Parramatta Rail Link – design and construction

Abstract

The Parramatta Rail Link is the largest publicly funded infrastructure project to be built in New South Wales, Australia, and the most significant addition to Sydney's heavy rail network in 80 years. The major infrastructure components of the work currently underway include 13 km of twin TBM driven rail tunnel, and four new underground railway stations. The underground tunnels are twin 7.2 m bored diameter with cross-passages every 250 m, and three crossovers. They vary in depth from about 15-60 m, and pass under commercial and residential properties, universities, and the Lane Cove National Park. Each of the stations is constructed in rock caverns, with passenger access to the surface by way of escalator, stairs and lift shafts. Service shafts and tunnels are also included within each station complex. The Parramatta Rail Link is located within the Sydney Basin. The majority of the route lies within the mid Triassic Ashfield Shale (lower Wianamatta Group), Mittagong Formation, and Hawksbury Sandstone. The geology at the station sites, and the insitu stress conditions are of critical importance to the support design of the caverns. The structural geology in the area between the cavern shoulder and the zone 5-7 m above the crown are critical to roof stability and support requirements. The station caverns are being excavated in mainly competent sandstone and shales, although weaker weathered sandstone and some shales occurs above the crown of the caverns. The station comprises a main platform cavern approximately 190 m long by 20 m wide at the springline with an arched roof and is 14 m high overall. The cross-section is asymmetrical and has a shape of a human brain. Adjacent to the platform cavern is a 50 m long concourse cavern. At each end of the station is a large service shaft about 32 m long by 18 m wide, by 30 m deep. Each of the station cavern detailed design was developed and included excavation layout and dimensions, excavation sequencing, temporary and permanent support, including all shafts and related tunnels. The design methodology for the principal components of the station caverns included a study of cavern shapes to determine the feasibility of large span arched caverns in these geologic conditions, and the relative costs when compared with the precedent practice of flat roof or trapezoidal sections, which indicated that despite no precedence for long span arched construction in the Hawksbury sandstone, the architectural shape was technically and economically feasible. Numerical modeling was performed to investigate the rock mass response. The modeling was performed for the platform cavern alone, for typical and adverse conditions, and included excavation and support sequencing. A strategy was developed to deal with the potential for high loads and shearing stresses. It was determined that the potential risks could best be managed through an observational approach, by establishing sufficient instrumentation from the surface and within the tunnel (in addition to normal convergence and surface monitoring), and to closely monitor the ground behavior as the excavation progressed. A ground movement criteria was defined to trigger additional ground support. At the time of writing, there are several hundred designers and construction personnel working on the project, and the contractor was turning over $A 20 million per month. There is excavation in three of the four underground stations, with roadheader excavation complete at the Macquarie Park station. The first TBM began excavation in September, and is named 'Babs Guulang' or the 'big wombat'. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.