Abstract
Lack of harmonisation between reliability-based design and the partial factor method in Eurocode 7 (EN 1997-1:2004) is preventing the widespread introduction of a risk-based concept in geotechnical design. This paper discusses how uncertainties are managed according to EN 1997-1:2004 and possible implications of not harmonising the current safety format with reliability-based design. One of several challenges highlighted is how EN 1997-1:2004 defines the characteristic value and design value. The characteristic value is therein defined based on a classical frequentist approach through a confidence interval. From a Bayesian point of view, the current definition does not treat the characteristic value as an uncertain variable. Consequently, the definitions of the characteristic value and design value in EN 1997-1:2004 feature weak connections between uncertainties in the geotechnical properties and the consequences of failure, as regulated by the target reliability index.
Highlights
Reliability-based design has been around for quite some time, as an eligible alternative to the partial factor method
The uncertainties that propagate into the probability density function (PDF) are rarely communicated on statistical grounds in the geotechnical literature
It is suggested that the slow introduction of reliability-based design in geotechnical engineering may be related to the lack of harmonisation between the Bayesian concept of reliability-based design and the frequentistically defined partial factor method in EN 1997-1:2004 (CEN, 2004)
Summary
Reliability-based design has been around for quite some time, as an eligible alternative to the partial factor method. It is argued that the statistical definition (i.e. the 5% fractile) of the characteristic value, in combination with a fixed partial factor, may limit the possibility to account stringently for varying uncertainties and to achieve a uniform reliability level in geotechnical design In both semi-probabilistic design and fully probabilistic reliability-based design, that is level II and III (EN 1990:2002; CEN, 2002), the most difficult issues typically arise during the process of establishing a probabilistic description of uncertain variables. This paper discusses how uncertainties in geotechnical properties are managed according to EN 1997-1:2004 (CEN, 2004) and the possible implications of not harmonising the current safety format with reliability-based design It introduces the definition of the partial factor method in EN 1997-1:2004 (CEN, 2004), followed by a review of the uncertainties that are accounted for in the statistical definition of the characteristic value (i.e. the 5% fractile). It is discussed (a) whether the 5% fractile is suitable to use to account for varying geotechnical uncertainties in the partial factor method, (b) whether the curse of small sample size is a valid argument to dismiss estimation of geotechnical property statistics and (c) how to bridge the gap between design with partial factors in future versions of the Eurocode and reliability-based design
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