Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 411

A further assumption in the derivation of the ξ values is that the structure to be supported
by the piles is not stiff and strong enough to transfer loads from piles where the ground is
weaker to piles where the ground is stronger. If, however, the structure is stiff and strong
enough to do so, then the ξ values may be reduced by the factor 1.1, provided they do not
become less than 1.0. This takes advantage of the Eurocode 7 requirement when selecting
characteristic parameter values, noted in Section 10.4.3.1, to take account of the ability of
the structure to transfer loads from strong to weak zones.
The fact that the ξ values reduce as the number of pile load tests and the number of pro-
files of tests increase reflects the fact that obtaining more information about the ground
properties results in higher, that is, less cautious, R c;k values and hence, more economical
pile designs with the same degree of reliability. This shows the advantage of carrying out
more load tests and obtaining more test profiles.
10.4.4 Partial factors, safety levels and reliability
10.4.4.1 Types of ultimate limit state and recommended partial
factor values
The values of the partial factors that are applied to representative actions and characteristic
resistances are chosen to obtain design values that, when used in calculation models, will
provide acceptable levels of reliability against the occurrence of an ultimate limit state and a
serviceability limit state. The following five different types of ultimate limit state are identi-
fied and defined in §2.4.7.1(1)P of Eurocode 7:
• EQU, which is loss of equilibrium of the structure or the ground, considered as a rigid
body, in which the strengths of structural materials and the ground are insignificant
in providing resistance
• STR, which is internal failure or excessive deformation of the structure or struc-
tural elements in which the strength of structural materials is significant in providing
resistance
• GEO, which is failure or excessive deformation of the ground, in which the strength
of soil or rock is significant in providing resistance
• UPL, which is loss of equilibrium of the structure or the ground due to uplift by water
pressure (buoyancy) or other vertical actions
• HYD, which is hydraulic heave, internal erosion and piping in the ground caused by
hydraulic gradients
Since the relative significance of, and also the uncertainties in, the actions and resistances
of materials differ for these ultimate limit states, separate sets of partial factors have been
established for each type of limit state. Recommended values for the partial factors are given
in Eurocode 7 to achieve structures with the appropriate degrees of reliability and hence
safety. Since setting the safety levels and choosing the required degrees of reliability for
structures in a country is a national responsibility, the values of the partial factors to be used
for geotechnical designs in a particular country are published by that country's national
standards body in its national annex to Eurocode 7.
The recommended partial action, material, and resistance factors in Eurocode 7 for use
in STR and GEO ultimate limit states, which are when the strengths of structural materials
or the ground are significant in providing resistance, are shown in Table 10.7. In the case of
pile foundations, the recommended partial resistance factors for bored piles are only given
in Table 10.7 ; however, Eurocode 7 also provides partial resistance factors for driven and
CFA piles. The partial factors for actions, soil parameters, and resistances are presented in