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ABSTRACT:A new experimental set-up using a differential pressure transducer was developed, that enables the monitoring of volume changes in cyclic triaxial tests on unsaturated soils. Calibration testswere performed in order to analyze the performance of the set-up, especially in terms of loading frequencies. Based on calibration results, a low frequency of 0.05 Hz was adopted for the tests carried out on the unsaturated loess from northern France. Five water contents were considered in the tests. The obtained results have confirmed the efficiency of the new system for volume change monitoring under cyclic loading. The effect of water content on the cyclic behavior of loess was clearly evidenced. Finally, some suggestions were made to improve the accuracy of the system.

Résumé Mechanical behaviour of natural loessic soils under cyclic loading AbstractWithin the action plan launched in 2002 by the French company for railway management (RFF) and the railway company (SNCF), and following on the instability problem observed on the northern high speed line, a laboratory study was undertaken aiming at identifying the mechanisms of instability related to the behaviour of the soil involved: loess. Intact samples were taken in block at various depths and different tests were carried out: geotechnical classification tests, collapse test in oedometer, fatigue tests at low and high frequencies, and liquefaction tests. The results showed a moderate collapse starting from the natural state (< 3.7 %). On the other hand, a high sensitivity to the water content was observed since a linear increase of collapse with the reduction of the water content was identified. The results of the fatigue tests at low frequency showed that on the whole the strains increase with the increase in amplitude of the deviator stress. The effect of frequency was found to be negligible. The liquefiable character of the soils was shown by both undrained monotonous shear tests and cyclic shear tests. Moreover, the results showed that it is possible to define a criterion of liquefaction either in terms of over pore pressure build-up or in term of axial strain. The liquefaction resistance of the soils seems strongly dependent on the density and the fraction of fines.

Organisation des recherches dans le cadre du plan d'actions LGV Nord-Caractérisation des limons sous différents états hydriques et sous sollicitations ferroviaires Résumé Dans le contexte de recrudescence de fontis survenus au cours de l'hiver 2000-2001 particulièrement pluvieux (cf. inondations de la vallée de la Somme), et dans le cadre du rapport d'expertise du Conseil général des Ponts, RFF et la SNCF ont engagé des actions de recherche visant à : (i) caractériser le comportement des limons sous différents états hydriques et sous sollicitations ferroviaires, (ii) déterminer les mécanismes de rupture des fontis et leur vitesse d'évolution. Ces actions de recherche ont été lancées dès 2002 sous forme de projet élémentaire. Au fur et à mesure de la production des résultats, d'autres projets, ou phases de projet se sont greffés afin d'assurer auprès de la DTT (Direction des transports terrestres) la complétude des réponses apportées. Le présent article dresse le panorama d'ensemble de ces actions en rappelant, action par action, les attendus, et les principaux résultats obtenus.

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