factor of 1.31. This is an example of how the use of pile load tests to verify a design