Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 397

- Ground profile
- Structural geometry

Nominal values
Geometrical allowances
e.g., overdig

Geometrical
data

- Ground
- Structure
- Groundwater
- Environment
- Seismicity

Partial factors, γ
Combination
factors, ψ
Allowable
deformations

Nominal values
Characteristic
values
Correlation
factors, ξ

Design complexity
Risk assessment
Geotechnical Categories
Comparable experience

Actions and
geotechnical
parameters

Verification
method

- Geotechnical investigations
- Field and laboratory tests
- Derived parameters

- Consideration of limit states
- Calculations
- Prescriptive measures
- Load or model tests
- An observational method

Figure 10.1 Geotechnical design triangle showing components of geotechnical design and, in italic type,
measures in Eurocode 7 to reduce risk and provide reliability.

the levels of ground and dimensions of geotechnical structures or elements should usually
be nominal values, that is, values chosen on a nonstatistical basis, corresponding to values
obtained from the ground investigation or specified in the design. The design value of a geo-
metrical parameter is generally equal to the characteristic value, since the partial factors for
actions and material properties include an allowance for minor variations in the geometrical
data and therefore no further safety margin on the geometrical data is normally required.
For situations involving water pressures and designing against limit states with severe con-
sequences, generally ultimate limit states, §2.4.6.1(6)P states that the design values of the
water pressures represented by geometrical data in the form of groundwater levels shall be
the most unfavorable values that could occur during the design lifetime of the structure and,
for limit states with less severe consequences, generally serviceability limit states, the design
values shall be the most unfavorable values that could occur in normal circumstances.
Where deviations in a geometrical parameter have a significant effect on the reliability of
a structure, the design value of the geometrical parameter a d is assessed either directly or
derived from nominal values using the following equation:

a d = a nom ± Δa (10.3)