Railway subgrade behaviour under flood conditions : an experimental study at full-scale

Railway track is constructed on compacted soil which is characteristically unsaturated, however, the analysis and design are not to be considered based on unsaturated soil mechanics. The main objectives of this research are to investigate track behaviour, particularly, subgrade behaviour in the event of both flooding and during the recovery period. The investigation includes hydro-mechanical behaviour and the cyclic wetting and drying effect on subgrade. Previous research has been carried out on superstructure but little information is available regarding subgrade behaviour in different conditions (saturated and unsaturated). Inadequate drainage design or blockage of existing drainage can cause substantial damage to rail track. Furthermore, the wetting and drying cycles, due to frequent flooding, changes the soil behaviour significantly; therefore, it is essential to have an understanding of subgrade behaviour in both unsaturated and saturated conditions. The design and evaluation of subgrade behaviour is primarily based on construction loading. Environmental changes and the impact on subgrade behaviour have rarely been considered during design and maintenance work. Railway track experiences cyclic wetting and drying due to seasonal variations which significantly influence track performance. To improve both short and long-term performance with minimal maintenance, it is essential to consider environmental changes during design and assessment of subgrade behaviour; especially, for the extreme case of flooding. A series of full-scale experiments was performed to investigate track performance during and after flooding and the subsequent recovery period. Soil suction, which is a key parameter in unsaturated soil, was measured using the filter-paper technique. Subgrade stiffness was measured by the plate load test. A relationship was established between subgrade modulus, moisture content and matric suction. In order to investigate the effect of cyclic wetting and drying, the pressure-plate and filter-paper tests were conducted to obtain the water retention curve. A track settlement model was proposed based on soil suction hysteresis. The model can predict track settlement based on whether, the track is following wetting or drying path. Soil suction has a significant influence on subgrade behaviour. This research, therefore, highlights the importance of taking into account suction hysteresis in the design of railway track and the assessment of maintenance work required. The results showed a novel way to investigate and assess subgrade behaviour