Design of a Temporary Retention System to Support Shaft Excavation for a Sydney Metro Project

As part of the Sydney Metro–Western Sydney Airport, Station Boxes and Tunnelling project, there was a need to excavate a circular shaft with a depth of 22 m and a diameter of 29 m. Notably, this shaft also incorporated a rock bench at its base to facilitate construction of four tunnel portals. A temporary retention system to support the shaft excavation comprised (a) Secant piles within the upper 11 m of the shaft. The secant piles were supported with a capping beam at the top and a ring beam at the bottom, (b) Eight piles situated on the sides of the tunnels, extending below the final excavation level to offer additional support to the retention system during tunnelling activities, and (c) Utilization of pattern rock bolts and shotcrete to stabilize the rock between the toe of secant piles and the final excavation level. To enhance stability of the rock bench and facilitate the breakthroughs of tunnel boring machines (TBMs), a pre-support system comprising spiling bar tubes and a shotcrete collar was employed. In the design of the retention system, various aspects were carefully considered, including loadings from ground, groundwater, and swelling rock, and interfacing with tunnels entering and exiting the shaft. This paper details the components of the retention system employed during shaft excavation and TBM arrival and relaunching activities. It outlines the design process involving finite element analyses to assess structural actions acting on the retention system elements. Finally, the paper discusses the instrumentation and monitoring program implemented to confirm the stability of the system throughout construction.