Support of 30-m-Deep Excavation for TBM Launch and Future Underground Station of Sydney Metro West

The paper describes the design and construction of an anchored secant pile wall used to support the deep excavation which enabled the launch of two tunnel boring machines (TBM) for the Sydney Metro West Project near White Bay on Sydney Harbour. Bored piling with segmental casing as well as continuous flight auger (CFA) drilling techniques were used effectively to construct a 10~25-m-deep secant pile wall. The wall was required to temporarily support a 30-m-deep excavation in the geotechnically challenging White Bay Alluvium and Paleochannel sediments underlaid by the Sydney Hawkesbury Sandstone. The sandstone surfaces on the east end, and at the west end diving to a maximum depth of 22 m resulting in a unique challenge. Parts of the Great Sydney Dyke also intersected the excavation box. Two different shoring types (diaphragm walls and secant pile walls) were considered. Case histories and lessons learned from similarly deep excavations nearby along the Sydney Harbour were investigated to inform the concept design. Secant pile walls were selected due to several key reasons detailed in this paper. The key advantages of secant piles were de-risking against geological hazards, as well as benefits for the overall construction programme (TBM launch) when compared against other techniques.