Use Of 3D FEM Model To Assess The Impact Of Deep Excavation On Existing Sydney Water Tank Stream

A residential-commercial development located in Alfred Street, Sydney (Circular Quay) was to be constructed adjacent to the heritage structure Tank Stream, currently owned by Sydney Water Corporation (Sydney Water), with a 6-level basement extending to approximately -17.3 m AHD (Australian Height Datum) resulting in an approximately 21 m deep excavation. The Tank Stream comprised a range of construction including brick oviform section, stone arch and twin concrete pipes employed to replace a damaged section. Due to the proximity of the proposed excavation to the Tank Stream, a geotechnical impact assessment was warranted. As part of the impact assessment work, a three-dimensional finite element numerical modelling (3D FEM) was carried out to assess potential impacts of the basement construction on the various components of the Tank Stream. The retention system was designed to satisfy a reasonably stringent criteria intended to keep oviform and stone arch components in compression. The paper discusses the development of the numerical model, the results of the analysis and the predicted ground movement arising from the construction and the measures taken to monitor and protect the Tank Stream. Inclinometer monitoring results are presented for comparison against the movements predicted from modelling.