In 140 AD The Roman Emperor Antoninus Pius ordered the construction of a new wall to bisect central Scotland.

The Antonine Wall as it became known, was really an attempt to push the northernmost boundary of the Roman Empire over 100 miles further north of Hadrian’s Wall which had been built 20 years earlier

This wall was no more than a 3m high mound of earth, but nevertheless it ran for 37 miles (60km) from the Firth of Forth in the east across to Old Kilpatrick on the River Clyde in the west.

Despite housing 19 forts and having hundreds of Roman soldiers stand guard, Caledonians continued their raids on the south and just 20 years after it was built the Antonine Wall was abandoned.

So it is perhaps not surprising that preservation of this embankment was not really at the forefront of the minds of engineers who were looking to design an iconic new structure, a structure that would reconnect two canals, the Forth and Clyde Canal, and the Union Canal.

These had been decommissioned in the 1930s but by the 1990s Scottish Canals sought to regenerate the waterways. It wanted to improve leisure activities along this corridor which between the major cities of Edinburgh and Glasgow.

So engineers needed a new boat lift, that was designed, and then had to be redesigned once engineers realised the historical significance of its location. It was a structure that was developed to mark the new millennium with engineering ingenuity and architectural flair; a structure that became truly iconic as the world’s first and only, rotating boat lift.

This is the fascinating design story of the Falkirk Wheel. A boat lift which is unlike anything else that had ever been built before.

Site conditions

Funded by the Millennium Commission the massive £84.5M investment was led by British Waterways and saw the restoration and reconnection of the Union Canal and the Forth and Clyde Canal in central Scotland. A connection that was particularly complicated because these canals did not meet at the same level.

“The height difference between the two canals at the location where they were to be joined is 35m,” says Ann Madsen, a designer of Ship Systems and a Senior Lecturer in Systems Power and Energy at James Watt School of Engineering at the University of Glasgow. “I’m a marine engineer by background training, I joined the merchant navy when I was 18. And I’ve never been able to quite leave engineering.”

It was through work at University of Glasgow that Madsen started to get her interest piqued into the Falkirk wheel, not how it works. But why it is as it is. What it is not, is a replication of the original Victorian solution.

“Now, our great Victorian engineers, this was no problem to them, and they joined the two canals together with a series of 11 locks,” says Madsen.

These locks took a whole day to pass through and used 3,500 tonnes of water just to move a single barge. It was not exactly user-friendly. For tourists and leisure activities, a simpler system had to be devised.

The engineers who looked at the problem decided to lift the boats up 35m in one step, there was nothing to stop them.

“And the whole design was based around that. And you look at the concept sketches, not just the first design that was function, but the next design that was form, it was still going up in a 35m step. It was maybe a bit of engineer’s hubris. They looked at the surroundings and said: ‘there’s nothing here of importance, we can do what we like.’”

Nothing there, apart from a two-thousand-year-old wall that crossed the route. As this structure was to celebrate 2000 years, courting public outrage by vandalising the Antonine wall was not an option. The designers had to think again, which introduced a lot of engineering changes.

The modern structure

Before we get on to the challenges, let us consider what was built in the end. The Falkirk wheel really is a unique structure and its evolution makes it a brilliant project to explore.

“And the whole design history of the Falkirk wheel is a function form function circle. And it’s only as it is, because it had to go through function, followed by form, back into function until it had a design that worked and a design people were willing to fund. So I love it for that. For teaching design students, it became this perfect case study.”

It is also a great example of engineering efficiency. It is a boat lift which is perfectly balanced so that each half rotation sees a boat lifted from the Union Canal, up 25m to the Firth and Forth, or vice versa. There have been a multitude of descriptions give to the wheel from a Celtic axe to a duck’s bill. Madsen thinks it looks more like a pair of glasses.

You drive your boat into one of the lenses of the glasses and then it rotates. Each lens is a perfect circle inside, and on the outside it has this wonderful elongation of form giving it the axe or duck’s bill shape that many observers see.

“Some people say it’s says it’s to look like a Celtic axe. Some people say it’s just accidental that it looks that way. And the actual boat lift itself, it appears visually, as it turns, to dig into the water at the bottom of the canal. It actually doesn’t – it’s an optical illusion. The boat lift itself never gets wet.”

Once the carriage is in place, a dam is created between the carriage and the canal lower basin. This dam is then allowed to flood with water. Once it’s flooded with water the sensors which allow the lift to know that the carriages fill with water, the dam is filled with water the basins, okay, there’s a ship there, then the doors are dropped, you can sell your canal boat in the doors come back up.

“Now these doors seal the canal basin and they seal the boatlift that you’re in. What they’ve got to do now is pump out the dam that’s between the two. So that’s pumped out and then released. So no water ever goes in the pit underneath the boat lift.”

And from an engineering perspective this is just one of the clever things about this wheel. Functionally, it’s really good that it’s dry, because it means there’s less stress is put on the Falkirk wheel itself.

“The whole thing becomes nicely balanced. If it was digging into water, we would have all sorts of imbalances going on.”

Engineers to the core

The physics of the wheel can be traced back to Greek mathematician, inventor and physicist Archimedes who discovered that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces.

“The whole thing is designed around Archimedes Principle, the simple principle that you will just as long as you are floating you will displace your weight in water. So if I have a boat lift with two carriages in it, and the two carriages have the same depth of water in them, they’ll weigh the same. So if I have my boat, lift two carriages, all the energy I need to turn it is energy to overcome friction. That’s all because everything is perfectly balanced all the time.”

The next design consideration was how to keep the boats horizontal as the lift turned.

“So the carriages that your boat is in, are forced to remain maintain a horizontal attitude as they go around. And this is done by a really simple but lovely cog system.”

This begins with a central cog which is fixed to the structure. A lot of people think it rotates, but it doesn’t. Everything rotates around it.

“Each of the boat carriages has a large cog as well. Between its large cog in the central fixed cog, there’s a small cog. So what happens as the boat lift is rotated around its axle, the centre cog doesn’t move. So the two small cogs do rotate. Those two small cogs, they force the cogs on the boat lifts to rotate, the ratios have all been set to the boats stay horizontal as they go around. So they’re very slowly turning, which means the water never undergoes a free surface effect because the water never sees any motion. So that’s the really nice piece of engineering.”

Yet this perfectly balanced dual boat lift didn’t always look like it does today. The original design team identified the rotating lift and used the Archimedes principal to design it, but their boat lift held four carriages and that resembled the spokes of a wheel.

“And their boatlift looked like a bicycle wheel. So imagine the front wheel of a bicycle, not even a cantilevered wheel like you see in the London Eye, but a proper wheel and an axle with two axle supports a full wheel, four characters in it looked a bit like a Ferris wheel. So when I look at this, I think it looks iconic. But it also looks very Victorian.”

Imagine the wheel at the top of a coal mine, not exactly futuristic. British Waterways wanted something more dramatic and iconic, so they brought in architectural firm RMJM

“And the first thing they did was they took away two carriages and made it a two-wheel structure. Then they said, ‘well, you don’t need a full wheel shape, you just need to be able to support the two carriages’. So that’s when it changed to look like a pair of spectacles. So now it’s starting to look a bit sleek.

“So you’ve now got this nice sleek structure was just to spectacles, and we thought, we need to give a bit of shaping. So that’s when they put on this what looks like this axe head shape, which is now become quite an iconic image for Scotland.”

The same thinking applied to another critical part of the Falkirk wheel structure, which was again designed by the original engineering team and then reconceptualised.

“These guys were engineers to their core, so they knew they were going to need an aqueduct to take the waterway at the top. So, when you look at sketches of their initial aqueduct, it was big and heavy look like a Roman aqueduct, but really, really solid. You can’t even think of this thing ever falling over.”

Engineers had created a workable solution to the problem, but it didn’t look like a piece of modern engineering to celebrate the new millennium. Once again, the new design team at RMJM set to work.

“The first thing they did was they got rid of the solid construction all the way and just put in these. I called them before they look a bit like magnifying glass posts.”

This is exactly what they look like, a row of concrete magnifying glasses with the deck of the aqueduct running through the base of the eye.

“These are very, very slim. And at the top, they have a full circle at the top. And the deck of the aqueduct is supported inside them. It looks stunning, it looks very slim, it looks slightly impossible. So this is what the architecture team did. And they created something really iconic. It was then handed back to engineers. And the first thing they said about the aqueduct was that’s unbuildable.

“What I love, though, is engineers were always too cautious. So if you let us design it completely, we’ll be very cautious. If you then let the more architectural people get involved. They’ll push it beyond where we’re happy. And then we’ll go ‘well, if you must have it like this, maybe we could do it this way’. And that’s what the engineers did. I’m not a civil engineer. But I know that the solution they came up with uses 40mm rebar, which I’m told is not a normal thing.”

Rebar being the steel used to provide tensile reinforcement to the concrete. And for rebar 40mm diameter is huge.

“So the engineers looked at what the architects had said, and made it work. So what I love is you started off with functional design. That would work but we’d never have got the money. We then got into form to have something that could get the money. But it had to go back to function to make it buildable now had a design that could be funded and would work and people would go out and look. Wow, that is just awesome.”

Funding was granted by the Millennium Commission in 1997 with construction beginning two years later in 1999.

Butterley Engineering fabricated the wheel at its site in Derbyshire and then transported it in 35 lorries up to Falkirk in Scotland where it was reassembled.

A simple solution to a major problem

It was an early example of modular construction and although all of this went well there was an incident that almost led to catastrophic failure just before the wheel opened, and it all stemmed from the design change.

Remember how the engineers had to preserve the Antonine Wall? Instead, a lock was built for the first 10m of lift before the wheel then lifted the boats up 25m. But no one had thought about what this design change would mean to operations.

“At this point, someone should have stopped and really looked at some of the initial safety and design arguments, because the whole original design and when they looked at it for safety wise and water flow wise, it was directly linking to canals. Now, you’re not directly linking to canals, you’ve got a locked in smaller body of water between the two canals. This now is going to be crucial for failure mechanisms for this canal.”

Which only became apparent, when a failure happened just before the wheel opened in April of 2002. Either sabotage or vandalism damaged the control system for the lock that makes the last 10m jump in height. And water was admitted into a trapped-in basin.

“A lot of water, the valves were just open and they were pouring water in. This now trapped in basin tunnel basin section was flooding and its normal overflow system was not able to cope. So the water started to tarp over this canal. The route the water took was straight down the hill towards the fork wheel, which at the bottom of the hill was racing down the hill. What’s at the bottom of the hill, the pit that the wheel is going to rotate into the water then raced into the pit and started flooding the pit.”

Luckily the water was stopped and the valves were closed on the Union Canal before irreparable damage was done to the Falkirk wheel.

“Because in that pit underneath the Falkirk wheel mechanism is where the high voltage connections are. The water stopped when it was eight centimetres below the 11kV busbar connections. There was a tidemark in there where it happened. The cost of this damage would be £350,000.”

And yet the solution, the measure that could have prevented this damage from ever happening, was incredibly simple.

“The solution they put in place, I could go to B&Q and buy for less than £1,000. Because what they realised is all they needed to do was divert the water as the water raced down the hill. If all he had done was had a lip around the pit that the wheel rotates into, and that lip being higher, then any obstruction on the water’s route into the bottom canal basin, the water would have diverted around the pit into the canal.

“So if you go to the wheel now you’ll find that there is a line of railway sleepers around the top edge of the pit just wooden railway sleepers at the perfect height, and they act as the diverter. Now this is the thing I love with design. This final design change of the 35m to the 25m was so far along the design process. I don’t know why but nobody stopped to think all the implications of putting this in place what would happen, and this is a massive safety implication here we almost hit the high voltage cables and you know the damage done. But it wouldn’t have taken much to have realised the only escape path of the water was down the hill straight into the pit.”

The Falkirk Wheel really lives up to its name as a circular design process, where function followed form and back again in a perfect circle. And this means that like most projects there are plenty of design lessons to learn along the way. What began as fundamentally a brilliant piece of simple engineering became a beautiful landmark thanks to the insistence of British Waterways that this become an iconic structure.

A design that engineers declared unbuildable was possible once the team decided to push the limits of the reinforcement so that the slender columns could hold the crucial aqueduct that feeds the boats into the wheel.

And let’s not forget the importance of thoroughly investigating the impact of design changes on operation so that all potential failure mechanisms can be mitigated.