Abstract : For a better understanding of the stability problems observed on the northern line of TGV in Picardy, many tests of mechanical characterization have been carried out on the natural loess which is the railway subgrade soil. Emphasis was made on its behavior under large number of cyclic loadings in unsaturated conditions, its collapse properties upon wetting and its susceptibility of liquefaction when saturation is reached. The results obtained showed that the layer located at 2.20 m presents the greatest risk of instability. A new liquefaction evaluation method was elaborated using results of common geotechnical testing. This method takes into account, in particular, the effect of initial unsaturated conditions. Application of this method to 4 different sites showed a good agreement with in-situ surveys. As far as the modeling work is concerned, an elastoplastic model with damage was developed to describe the main phenomena observed such as collapse due to suction decrease, damage and liquefaction due to cyclic loading.