Novel pile design for multi-level car park above twin rail tunnels

Cherrybrook Station is one of 8 new stations constructed as part of the Sydney Metro Northwest project. The station is delivered under two separate contracts: the twin tunnels and station box excavation were constructed first as part of the Tunnels and Station Civils (TSC) contract, while the station and associated rail infrastructure is currently being constructed as part of the Operations, Trains and Systems (OTS) contract. As part of the OTS works, a multi-level commuter car park will be constructed at the city end of the station, directly above the existing twin tunnels and adjacent to the station box excavation. For the foundations of the car park, a novel pile design was developed to mitigate the risk of impacting the existing tunnels by ensuring that the relatively large axial loads imposed by the car park are transferred to the high-strength rock below the tunnels, while allowing the ground above the tunnels to provide lateral support to the pile to resist the relatively small horizontal loads. This paper presents this novel pile design, along with details of how the interfaces with the existing and future works were addressed in the design.