Precision Tunnelling Under Heritage Building in Sydney CBD

Adrian Callus

The design of a pedestrian tunnel has been completed as a part of the integrated station design of Metro Martin Place (MMP). This tunnel connects two deep station entrance shafts (up to 28m deep) and is located immediately below a hundred-year-old heritage building in Sydney CBD area. The heritage building is ornately finished and therefore extremely sensitive to ground movement. During detailed design, it was identified that Mass Concrete Backfill (MCB) supporting the building foundations was found to extend within the tunnel profile. Numerical modelling was carried out to assess the design of tunnel support for ground and building loads. It has also been used to estimate the surrounding ground deformation and foundation settlement of the building. The tunnel is mainly formed within Hawkesbury Sandstone impacted by the Martin Place Joint Swarm. The ground model including overall stratigraphy, in-situ stress condition, and rock/joint parameters was developed according to available borehole information, surrounding tunnel and excavation mapping, and past project experience. 3DEC software package was utilised to develop a local model and a global model simulating the interaction between rock and joints due to the tunnelling using determinate Discrete Fracture Network (DFN). The local model assessed sensitivity of MCB in terms of settlement due to different rock-MCB interface parameters. The global model captured overall ground deformation and considered the effects of staged construction for the entire project site. The numerical results formed the basis of final tunnel support design, the impact assessment of the heritage building and monitoring strategies to minimise the impact on the building above and provide safe design.

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