Characteristics and Variability of the Martin Place Joint Swarm — A Sydney Metro Case Study

The Martin Place Joint Swarm (MPJS) is a well-known, but poorly defined, regional geological structural zone extending through inner Sydney. The recently completed Sydney Metro City & Southwest (Sydney Metro) Tunnels and Station Excavation (TSE) project provided a unique opportunity to study the MPJS across large continuous excavations within both the new Pitt Street and Martin Place stations. Both stations involved complex geometries of running tunnels, station caverns, interconnecting adits and station shafts. These excavations allowed the MPJS to be observed across significant geospatial extents thereby exposing the structure’s inherent variability with respect to its character, lateral and vertical continuity, and stratigraphic dependency. The encountered geological features further advanced our understanding of the major vertically-persistent structures comprising the MPJS as well as strata-bound structures associated with the regional MPJS domain. Observations of displacements across significant fault structures are discussed, including ratios of strike- slip to vertical displacement magnitudes being up to 7H:1V.

This paper presents a summary of key observations from the Pitt Street Station and Martin Place Station excavations in Sydney’s CBD. The intent is to knowledge share and contribute to existing publications and the current understanding of the MPJS within the engineering industry. Some practical construction considerations are also recommended based upon experiences from Sydney Metro. As future city developments will demand more extensive and deeper underground spaces, it is important to document findings from completed projects so that others can consider potential impacts on their planning, design, and construction phases.