Comportement mécanique des sols lœssiques naturels sous sollicitations cycliques

Abstract

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.