Investigation of interlayer soil behaviour by field monitoring

As opposed to the new ballasted railway tracks where a sub-ballast layer is often emplaced, the conventional railway tracks were constructed with ballast directly emplaced on the natural sub-soils. Thereby, a layer of mixed materials, namely interlayer, was formed over time mainly by interpenetration between ballast and sub-soils. As this layer plays an important role in transmitting load to the sub-soils, its behaviour under the effects of dynamic loading and climate changes is of primary importance for the stability of tracks. In order to understand the behaviour of the material in such interlayers, field monitoring was performed at a selected site in France, namely Moulin Blanc. Firstly, site investigation was done by borehole, allowing the interlayer location to be identified. Secondly, the interlayer was instrumented with suction probes, temperature sensors and accelerometers at different depths. Two piezometers were also installed for water table monitoring, and a weather station was installed for air data monitoring. The data recorded allowed the suction changes with water evaporation and the behaviour of tracks under the effect of temperature to be analysed. Moreover, the recorded data of acceleration allowed assessing the double-integration method for different kinds of train. It was observed that the site is out of the hazards related to freeze/thaw and the double-integration method can be applied to determine the particle velocity and displacement provided that appropriate filters are used. Note however that further study is needed to confirm this point with comparison between the measured particle displacement and the calculated one.