Influence of loading path on the undrained behaviour of a medium loose sand

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The results of an experimental programme aimed at studying the undrained behaviour of Hostun sand are presented in this paper. Specific conditions concerning the initial relative density (medium loose arrangements) and the loading paths (compression and extension under monotonic and cyclic loadings) were considered in the test programme. Monotonic tests carried out in both drained and undrained conditions show a significant difference in behaviour between compression and extension. It is observed that, in undrained conditions, Hostun sand is weaker in extension than in compression. In compression, the material is stable (dilatant) and the phase-transformation state controls the mechanical behaviour. In extension, the experimental results show an unstable behaviour (contractant), with monotonic, liquefaction-induced instability in undrained conditions. The results of cyclic tests, carried out with one- and two-way stress reversals, show a good correlation with the results of monotonic tests. The loading path strongly influences the undrained mechanical behaviour of the sand, mainly by inducing liquefaction in extension. This situation suggests that differences in soil fabric, caused by the sample preparation technique (air pluviation), can influence the sand behaviour by inducing a significant contraction in extension. By further analysing the cyclic results, it is shown that, during unloading, the stress paths reflect the transverse isotropy (orthotropy) of the sand, with stiffer elastic characteristics in the vertical direction than in the horizontal direction.Key words: liquefaction, cyclic mobility, sands, triaxial test, anisotropy, loading path.