For the Sydney Metro – Western Sydney Airport , station boxes and tunnelling works package, CPB Contractors Ghella (CPBG) engaged Robert Bird Group and SMEC to undertake detailed design.
The 29m-diameter, 32m-deep circular maintenance shaft for the Bringelly Services Facility required a system that could be constructed and excavated within six months.
The shaft provides vertical access between the metro tunnels and the ground surface during construction and for permanent maintenance.
SMEC was responsible for the assessment of the geotechnical aspects while Robert Bird undertook the design of the shaft structure.
As well as meeting a tight time frame, the companies needed to maintain cost-effectiveness while ensuring adherence to performance specifications. Sustaining a minimum clear internal opening throughout the project duration was also required.
Site investigations found the ground conditions at the site could apply significant forces, making it suboptimal to use a conventional retaining structure such as secant piles, a diaphragm wall or a lined shaft with closely spaced ring beams. Robert Bird Group and SMEC designed a unique retention structure configuration to promote as much flexibility as possible.
To validate the design, SMEC developed a 3D finite element model of the shaft using Plaxis 3D software. This enabled intricate evaluations, such as ovalisation due to ground variability, stresses and tunnelling activities.
Robert Bird says the solution met the design objectives with substantially less steel and concrete than common alternatives. It provided savings in cost, time and greenhouse emissions, offering consequential programme, budgetary and broader scale improved sustainability outcomes.
“The innovative design employed on the temporary shaft retention structures holds promising applications for future endeavours by SMEC, Robert Bird Group and the broader industry, particularly in the realm of road and rail tunnel construction,” says the company.
For the industry, the design approach offers a standardised solution for tunnelling works in similar geological conditions, creating a template for addressing challenges related to weak rock deposits, locked-in rock stresses, and swelling issues.
Its adaptability potentially influences future design standards and regulations. “As the industry seeks sustainable and efficient solutions, the incorporation of this innovative approach can set a precedent for environmentally conscious and resource-efficient tunnelling practices,” Robert Bird says.
Robert Bird project director Bona Bucalina said the two companies worked closely with the contractor to develop numerous options for the shafts “until we landed on a solution that maximised savings, minimised waste and programme, while using commonly used construction methods and equipment”.
The Western Sydney Airport, station boxes and tunnelling works package involves the design and construction of 9.8km of twin tunnels and excavations for stations at St Marys, Orchard Hills, Airport Business Park, Airport Terminal and Bradfield (Aerotropolis) and services facilities at Bringelly and Claremont Meadows.
