Subgrade ground improvement using in-situ stabilisation for track formation in Melbourne

The Southern Program Alliance (SPA) is one of the alliances formed in Melbourne to remove level crossings, construct track duplications and upgrade the rail network as a part of the Victorian government initiative to improve rail infrastructure. An integral component of a rail infrastructure project is the construction of track formation. The essential requirement for rail formation is to satisfy the bearing capacity (strength) and settlement (serviceability) requirements for rail loading. The strength and serviceability requirements are a function of track design, axle-load, speed, and, notably, subgrade characteristics. For the SPA projects to date, where existing subgrade conditions could not satisfy these requirements, either the weaker subgrade was excavated and replaced by structural fill material, or subgrade improvement using in-situ stabilisation was explored. Subgrade improvement typically minimises the excavation and earthworks required for the construction of track formation and provides significant sustainability, cost, and time benefits to the project, without compromising the functional requirements. Subgrade stabilisation methods using admixtures (lime and cement) were considered and/or adopted for differing surficial geological deposits, including alluvial deposits, residual soil of Silurian origin and Tertiary sediments of South-East Melbourne. The design strategy was to verify the applicability of admixture ratios through laboratory testing, whilst further undertaking quality assurance (QA) measures through the construction phase. To assess the depth of stabilisation required, both empirical and finite element analysis design methods were undertaken. This paper summarises key design, laboratory testing and construction considerations for subgrade improvement works undertaken for a rail track duplication between Diamond Creek and Wattle Glen in Melbourne’s North-Eastern suburbs.