Laying the foundation for the Sydney Light Rail

The Sydney Light Rail project alignment covers the typical geological and geomorphological settings for the Sydney and Botany Basin. This comprises three distinctive zones: (1) manmade fill; (2) recent Quaternary aeolian, alluvial and estuarine sediments, which overlie; (3) residual soil and weathered to fresh bedrock. When investigating subsurface conditions within a metropolitan area, various constraints can have impacts on the scope and extent of geotechnical investigations that can be safely and practically completed. For the SLR project, investigation of the subgrade conditions along the alignment was significantly constrained due to the route which follows existing roads through the CBD and eastern suburbs. Constraints for working in these areas included complex traffic management planning to avoid disruptions to the community and also an equally challenging network of underground utilities and other service tunnels. These constraints required thorough planning for traffic management and consultation with utility service providers, thus significantly reducing the amount of work that could be completed within the program. A robust geotechnical validation regime was developed so that the in-situ subgrade could be re-assessed prior to track slab construction, to mitigate the limited extent of geotechnical investigation undertaken. The objective of the site validation regime was to verify the inferred design subgrade California Bearing Ratio values along areas where geotechnical investigation was constrained. The site validation regime included nominally spaced plate load testing, dynamic cone penetrometer testing and laboratory CBR testing to assess the subgrade CBR value. This paper explains the challenges faced in validating in-situ CBR in different subgrade conditions, appropriateness of published DCP to CBR empirical methods and the approach using calibrated site specific DCP-CBR correlations.