Collabrative engagement for better city outcomes: Building within the Sydney Metro First Reserve at 55 Pitt Street

Early and ongoing collaboration between Sydney Metro (SM) and the property developer was key to develop a robust design to ensure that a beneficial outcome was achieved for all stakeholders involved. The project went from being a sterilised site due to complex geometry constraints and additional limitations imposed by the SM protection reserves to a successful development application following an agreed approach for concession from SM to build within the first reserve. 55 Pitt Street is a new 53-storey commercial tower located at the northern end of the Sydney CBD. The building is located immediately above one of the Sydney Metro City and Southwest (SM C&SW) running tunnel, with the basement excavation extending into the tunnel’s first reserve. Through collaborative and ongoing engagement with SM, an agreed foundation design and permanent structure located within the first reserve and a minimum 2.5m offset from the extrados of the tunnel lining was successfully developed and constructed. The design consisted of a bridging structure – spanning the running tunnel – supported by foundation piles with diameters up to 2.4m designed for a maximum ultimate load of 165 MN. The engagement process involved impact assessments of the excavation sequence and foundation solution as well as implementation of mitigation and monitoring plans. Numerical modelling was carried out to estimate the overall ground movements and demonstrate the overall impact to tunnel lining. The development also had direct interfaces with the Telstra Tunnel, heritage listed Tank Stream and Ausgrid easement and substation each requiring individual assessment and engagement with their respective asset owner. The Site is mainly formed in alluvial soils overlying Hawkesbury Sandstone and adjacent to the Pittman LIV Dyke which is located along the Site’s southern boundary. The ground model including overall stratigraphy, in-situ stress condition, and rock/joint parameters was developed according to site specific site investigations, surrounding excavation mapping, and past project experience. A range of numerical packages (RS2, 3DEC) utilising continuum and discontinuum methods were utilised to develop localised and district scale models to simulate the proposed construction sequences and effects of the surrounding excavations. The numerical modelling results formed the basis of the impact assessments, monitoring strategies and a basis for a design which allowed the development to proceed and be successfully constructed.