Sydney Metro site investigation for the Sydney Harbour tunnel crossing

Sydney Metro is Australia’s biggest public transport project. Stage 2 of the program, the Sydney Metro City & Southwest project, will extend metro rail from Sydney’s North West, under Sydney Harbour, through new underground CBD stations and beyond to the South West. A critical element of this project is tunnelling beneath Sydney Harbour. At the commencement of the concept design development, very little geophysical, geological and geotechnical information was available that covered the area of interest in Sydney Harbour. The harbour geology was therefore identified as a critical design risk, and geological information was urgently required to inform the tunnel and track alignment designs. The metro alignment and tunnelling approach were progressively refined during the site investigation stages, as the geological model was developed. This paper summarises the site investigation stages (desktop analysis, geophysical testing, cone penetration testing and boreholes with HQ coring) and describes the implications for the design development of both the tunnel alignment and metro stations, as the harbour geological model was established and refined. It highlights the importance of good coordination between all design disciplines and the geotechnical team during the site investigation process, to ensure that both the site investigation is adequate for the design and that the design can capture and guide the site investigation.