Unsaturated soil conditions in railway foundation materials for increased axle loading

Increased axle loading in the freight railway industry has become a thematic topic due to its associated advantages. However, increased axle loading leads to increased stresses on railway track components, including the track foundation, which is mostly unsaturated as a shallow foundation structure. It is therefore important that the engineering behaviour and performance of each track component should be well researched and understood for increased axle loading. This study aims to explore the effect of unsaturated soil conditions on railway foundation materials for the implementation of increased axle loading. The study was carried out by conducting cyclic triaxial tests on the subballast and subgrade material equilibrated to unsaturated soil conditions at matric suctions of 50, 100 and 225 kPa using axis-translation. Finite element modelling was used to transform axle loading of 20, 26, 30, 32.5 and 40 tonnes per axle to triaxial stresses, which were used as input parameters in the experimental work. Based on the test results, the shear strength of unsaturated soil samples was found to be higher than that of the saturated soil samples due to strain-hardening. The increase in the shear strength as a function of matric suction was found to be logarithmic. Furthermore, the deterioration of the resilient modulus during cyclic loading was found to be slower in unsaturated soil samples as compared to saturated soil samples. In unsaturated soils, the permanent strains decreased while the resilient strains increased as the matric suction was increased, which was indicative of resilient behaviour. However, load-collapse followed by strain-softening occurred in unsaturated soils. It was therefore concluded that unsaturated soil conditions are beneficial for the sustainable implementation of increased axle loading on railway foundations, provided the imposed loading is below the load-collapse threshold stress.