Management of complex geological structures using engineering geological models during tunnel construction: The Luna Park Fault Zone, Sydney
This paper describes the evolution of an engineering geological model developed as part of Construction Phase Services for the Sydney Metro City & Southwest Tunnel and Station Excavation project.
Barangaroo Station, located on the southern side of Sydney Harbour, comprises a 34 m deep temporary construction shaft, 35 m long crosscut adit and 230 m long crossover cavern adjacent to the north of the 28 m deep station box excavation. The site is founded within the Permian – Triassic Hawkesbury Sandstone and transected by the Northeast – Southwest trending Luna Park Fault Zone (LPFZ).
The LPFZ was anticipated to occur at the Barangaroo Cavern. During construction, the primary LPFZ plane was initially identified in probe holes before daylighting in the temporary shaft and crosscut adit. In total, the fault zone was exposed for approximately 70 m along the strike and encountered in each of the three respective underground excavations. The total zone of structural deformation was excavated over a width 50 m along the wall of the crossover cavern, equating to a true thickness of 30 m. The structure included packages of normal and strike-slip dominated displacements which combined with high angle reverse faults, Riedal shears and orthogonal transfer faults.
A three-dimensional engineering geological model was developed within a construction environment and offered rapid reproduction of encountered conditions in a format suitable for design verification during Permit to Tunnel meetings. Geotechnical borehole and field mapping records offer detailed insights into ground conditions. However, they can be difficult to visualise in two dimensions. Understanding the details of a ground model is essential in reviewing support systems that can be readily applied (and constructed) to the variable conditions encountered within a complex structural environment. The resultant model offered high-resolution imagery and interactive three-dimensional elements that could be queried and interrogated, offering increased value to numerous stakeholders.
Beyond documenting the engineering model development during construction, this paper also intends to share our observations and contribute to existing publications and the current understanding of the LPFZ within the engineering industry.