Sandstone Pedestrian Tunnel Project
Abergeldie successfully delivered the design and construction of a new pedestrian tunnel in Sydney’s central business district. The tunnel, which passes below the historically significant and heritage-listed Bennelong Drain and above the Sydney Metro tunnels, connects two heritage-listed buildings and will serve as both a pedestrian thoroughfare and a route for critical building services. The tunnel was constructed utilising the staged mined excavation method. The project was completed ahead of schedule and within budget.
Scope of works
The scope of works included:
- Construction of the main pedestrian tunnel with a horseshoe profile, measuring approximately 4.2m wide, 4.2m high and 45m long.
- Excavation of a 5.3m deep stair shaft, measuring 4.7m wide by 6m long.
- Construction of a stair stub tunnel linking the main tunnel and stair shaft, approximately 4.7m wide and 4.3m high.
- Installation of site infrastructure and temporary works.
Protecting heritage and structural integrity
Delivering the tunnel from within and beneath two heritage listed buildings, below the historically significant Bennelong Drain, and above the Sydney Metro tunnels, required exceptional care. In addition to the construction method, a rigorous instrumentation and monitoring regime was designed and implemented, including tilt sensors, façade tracking, settlement & convergence monitoring and manual in tunnel checks, ensuring vibrations and displacement stayed within strict design limits, protecting the sensitive surrounding assets.
Innovative techniques used within restricted access and work areas
The restricted access and operating areas demanded compact, carefully selected equipment to ensure target productivity was achieved in the high strength and highly abrasive Sydney Sandstone. A S65 Mitsui roadheader, small enough for the shaft yet large enough for the tunnel profile, and powerful enough for the geology, was used alongside a robotic shotcrete rig, pneumatic rock bolting rig, mini excavator, and skid steer. Over 310 rock bolts and 362m³ of steel fibre reinforced shotcrete provided ground support, with shotcrete pumped nearly 100m from the street level above through the heritage structure. Beneath the building and Bennelong drain, advance lengths were restricted to 1.25m, with probing and endoscope holes used to assess conditions ahead and to enable adaptation of the ground support in real time. The stair shaft excavation was progressed through the building’s foundations, where rock was fractured without percussion using saw cutting, grid coring and mechanical splitting to safeguard both the ornate façade and the structural integrity.
Careful coordination and collaboration
The tunnelling works ran in parallel with the building’s fit-out, requiring constant coordination with a multitude of other contractors and builders. With no space for a muck bay, more than 2,000t of spoil was removed in 8t skip bins via a shared tower crane, with extensive coordination required to avoid congestion. Breakthrough of the tunnel into the basement of an operational hotel was carefully planned to avoid disruption, with dust, noise and vibration mitigated by various means, including retaining an existing shotcrete wall.
This technically challenging pedestrian tunnel demonstrates our capability to deliver complex infrastructure in highly constrained urban environments. Through close collaboration with stakeholders, innovative design and construction methods and careful planning, the team delivered a high-quality outcome that will serve the building occupants well into the future.