Probabilistic Evaluations of Prescribed Safety Margins in Eurocode 7 for Spread Foundations

According to the design code Eurocode 7, analysis procedures require reaching a prescribed safety margin, based on the conditions of levelling design action and design resistance. Such semi-probabilistic procedures do not result in a consistent equivalent value of the Overall Factor of Safety (OFS), neither in individual analysis nor in different tasks in geotechnical engineering. Furthermore, the implementation of different calculation approaches in Eurocode 7 also does not guarantee an equal probability of the occurrence of a relevant limit state. A comparative analysis is conducted for an example of a centrically loaded spread foundation on homogenous, isotropic, and coarse-grained soil, according to procedures in Eurocode 7, Design Approach 3. An algorithm is developed to estimate failure probability, taking into consideration the relevant statistical characteristics of each calculation parameter. A significant influence of the statistical characteristics of the relevant sample is emphasized. The degree of required modification of the equivalent Overdesign Factor (ODF) and the Overall Factor of Safety (OFS), based on the criterion of the required reliability index beta and failure probability pf , is quantified.

Automated summary

According to Eurocode 7, analysis procedures in geotechnical engineering must achieve a prescribed safety margin based on design action and resistance conditions. However, these semi-probabilistic procedures do not provide consistent values for the Overall Factor of Safety (OFS) and do not guarantee equal probabilities of relevant limit state occurrences. This study focuses on a comparative analysis of a centrically loaded spread foundation to quantify the required modification of the equivalent Overdesign Factor (ODF) and OFS based on the required reliability index beta and failure probability pf.