Excavated through sandstone while protecting heritage buildings, live infrastructure and one of Sydney’s oldest, still-active stormwater drains, the secret pedestrian tunnel nine metres below Loftus Street is a small engineering marvel.
Typically, after completing a major project, engineers know they’ll enjoy proudly pointing out their work to their children and grandchildren every time they’re in the area.
But for Matt Seelin, Construction Manager at Built, his work on the pedestrian tunnel between the former Department of Education building, now Capella Sydney, and the former Department of Lands building, now Lands by Capella, will remain a mystery.
“We all want to walk past a building or piece of infrastructure and say, ‘I built that’, but very few people will ever see this tunnel,” Seelin said.
“It’s a subterranean pedestrian tunnel link that connects the two buildings for two primary purposes. The first is as a back-of-house pedestrian thoroughfare for hotel staff and for VIP guests. The second is as a reticulation pathway where the essential services housed in one building can connect to the other.”
The tunnel, 45 m long with its base nine metres below street level and with 4.5 to 7.5 m of ground cover between the road surface and the tunnel crown, is 4.5 m high and 2.5 m wide.
Dug generally through medium-strength Hawkesbury sandstone and adjacent to the GPO Fault Zone, it provides a clear pedestrian corridor 2.6 m high and 2.4 m wide, with a 900 mm zone above reserved for mounted services.
Small tunnel, big challenges
The more that design engineers considered this project, the more complex it became.
The tunnel would run under two heritage-listed buildings, below a CBD street and beneath a live hotel, in a Metro secondary exclusion zone and dangerously close to the Bennelong Drain, a heritage-listed brick stormwater channel built in 1857.
Of course, it would also need to negotiate its way below a dense set of pre-existing underground services.
“First principles for us is to understand what’s around us,” Seelin said. “We mapped all of the structures, identified site geology through investigations and then we also undertook a 3D services scan of all the roadways, tying in all of the as-built information we had from various parties, so we could create a 4D model that gave us an understanding of the needle that we had to thread.
“Step two was understanding how the tunnel would interface with both buildings and figuring out ways to get access to build the tunnel. We needed open-air access to create access shafts to facilitate construction works. So, there was detailed planning undertaken to locate and profile the tunnel with the aim to make it as short as possible.”
Because of the heritage assets, including the Bennelong Drain and the two buildings, deflection and vibration limits were “extremely low”, Seelin said.
Vibration limits were capped at three millimetres per second, the lowest possible with respect to vibration standards in Australia. Also, no more than 1.93 mm of deflection for the Bennelong Drain asset was allowed, the hard cap set by the Authority, Sydney Water, via the Specialist Engineering Assessment process.
Because of deflection, vibration and access challenges, instead of a tunnel boring machine or traditional pneumatic excavation techniques, the team chose to utilise a roadheader, which is a track-mounted excavator with a cutting head mounted to a rotating boom.
“Essentially imagine a big pineapple attached to a rotating boom,” Seelin explained.
Digging deep for a breakthrough
Six months was spent establishing the vertical shafts, with each shaft separated by an outrigger wall supporting a heritage listed 75 m high clock tower.
“We had to completely undermine the outrigger wall to construct the access shaft and the lift shaft,” Seelin said. “To do that we had to progressively excavate in drops varying between 500 mm to a metre on alternating sides of the existing foundation, construct concrete pad footings, install structural primary transfer beams on either side of the footing, then insert needles through letter box openings approximately 0.5 m by 0.5 m, and progressively install various hydraulic jacks as part of jacking system.
“The jacking system included four 200-tonne hydraulic jacks and eight 60-tonne hydraulic jacks that we used to engage the foundation, pre-deflect the transfer structure and support the outrigger wall to then enable us to completely undermine the foundation, connecting the access shaft and the back-of-house lift shaft and allowing excavation works to continue.”
The tunnelling itself, the excavation and primary lining, took another five months, with three months following that to complete membranes, secondary linings, floor slabs and other finishes.
Seelin says like any project, there were challenges to work through.
“Undermining and underpinning the outrigger wall was extremely complex and extremely serious. No matter the trust you have in your engineers, and how many calculations and analysis is done, at the end of the day you still find yourself holding your breath when you complete the jacking process, disengage the existing foundations and complete the load transfer into the new structural system.
“Additionally, to cap the access shaft upon completion of tunnelling works, there was also a ring beam that ran around the full perimeter of the access shaft that would receive the future prestressed, precast planks spanning over the top of the access shaft, designed to take approximately one metre of soil in the courtyards. So, with the transfer structure, ring beam and existing building foundations located in close proximity to the shafts, the engineers had to address a heavy concentration of load directly above the main tunnel shaft.”
Innovation, Seelin said, came less in the design and construction of the tunnel itself and more from the methods around the build, including the perfectly timed and planned series of crane and truck movements to get materials in and out.
“Thankfully, through months of extremely invasive work, we had no neighbourhood complaints, no authority complaints and minimal disruption to the hotel’s operations, meaning the project was an extreme success.”
A moment to remember
Most people will never get to see the results of the project team’s work. But Seelin did manage a proud moment with his children, when he was allowed to walk them through the private tunnel.
“They were the first kids to ever enter the tunnel,” he said. “That was actually a pretty proud moment for me to have my family and my team’s families come to site and see what has been constructed.”
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Fascinating article about the engineering required to build the tunnel. How about a brief recognition of the engineers who designed it?