Unsaturated railway track-bed materials

Railway track-bed materials are mostly in unsaturated state, and their hydro-mechanical properties depend strongly on their water contents or suctions. In France, problematic soils such as collapsible loess and swelling marl can be origin of instability problems for new lines for high speed trains, while the hydro-mechanical behaviour of interlayer soil formed mainly by interpenetration of ballast and subgrade soil is of concern for conventional lines. In the latter case, the main challenge relies in the large variability of the interlayer soils owing to the variability of natural subgrade soils involved in the railway network. In this paper, a study on interlayer soils is presented, that covers a large spectrum of aspects: geotechnical/geophysical site characterisation, laboratory investigation into the hydraulic behaviour and mechanical behaviour as well as the mud pumping/interlayer creation phenomena, and field monitoring. The combined effect of water content and fines content were emphasized in the laboratory investigation, whilst interaction between atmosphere and track was focused on in the field monitoring. The results show clearly that it is of paramount importance to consider the unsaturated aspect of track-bed materials when analysing the overall behaviour of tracks, in particular when clay is involved in fine fraction.