Statistical determination of multivariate characteristic values for Eurocode 7

Eurocode 7 recommends that when statistical methods are used, the characteristic value of a geotechnical parameter can be selected as the 0.05 quantile. However, Eurocode 7 does not state how to select characteristic values when there are multiple correlated input geotechnical parameters, e.g., cohesion and friction angle. One possible interpretation is that the 0.05 quantile has to be applied to each geotechnical parameter. Another possible interpretation is that the 0.05 quantile should be imposed on the overall factor safety (FS) or the performance function G = FS − 1. This study shows that the first interpretation may lead to overly conservative designs when there are multiple correlated geotechnical parameters. The second interpretation does not have this issue, but it is challenging to implement the second interpretation in practice because it requires the ability to compute the cumulative density function (CDF) of G and also control this CDF. The challenge is practical rather than theoretical; otherwise a fundamental shift to direct probability-based design would address this issue. This paper examines how multivariate characteristic values can be selected in a sensible way and aligned to the non-probabilistic basis in Eurocode 7. One promising proposal is to apply a simplified version of the quantile-value method (QVM) in conjunction with the concept of the “effective random dimension” (ERD). The resulting design verification format is identical to the current one in Eurocode 7. The key difference is that the quantile defining the characteristic value is not fixed at 0.05, but it is adjusted through calibration by the code drafting committee (not practitioner) using the ERD-QVM method.