The deformations of fine-grained soils due to water content variations are responsible for many damages to civil engineering structures. In the framework of a research program aiming at optimising soil uses in road and railways embankments, the present study was aimed at evaluating the influence of soil nature and initial conditions on the risk of disorders due to wetting. This question was analysed by defining a microstructural model, based on the organisation of the clay fraction in aggregates. Thanks to recent technological developments in scanning electron microscopy and mercury intrusion porosimetry, fundamentals hypothesis were suggested in order to determine the model parameters from conventional geotechnical parameters. 13This quantitative microstructural model was used to describe the free swell deformations and deformations due to wetting under vertical stress, by studying the influence of initial inter-aggregate volume. Thanks to the microstructural model, the changes in intra-aggregate and inter-aggregate volumes were quantified and a new methodology was developed for the design of roads and railways embankments in sensitive contexts (areas liable to flooding, high embankments...).

Limit analysis has been firmly established over the past decades and is now considered to be an accurate method for predicting failure mechanisms and for calculating limit loads. A new finite element procedure has been developed, and is briefly described in this paper, to determine automatically the optimal failure mechanism for geometrically complicated structures under applied loads. In limit analysis, the objective is generally to determine the collapse load, whereas the factor of safety is calculated in methods of slices. This paper also shows how the factor of safety introduced in the limit equilibrium analysis can be systematically incorporated into limit analysis. Several computations of stability of embankments were carried out to compare the limit analysis results with those obtained by methods of slices based on the limit equilibrium analysis. Through this investigation, some differences and similarities between these two theories can be demonstrated. The numerical results demonstrate that limit analysis by the present finite element method generally gives factors of safety more accurate than or as accurate as those obtained by methods of slices.

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