Safety margin in serviceability limit state according to the 2019 draft of Eurocode 7: Applicability to embankments on fine-grained soils.

Abstract

Determination of characteristic values of soil properties is an essential step in the verification of serviceability limit states such as total settlements of embankments founded on fine-grained soils. These values provide a safety margin that accounts for sources of uncertainty such as inherent spatial variability of the properties within the soil mass and sampling uncertainty related to the extent of the investigations. However, modification of soil parameters results in deviations from the actual stress-strain behavior of soils that might be significant for certain soil conditions and characteristics. For instance, the nonlinear strains exhibited by soft Finnish clays in oedometer tests that are accounted for by the tangent modulus method.

The aim of this thesis was to quantify the safety margin obtained from characteristic values in settlement calculations. Four cases from Finland with different soil conditions were analyzed. Several total settlements analyses were carried out to compare values of settlements calculated from characteristic values with values from the mean of data sets of derived soil properties (i.e., best estimates). The analyses were made as functions of the range of coefficient of variation reported in the literature for the relevant soil parameters used in the calculations. Two different deformation models were used to calculate the settlements: tangent modulus method and compression index method. The European Committee for Standardization (CEN) is planning to publish a revised version of Eurocode 7 where a statistically based equation for the determination of characteristic values will be included. This equation was used for the determination of characteristic values in this study. Likewise, an alternative equation was also used for one case in which, a purely statistical approach cannot be applied.

The results show that large conservatism is induced by the strong nonlinearities of the stress-strain behavior of some soft Finnish clays when characteristic values of soil properties are used in settlement predictions. The results are especially critical for the upper bound of the range of the coefficient of variation selected. The characteristic values fail to provide a harmonized safety margin among the cases because of the different stressstrain behavior features. Therefore, characteristic values of soil parameters used in the tangent modulus method have to be estimated cautiously to avoid unnecessary safety margin. Likewise, the equation from the revised version of Eurocode 7 should be avoided in cases where limited samples are available because settlements are considerably overestimated.