Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 401

materials, such as steel and concrete, and have higher coefficient of variation (COV) values.
However, as well as varying locally, the properties of soil and rock also vary spatially. The
spatial variation maybe taken into account using the autocorrelation distance, which is the
separation distance at which the correlation between the soil properties can be considered
to be relatively weak, or the scale of fluctuation (SOF), which is the distance over which the
properties are relatively strongly correlated. The SOF is equal to about twice the autocor-
relation distance (Schneider and Schneider, 2013).
The first requirement in §2.4.5.2(1)P for selecting the characteristic values of geotechnical
parameters, arising from soil being a natural material, is that they shall be based on values
derived from laboratory and field tests, which shall be complemented by well-established
experience. In structural design, where a large number of samples of the structural material
can be tested and where the variability of the material in the section being designed is usu-
ally small, the results of individual tests are representative of the behavior of the material in
that section, and hence the characteristic value is a particular fractile of the test results. In
geotechnical design, however, only a few tests are carried out on a very small portion of the
total volume of soil involved in a particular limit state and hence, the characteristic value
of a geotechnical parameter is not defined as a particular fractile of test results. Instead,
Eurocode 7 states in §2.5.4.2(2)P that the characteristic value of a soil parameter shall be
selected as a cautious estimate of the value affecting the occurrence of the limit state. Thus,
the characteristic value is normally a cautious estimate of the mean value on the failure sur-
face, not a particular fractile of the test results.
When selecting the characteristic value of a geotechnical parameter, Eurocode 7 requires
in §2.5.4.2(4)P that the following factors be taken into account:
• Geological and other background information, such as data from previous projects
• Variability of the measured property values
• Extent of the ield and laboratory investigations
• Type and number of samples
• Extent of the zone governing the behavior of the structure at the limit state being
considered
• Ability of the structure to transfer loads from strong to weak zones
The wording in Eurocode 7 to define and select characteristic geotechnical parameter
values is a major improvement on previous geotechnical codes of practice, most of which
provided no guidance on how to select soil parameter values. However, the definition of the
characteristic value in Eurocode 7 has purposely been worded so as not to be prescriptive.
How cautious an estimate the characteristic parameter value should be has been left to the
designer to decide, taking into account the factors listed above, which include the designer's
experience of the particular ground conditions, the nature of the structure being designed,
and the anticipated failure mechanism. However, selection of the characteristic value of a
soil parameter also needs to take into account the extent of the zone of ground involved in
the limit state and prior knowledge of comparable ground behavior. This is why Eurocode
7 defines the characteristic value of a geotechnical parameter as a cautious mean value that
is selected by the designer, that is, it involves subjective judgment.
Due to the subjective nature of this process, when a number of geotechnical engineers
are presented with the same data, they will generally select different characteristic values,
reflecting their different assessments of the design situation and interpretations of the char-
acteristic value. Bond and Harris (2008) found this when they asked geotechnical engi-
neers to assess the characteristic parameter values of London and Lambeth clays from the
results of Standard Penetration Tests carried out in these soils. The characteristic SPT values