Ground Control For A Deep Basement Excavation In Sydney’s GPO Fault Zone

The use of the underground space in major cities often involves complex systems with significant interaction between new excavations and existing structures such as buildings, services and tunnels. Such a complexity significantly increases when excavating in poor quality or unstable rock masses. Such ground conditions can be found in a fault zone or areas with high locked-in horizontal stress, both commonly observed in Sydney. This paper presents the geotechnical design challenges and construction outcomes of a deep excavation for a 38-storey mixed-use tower with 3 basement levels in the Sydney CBD. The challenges included excavating in the immediate vicinity of heritage listed buildings and rail tunnels built circa 1930. Two distinct excavation zones were inferred during the geotechnical site investigation, including both poor rock mass related to the GPO fault zone and good quality sandstone. An additional challenge was imposed by a 14 m deep excavation of a vehicle lift shaft with unsupported horizontal spans of 10 m, i.e. without internal support such as anchors or struts. Details of the design approaches and methods of analysis are discussed. These included a 3-Dimensional (3D) finite element (FE) analysis for prediction of ground movements and impact assessment. A structural frame model was used to simulate the effects of waler beams in 3D; hence it could be used as an input in the 2-Dimensional (2D) finite element model of the excavation. Finite Element Limit Analysis (upper and lower bound theory) was also adopted to estimate global factors of safety. A comparison between Class A predictions of ground movements developed during design and impact assessment stages and onsite measurements taken during and after excavation will be discussed. These include field data from two inclinometers, one horizontal extensometer installed in the vicinity of the rail tunnels and survey targets around the perimeter of the excavation. Photos taken during construction are presented to illustrate the challenges and successful outcome. These include some snapshots of the GPO fault zone, completed excavation with multiple ground support and ground control measures.