Keywords: full scale test, flooding, subgrade, soil suction.

Railway track behaviour and maintenance work depends on the behaviour of substructure performance and support. The underlying subgrade soil plays an important role because the track performance, maintenance, and stability are all dependent on the variation and magnitude of subgrade stiffness. Track geometry deterioration is mainly considered as a result of substructure settlement. Therefore, it is important to evaluate the hydro-mechanical response of the subgrade in different conditions. Track design and assessment still persist empirically from a geotechnical point of view and have rarely been investigated based on modern soil mechanics. Recently, with observed increases in precipitation levels in the UK, the railway tracks are experiencing repeated flooding events. As a consequence, maintenance cost has increased extensively. Despite clear evidence, the enormous effects of flooding to the railway foundation have rarely been investigated. This research focused on the investigation of subgrade based on unsaturated soil mechanics. Large scale tests were performed using the Geopavement & Railway Accelerated Fatigue Testing (GRAFT-I) facility at Heriot-Watt University, together with suction measurements using the filter paper method.

The result improved the understanding and evaluation of subgrade behaviour in unsaturated and saturated condition. It shows significant softening of subgrade arising from soil suction loss by wetting. During the recovery period (after two weeks), the track gave a very poor performance as a result of the movement of water through other layers.

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