Hydromechanical behaviour of a natural loess

The hydromechanical behaviour of an unsaturated natural loess from Northern France has been studied. Collapse-on-wetting phenomena under constant external load are observed in the in-situ conditions, related to some stability problems detected on the loess foundations of the high speed train line that links Paris to Brussels. A new triaxial apparatus with complete local monitoring of unsaturated properties (strains, water content and suction) of this loess has been developed. The water retention curve of loess, obtained by using an in-house constructed high capacity tensiometer and the filter paper method, shows a special behaviour, with a non-hysteresis zone detected around the natural hydric state bordered by two hysteresis loops at the dry and wet sides of the curve. Further analysis of the water retention curve and of the hysteresis phenomena were conducted by means of a microstructure study made by mercury intrusion porosimetry and scanning electron microscope observations. The complex structure of loess is highlighted, composed by a metastable arrangement of dry silt grains (possibly related to carbonate re-crystallisation) and a heterogeneously distributed clay fraction. The hydromechanical behaviour study includes the evaluation of the collapse sensibility of loess at stress states close to the in-situ one, by a series of suction-monitored oedometer tests performed at different constant strain rates (CSR tests). A maximum of collapse and a time-dependent behaviour of loess were observed. A triaxial testing programme at different water contents with suction measurement, allowed to obtain the yield surface of loess in a q/p diagram, and the water content and suction evolution upon loading and to confirm the suction hardening in this material