Civil Engineering Reference
In-Depth Information
Bond and Harris (2008) give guidance on establishing a cautious estimate of a geotechnical parameter
whilst illustrating the variation in values that can result. Addressing the same issue, Simpson (
2011)
adds
interpretation to the Eurocode text to aid users of the code to understand the processes involved. It can
be argued that only a skilled and experienced geotechnical engineer can interpret test results and other
factors rigorously enough to establish a reliable cautious estimate. To help address this, Bond (
2011)
has
developed a procedure for determining the characteristic value based on simple statistical methods which
goes some way in helping designers overcome the challenges in establishing the cautious estimate. Fur-
thermore, Schneider and Schneider (2013) present a simplified statistical approach based on the mean
and standard deviation or coefficient of variance of a soil property, combined with consideration of the
vertical extent of the influencing failure mechanism, to determine the characteristic value.
Also helpful is an illustration by Hicks (2013) on the potential use of the random finite element method
in determining characteristic values by using it to quantify the combined effects of spatial averaging
soil properties along a failure plane, with the fact that failure planes tend to follow the path of least
resistance.
In offering alternatives to the cautious estimate approach, Eurocode 7 Part 1 states that both statistical
methods and standard tables of characteristic values can be used if sufficient geotechnical measurements/
results exist. However, as stated above, the likelihood of large enough data sets of geotechnical test
results existing is small.
5.4.2 Partial factors of safety and design values
The calculation method prescribed in Eurocode 7 Part 1 is the limit state design approach used in con-
junction with a partial factor method. The use of partial factors of safety, ensure that the reliability of the
various components of the design are assessed individually, rather than assessing the overall safety of the
system as is the practice with a global factor of safety approach. This means that partial factors of safety
are applied to all actions, material properties and resistances for each limit state being checked. There
are thus many partial factors of safety that have to be considered.
Partial factors are denoted by the general symbol,
γ
.
(Note: this symbol is, of course, already used by geotechnical engineers to represent 'unit weight'
(known as weight density in Eurocode 7) and thus a bit of care may be needed initially when using this
Greek letter gamma to represent the partial factor.)
Partial factors on actions are denoted by the symbol,
γ
F
.
Partial factors on material properties are denoted by the symbol,
γ
M
.
Partial factors on resistances are denoted by the symbol,
γ
R
.
Partial factors for specific parameters are identified by the subscript; e.g. partial factor of safety on coef-
ficient of shearing resistance, tan
φ
is denoted by
γ
φ
.
The partial factors to be used for the different limit states are provided in Annex A of EN 1997-1:2004.
The National Annex can offer national choice for each partial factor.
The verification of any limit state involves an assessment of the effect of the
design
actions against the
magnitude of the
design
resistance being offered by the structure or the ground. These design values are
obtained by combining the characteristic values with appropriate partial factors of safety. Once the design
values have been established, the geotechnical analysis is performed to check that the effects of the
design actions do not exceed the design resistance.
5.4.3 Design values of actions
The sequence of taking a characteristic value of an action through to the design effect of the action
involves multiplying the characteristic value (F
k
) by a correlation factor (
ψ
) (in accordance with EN 1990:2002)
then multiplying the resulting representative value (F
rep
) by a partial factor of safety (
γ
F
) to yield the design
