Obtaining the characteristic value

EN 1990 defines a characteristic material property as follows:

where a low value ... is unfavourable, the characteristic value should be defined as the 5% fractile value; where a high value... is unfavourable,... as the 95% fractile value [en 1990 ยง4.2(3)]

This definition works well for man-made materials, such as steel and concrete, but - as discussed below - fails to account for the remarkable variability of geomaterials and for the practical difficulty in obtaining measurements of relevant material properties in soil and rock.

5.3.1 Problems applying statistics to geotechnical parameters

Geotechnical engineering poses many challenges, not least of which is the need to determine ground conditions at each and every construction site. The adequacy of many site investigations leaves much to be desired, resulting in designs being prepared on the basis of insufficient information. Furthermore, many geomaterials are highly variable in nature, which makes it inherently difficult to establish not only their location but also their mechanical and chemical properties.

'The sources of uncertainty may be classified into two main types: (1) those that are associated with natural randomness [aleatory]; and (2) those that are associated with inaccuracies in our prediction and estimation of reality [epistemic].'6

It is not atypical for important design parameters to vary across a wide range of values, as illustrated in Figure 5.7. In addition, studies7 of the coefficient of variance of various soil properties have shown it is typically much higher than for man-made materials (see table below). This is an example of aleatory uncertainty.8

Figure 5.7. Range of soil and rock strengths encountered in nature, compared with strengths of man-made materials

Coefficient of variation (COV) of geotechnical and man-made materials9



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