While tourists flock to marvel at the architectural brilliance of Sydney's Opera House, few know about the one-of-a-kind concrete structure only a few metres below them.

Advantages of the double helix: Double helix design reduced footprint of 7,900 square metres to 2,950 square metres

Double helix design reduced footprint of 7,900 square metres to 2,950 square metres Gave parkers a shorter average travel distance to find a park

Gave parkers a shorter average travel distance to find a park Resulted in a shorter pedestrian tunnel

Resulted in a shorter pedestrian tunnel Enabled the car park to have more efficient ventilation

Enabled the car park to have more efficient ventilation The last car parking space to be taken was now at the top, not in the bowels of the Earth

Nestled beneath Sydney's Royal Botanical Gardens lies a double helix concrete car park, sinking 12 storeys underground.

Known as Bennelong Point Parking Station, the Sydney Opera House car park has just 7 metres of soil and rock between it and the surface of the gardens, built within a man-made sandstone cavern 42 metres into the Earth.

It has become famous within the architectural and building world for its unique design as both a structure, and as a car park.

Curious Sydney was asked by a curious Sydneysider why the Sydney Opera House's car park was built as a spiral.

One of the key people involved in the construction, geotechnical consultant Philip Pells, says the story behind the car park was anything but ordinary.

"There is nothing else like it in the world, and there won't be anything like it for quite a long time to come," Dr Pells said.

An architectural drawing of the Sydney Opera House car park which shows the double helix design. ( Supplied )

Dr Pells and his colleagues at Coffey & Partners were brought on to the project to ensure the integrity of the car park caverns structure.

"The Sydney Opera House is one of the world's most famous structures, it is unique and there's nothing else that comes close to it in the world," he said.

"The car park of the Sydney Opera House, whilst hidden underground, is equally unique.

"This great big doughnut-shaped cavern was created as a huge open space and then this concrete structure was built inside it.

"Nothing like this had been done before, and still today it's the widest shallow cover rock cavern in the world.

"It came together as a serendipitous design, a solution to something that had puzzled people for years."

One of a kind

An aerial diagram of the Sydney Opera House car park. ( Supplied: Pells Consulting )

In February 1990, the NSW Department of Works awarded the car park tender to Enacon Parking Pty Ltd and its simple rectangular two-cavern car park proposal.

That original design was in line with the standard car park styles found in Australia and overseas.

But something happened to completely alter that plan and changed how the underground car park would come to be built.

The challenge with the standard design was the legal requirement for a certain number of fire escapes that would cut into the number of car parking spaces available.

It would also force drivers to travel 12 storeys down into the bowels of the Earth to find a parking spot.

A blueprint for the Sydney Opera House underground car park. ( Supplied: Pells Consulting )

The story goes that the double helix spiral car park came to be through pure chance, sparked by a couple of beers shared between an architect and engineers on a Friday night.

"It is one of these extraordinary bits of engineering serendipity which occurred through a couple of people sitting around together and having a chat," Dr Pells said.

He said while the three key people behind the project sat musing about key design issues, they stumbled upon both a solution to their woes, and a unique exciting way to build the structure.

The following Monday, Dr Pells turned up for work unaware of the get together, and found a fax on his desk covered in sketches.

"I got these somewhat bizarre sketches, [with a note] saying 'Could we do this?' And I looked at this thing — I'd never seen anything like it," he said.

"It took me a while to work out what on Earth they were doing, and when I saw it I realised this was just pure genius."

Two versions of the same story

Dr Pells said the key person in the grand scheme of things was Neil Fimeri of Enacon, an engineer and the project director.

However when Mr Fimeri thinks back to how the car park ended up spiral, his story differs slightly. He said from his memory the meeting over beers was the cumulation of lots of hard work.

Philip Pells, a geotechnical consultant who was part of the team behind the Opera House car park construction. ( ABC News: Laura Brierley Newton )

Ron Barelle, the architect on the project, had been tasked with trying to figure out how to cut the number of fire escapes in the design.

The problem was by law they had to ensure there was one at a distance of 60 metres from wherever a person was within the car park.

The original rectangular design demanded a number of them, each one a set of stairs that went all the way to the surface — and each staircase knocked out valuable car spaces.

"It solved myriads of problems — but it was all driven by trying to find a solution to not having to drive hundreds of meters of small tunnels to make people escape in the event of a fire," Mr Fimeri said.

Mr Barelle proposed the idea of the circular structure as a way to solve the fire escape problem.

Construction of the Opera House car park took workers 42 metres underground. ( Supplied: Pells Consulting )

From a few beers, to a double helix

Mr Fimeri said it was at that point he met with Mr Barelle and another engineer, Tony Barry, at a local pub to discuss the new idea.

Construction machinery had to be lowered down into the hole and then lifted back out. ( Supplied: Pells Consulting )

Mr Barelle's idea came from a recent trip to Paris, where he had seen a self-ventilating above-ground single helix carpark.

As they continued to discuss the design they realised that a circular structure would be much stronger, and it would quickly solve other flaws within the original design.

"Then they thought well hang on, if we have a spiral that goes spiralling all the way down to the bowels of the Earth people are going to have to drive down 12 storeys to get to the bottom, and that's pretty painful," Dr Pells said.

"So then they thought what if we make it into a double helix, so you only go down six and you cross over to the other part of the helix which goes up?

"Now you only go down six levels and back up six levels. And lo and behold the last parking place that's taken is right at the exit."

Of course once everyone realised the double helix structure would work, they were left with the fact that the new design was not what the Government had signed off on.

Mr Fimeri said the Government then went to the Independent Commission Against Corruption (ICAC) to find out whether or not the changed design would make it necessary to go and retender it.

"There was this whole question of whether or not that was an unfair advantage," he said.

"ICAC fundamentally ruled that the whole process was designed to encourage innovation and this was exactly what it was meant to be."

Above the Sydney Opera House car park on the 'roof', with just 7 metres of soil between it and the Botanic Gardens. ( ABC News: Laura Brierley Newton )

Dr Pells said he doubted a complete redesign getting approval from a government with relative ease had ever happened before, and would likely never happen again.

"Things came together at the right time with the right people," he said.

"They had come up with a solution for that car park in that site which was just brilliant."

A fire escape tunnel in the car park shows the original sandstone that had to be dug through. ( ABC News: Laura Brierley Newton )

Constructing the 'donut'

The people with the overall responsibility for the design in terms of geometry and the structure inside were Tony Barry and Warwick Colefax, from the firm Rankine & Hill.

Car park facts: First and only underground double helix structure designed for such a purpose

First and only underground double helix structure designed for such a purpose Possibly the widest shallow-cover rock cavern in the world

Possibly the widest shallow-cover rock cavern in the world Cavern is 36m deep, 72m in diameter, base 26m below sea level, ceiling 7m below Botanic Gardens

Cavern is 36m deep, 72m in diameter, base 26m below sea level, ceiling 7m below Botanic Gardens Central sandstone pillar is 36m in diameter — nicknamed 'the donut'

Central sandstone pillar is 36m in diameter — nicknamed 'the donut' In addition to the main cavern, 16 tunnels were excavated

Mr Barelle, originally tasked with designing the pedestrian tunnel to the Opera House forecourt, also ended up playing a vital role coming up with the concept.

The car park was designed to have a lifespan of 50 years. ( ABC News: Laura Brierley Newton )

The entire project took 27 months from start to finish, coming in under the original timeframe proposed of 36 months — as well as coming under budget, costing just under $45 million.

And Mr Fimeri said there was little to no media coverage.

"It was deliberately hidden by the Government… parking has always been a sensitive issue anywhere and creating car parking at the end of Macquarie Street was one of those issues," he said.

At the same time as the car park's construction, the Harbour Bridge tunnel was also under construction — which worked in the engineers' favour during the car park construction.

"Most people thought what we were doing was in fact the harbour tunnel," Mr Fimeri said.

Dr Pells said the key feature of the cavern was the roof, which instead of being supported by a formed concrete arch, was internally reinforced by 2,000 steel anchors.

Basically long strong steel poles buried into the ceiling, clamping the soil and rock together — but he said they would not last forever.

"The whole structure was required to have a 50-year design life, and so the roof anchorage system was designed for the 50-year design life," he said.

Construction workers bury one of the 2,000 steel anchors which reinforce the roof of the car park. ( Supplied: Pells Consulting )

He said when that time was up, the entire structure would need a careful engineering review to establish what needs replacing — no easy feat with the anchors buried in rock.

"If I was doing it I wouldn't bother to check, I would just put more in," he said.

An article announces the new Opera House car park. ( Supplied )

As to why there have yet to be other similarly designed underground car parks built, Dr Pells said the design came about largely due to the car park's unusual location.

Most underground car parks are built beneath buildings, which in nature are rectangular or "funny shaped squares of sort".

"It all came together as the right thing in the right place. The piece of land here matched a circle. It didn't have a building associated with it," he said.



What happens in 2043?

Wilson Parking is the current lease-holder of the Opera House car park, after buying the lease from the original owners in May 2014 for $80 million.

The design expiration date for the car park was 50 years — so in 2043 the lease returns to the NSW State Government.

But what happens then?

Wilson Parking's general manager Peter Witts said the hope was that when the time comes they would be able to extend the lease.

However, technically the original lease states at the end of the 50-year time period the car park would be filled in — "returned to its original state".

But no-one involved in the construction, or the current lease holders, believes that will ever happen.

"Basically the whole idea was like anything, if you lease something then you're meant to restore it," Mr Fimeri said.

"In practice that will never ever happen, but that was the condition that was there."

Visit the car park and you will see there are two of every level — so two of level three, level four, level five and so on. ( ABC News: Laura Brierley Newton )

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