The trench is deep and wide and in the gloom the walls are seeping, sometimes water, sometimes a greasy mud. You climb down clanging metal stairways, past gangways on scaffolds, clinging to the sides, down and down to the muddy concrete floor.

The damp is everywhere, all the steelwork filmed with rust, a lone pigeon sweeps through the air.

The noise. A clang of hammers from up where the tunnel is near-completed, towards Aotea. At the harbour end a bulldozer and two diggers grind and roar their way over the sludge. Trucks growl from above, whistles shriek, an air pump stammers away. There's a constant rhythmic crashing coming from some machine in some far corner, out of sight but never out of mind.

There's a constant rhythmic crashing in the trench under Albert St. Photo / Greg Bowker
There's a constant rhythmic crashing in the trench under Albert St. Photo / Greg Bowker

And there's silence, although the noise never stops, but it's there, you're in a vast empty shell under the ground and the silence creeps up around you, in this place it is the natural state.

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The hammers were a surprise. There are jacks that hold up the steelwork, coaxed into place with hydraulics but then brought true to the last millimetre with bashes from the hammer. Is that the best way to do it?

"That's the way we're doing it," said Thomas Jefferd, a City Rail Link project engineer. "It works pretty well."

Underfoot there's a litter of what looks like cable ties, but they're steel, used to tie the scaffolding together. On scaffolding platforms against the wall a gang of Filipino men are doing exactly that, tying every join in the steel mesh that will reinforce the concrete.

"It's skilled work," said Jefferd. "They're a very good team."

Part of the trench dug before the concrete shell for the rail tunnels can be poured. Photo / Greg Bowker
Part of the trench dug before the concrete shell for the rail tunnels can be poured. Photo / Greg Bowker

There's a bloke on a Skilsaw too, slicing up little strips of wood. Wedges or battens for something, or possibly just to add to the noise. It's like a backyard operation, right in the middle of the biggest project ever.

They've had a barbecue down here, the occasional breakfast. Scott Elwarth, head of construction, said they were looking forward to a sitdown banquet in the tunnels, when they're done. It's a rite of passage, so to speak.

"It's pretty simple," said Mike Potts, a site supervisor with a shy grin. He meant simple in concept. This part of the project was cut-and-cover: you dig a big trench up the street and pour a concrete shell into it, which will have the rail tunnels inside, along with the tracks and electrics and everything else to make the trains run. They call it the tunnel box.

Simple to say, complicated to do. "The hardest thing," said Potts, "is the logistics." Not the work but the planning, getting everything in place to do the right jobs at the right time.

Before they started the tunnel box they had to secure the tunnel. They lined the trench with concrete piles and Stresscrete and braced it all with giant steel wales, steel joists that run along the sides, and struts, which are cylindrical and span the trench. The framework that stops it all folding in on itself.

They set down a concrete floor. They set down rails and put up a section of formwork, boxing for the concrete pour, shaped like a hipped roof, the sides slotting over the rails.

Now they're moving from the south end down towards the seaward end, prepping the walls, tying up the mesh, pouring the concrete. When it's set they roll the formwork down on the rails and repeat. They move it along two sections and then back one, for stability. They pour 12 metres of wall a week, pumping concrete from 20 trucks a day. Then they backfill and re-establish the street above.

At the top end they've nearly finished. The twin holes of the grey tunnel box – the trains will run both ways – recede into the gloom, strip lights along the sides. Men are down there with brooms, sweeping it clean.

The engineers are quiet types and sure of themselves. Elwarth and Jefford, Potts, all three of them soft-spoken and softly bearded, proud of their work and pleased for you to know it.

CRL chief executive Sean Sweeney's eyes light up when he talks about the project. Photo / Greg Bowker
CRL chief executive Sean Sweeney's eyes light up when he talks about the project. Photo / Greg Bowker

Sean Sweeney is proud too, but he isn't like them. He's the new CEO, a ferret of a man, sharp-featured, crop haired and eyes gleaming, small and thin with a voice like jagged teeth.

And a romantic. His eyes light up when he talks about the project, what it will do for Auckland, how exciting it is to be doing this job, in this city.

It's been a long time coming. He worked on Te Papa, 25 years ago, and then he left because there wasn't anything else big to do. No construction projects in the whole country worth over $100 million.

Now there are nine over $500 million. He called the scale of work here now "massive and unprecedented". He's been in the job four months; has the CRL been a surprise?

"I knew it," he said, "but I'm seeing it now in fine grain. It's not easy, but you don't expect it to be. Do not expect it to be rolled out early."

The tunnel box is close to completion towards the site of Aotea Station. Photo / Greg Bowker
The tunnel box is close to completion towards the site of Aotea Station. Photo / Greg Bowker

Or cheaply. They've had disruption and delay, the Fletchers problem, but now they're back on track with two joint-venture consortiums chosen as preferred bidders for the main part of the project – boring the long main sections of the tunnels and building the stations. They're waiting to receive their bids.

"We don't know what they'll tell us," said Sweeney. The budget is $3.4 billion. That could rise, because that's what budgets do. Or it could fall because sometimes technology finds new and cheaper ways to do things.

"We don't know," he said. "We have to wait and see."

There are local firms in the joint ventures but the expertise to work at scale comes from overseas. Sydney and Melbourne both have $60b of infrastructure work under way right now, which makes the CRL "just another job in this part of the world".

It hasn't been just another job at Britomart. The old post office building that's now the railway station is held up by pillars, sitting on piles driven deep through the reclaimed land into solid ground. Or, it was. Those piles were in the path of the new tunnels running west from the head of platforms 1 and 5, so they had to be cut out.

Works continue for the City Rail Link as the tunnels start to take shape under Albert Street. Photo / Greg Bowker
Works continue for the City Rail Link as the tunnels start to take shape under Albert Street. Photo / Greg Bowker

Four jacks around each pillar, then they lifted each one by two millimetres, cut out the pile, welded in enormous horizontal steel joists and settled everything back on top. Took them two years.

They're digging down again now, well below sea level to the level of the tracks, an excavator with a pick on the end, that's the endless bashing racket you can hear from everywhere on the site.

Compacted fill. It comes away black, foul, full of bricks and long-rotted timbers: Customs St was once the shoreline and this is the rubbish used in the reclamation. There's no strength to it. A truck on caterpillar tracks rumbles out from under the building, piled high with a load that wobbles like jelly.

There's a vast hole here where Queen Elizabeth Square used to be, with a green fluoro strip painted on the far wall. Behind that strip is the roof of the tunnel box under the vast new Commercial Bay project. Right under H&M.

They'll break through soon, and the whole section from the existing station up to Wyndham St will be joined up. Ready for the last stretch of tunnel boxing, the infill, the restoration of the streets, the installation of the tracks and systems electronics.

The view up from the vast hole in the ground where Queen Elizabeth Square used to be. Photo / Greg Bowker Greg Bowker
The view up from the vast hole in the ground where Queen Elizabeth Square used to be. Photo / Greg Bowker Greg Bowker

Ready for the next stage: not cut-and-cover but boring through the rock, under the upper part of Albert St to the new Aotea Station, under Karangahape Rd and a new station there, under Spaghetti Junction and out to a greatly changed station at Mt Eden.

And one day the trains will come, electric snakes in the tunnel-boxed darkness. We'll be riding them in 2024.

The City Rail Link

• Albert St trench: completed late 2019.
• The trench has 29 sections of tunnel box, each 12 metres long.
• 60m3 concrete for side walls of tunnel box and 28m3 for central wall.
• 50,000m3 of fill required for the Albert St trench: 20 Olympic pools' worth.
• Britomart: completed 2020.
• Stations and tunnels, systems and Western Line upgrade: completed 2024.
• More than doubles the capacity of the existing rail network.
• Speeds travel times to and from the city: 17 minutes saved between Henderson and Aotea.
• Budget $3.4 billion.
• 1600 jobs during peak construction: tender companies required to upskill the local workforce with a focus on Maori, Pasifika and youth, and to support local businesses in the supply chain.
• Unique technical manual called Mahi Rauora Aratohu provides "guidance for our work on the health of all things". Comprehensive sustainability targets established through Infrastructure Sustainability Council of Australia framework.
• The 2 shortlisted joint-venture tenderers are: 1) Downer NZ, Vinci Construction Grands Projets S.A.S., Soletanche Bachy Intl NZ, AECOM, Tonkin & Taylor, WSP Opus; 2) CPB Contractors, UGL (NZ), Beca, McMillen Jacobs and Jacobs NZ.