In order to better understand some stability problems observed along the TGV (high speed train) line between Paris and Lille, a geological presentation of the loess deposits of Northern France is provided, followed by a detailed geotechnical characterisation of a loess profile. The characterisation, that also included microstructure investigations, has been carried out on 4 good quality block samples carefully extracted at various depths, at 140 km from Paris along the TGV line. Special attention has been paid to collapse susceptibility investigated using the simple and the double oedometer methods. The effects of water content on the magnitude of collapse was also examined and the relevance of various collapse criteria is discussed. It is confirmed that the collapsibility is related to the density, the initial water content and the carbonate content of the loess. For the covering abstract see ITRD E128041.