Role of Fabric on the Shear Behaviour of Subgrade Soil under Cyclic Loading

In Australia, most rail systems are constructed along the coastal line, traversing soft soil deposits that can cause a series of track instabilities, including excessive plastic deformation and mud pumping. For this reason, the subgrade is considered one of the most critical components of the railway infrastructure, and the accurate prediction of its undrained shear behaviour is of utmost importance to ensure track integrity and safety over time. The subgrade is composed of naturally deposited soil, which may be modified using compaction or consolidation techniques to improve its bearing capacity. In each method, the soil presents a unique arrangement of particles, groups of particles, and voids (referred to as fabric), which govern the response of subgrade soils to the substantial loads imposed by moving trains. For this study, samples of clayey subgrade soil were collected from a site where track degradation had been reported. The soil was reconstituted using slurry consolidation and compaction methods to create different fabrics. A series of undrained cyclic triaxial tests were then carried out to investigate the influence of specimen preparation methods on the shear behaviour of soil. Differences in soil fabric were assessed through X-ray microcomputer-tomography (micro-CT), providing insights into the variations observed in the cyclic response. Based on the findings, it is evident that fabric plays a crucial role in the shear behaviour of subgrade soils and should, therefore, be considered in railway infrastructure design.