Wheels within wheels

Next month the world’s first rotating canal boat lift will reconnect two of Scotland’s waterways.

Scotland’s Falkirk Wheel, the world’s first rotating canal boat lift, is a story of art and engineering. Due to be opened by the Queen next month, it is the first phase of British Waterways’ £500m upgrade of the UK’s canal system and reconnects two 200-year-old canals between Glasgow and Edinburgh.

British Waterways wanted a stunning structure to mark the new millennium. It had a concept for a Ferris wheel to lift boats in two pairs of gondolas between the two canals.

When it put this exemplar design out to tender in early 1999, it hoped for something much more visually exciting. But all it received were tenders based on the exemplar. So in August 1999 British Waterways asked its preferred main contractor, Morrison Bachy Soletanche, and the architects and engineers involved, to start again from scratch. Here the three collaborators tell their stories.

1. The client George Ballinger, engineering manager, British Waterways, Scotland.

‘We looked at overhead monorails, tilting water tanks, water cylinders, giant spoons that worked like a seesaw. We had wheels that ran at right angles, and inclined planes and vertical lifts. Eventually we picked on a wheel with four hanging gondolas, two at the top and two at the bottom, with quite a lightweight structure in between.

And we envisaged a wheel with a rim, since that is the easiest position to drive a wheel from.

‘Then we did some detailed design and got a workable exemplar. But we felt it didn’t have quite the wow factor of the original concept. Eventually we said to Morrison Bachy Soletanche, let’s start again. And that’s how we got the current wheel. ‘When you look at it now compared with the original schematics it has remained quite true to the architect’s original vision. We did have a glass walkway through the central hub, but we abandoned that because it was difficult to make it work and added little benefit. Otherwise, that’s all that has changed. We’ve had a number of people say they’ve seldom seen anything built that looks so much like the original architect’s impression.

‘The wheel is not just for people who have boats. With the amphibious buses, visitors will be driven around, splash into the water, come through the tunnel, along the aqueduct, spin round the wheel and be dropped off at the shop in the visitor centre.’

2. The architect Tony Kettle, director, RMJM

‘The exemplar design wasn’t going to thrill anyone. So we held an internal competition within RMJM and three groups of us came back with some proposals. They included an elevator disguised as a circular waterfall where you drove your boat in at the top and you appeared, voila!, at the bottom. Another idea was a balance beam a bit like a seesaw.

‘My starting point was to look at the whole canal route. It is a linear, narrow route, a spine across Scotland. But this spine was broken and I wanted the wheel to be a celebration of it being joined back together again.

‘Think of a spine and you think of elegant, organic structures like fish bones. So I began to develop the aqueduct as an organic spine-like form. I wanted it to seem to float through hoops. And I wanted the wheel to be a celebration of moving between this aqueduct and the basin below. This was done by giving it direction. The hooked leading edges on the arms are all about rotation. It was as much a sculptural idea as a piece of engineering.

‘I was asked to convert it into a mechanical object in collaboration with Butterley. By making the gondolas narrower and longer and using two instead of four as in the exemplar design, I could make the aqueduct much narrower and more elegant. It also meant I could surround the end of each gondola with a large cog and have a simple gear system for keeping the gondolas level. Although simple, the cogs were a difficult concept to understand and my daughter made a little Lego model of it to try it out.

‘The day we presented the design there was a partial eclipse of the sun. Everybody went to look at it before my presentation. You couldn’t have had a better setting. ‘The only changes have been to the detail. The main masses have turned out as I hoped. Because the wheel is bolted together, we worked with Butterley to make the connecting flange plates as organic shaped as possible. I also wanted the waist of the wheel – the hub – to be as narrow as possible. It’s a very sexy curve.’

3. The engineer Colin Castledine, sales and marketing director, Butterley Engineering.

‘In August 1999 we had a call to say Morrison Bachy Soletanche had achieved preferred contractor status, but that the exemplar design was going to be abandoned. British Waterways felt it didn’t represent an icon structure for the 21st century and a celebration of 200 years of engineering achievement in Scotland. They wanted one last attempt to see if they could get something outstanding.

‘Early in September we attended a brainstorming session with everybody involved. Nothing was ruled out. Nothing was ruled in. No preconceived ideas. And there were some pretty hair-brained ideas, especially from the architects.

‘Tony Kettle’s view was that the aqueduct and the wheel were a single item. It had always been seen as an aqueduct with a wheel. That was the leap forward – Tony wanted to produce an architectural structure coming out of the tunnel that just happened to include a wheel.

‘We went away with several different ideas to see what we could do engineering-wise, and within budget, to achieve what was required architecturally. The day of the final presentations was one of those rare occasions when people came together with a common objective. Nobody got upset if their ideas weren’t liked.

‘Tony brought a Lego model with him to show his idea for stabilising the gondolas during rotation and it won. It is not an architectural solution. It is not an engineering solution. It is a unique combination of both.

‘We made an early decision to bolt the wheel together. As it rotated it went through a total stress reversal and the fatigue factor would have made a welded structure too heavy. Bolting also meant we could build it in modules. We developed flange plates with an architectural curvature to them enable the structure to stay within the architect’s vision.

‘It was also difficult to put hydraulic power on the rotating gondolas for opening the gates. So we have a mechanism like an aircraft refuelling boom on the aqueduct wall and in the basin that projects into a manifold on the gondolas to power up an accumulator and open the gates.

‘There are 10 hydraulic motors rated at 7.5kW each, but only four are normally used – the rest are for extreme conditions such as out-of-balance and high winds. It costs about £10 a day to power the wheel.’

Anatomy of a wheel

The Falkirk Wheel interchange consists of an extension to the Union Canal running through a 145m tunnel underneath the Edinburgh-Glasgow railway and out on to a narrow 100m aqueduct 20m above ground. This leads to the boat lift which stands in a dry circular basin below, surrounded by water. A lock connects the basin to the Forth and Clyde canal.

The wheel’s two water-filled gondolas will take four boats up and four boats down the 25m vertical drop in 15 minutes. A visitor centre will offer tours of the wheel in four 30-seat amphibious buses.

The wheel consists of two massive 1.4m-thick steel arms 35m across revolving around a hub driven by hydraulic motors. The ends of the arms support two gondolas (25m-long, 6.5m wide and 1.5m deep), each containing 300 tonnes of water. The dry well at the base of the wheel allows the arms and gondolas to rotate without touching water. When the wheel is fully loaded it weighs 1,800 tonnes. It is bolted together to counteract high-fatigue stresses using 15,000 high-strength friction grip bolts.

Boats enter and leave the gondolas via remotely powered lock gates at the ends of the gondolas. In operation, the wheel will be maintained in as close as possible to perfect balance, and typically requires the power of just two electric showers to turn it.

As it rotates, five unpowered ‘follower’ gears on each arm keep the gondolas horizontal. A static central gear is fixed around the hub. Two smaller gears revolve around this as the wheel turns. They in turn rotate two large gear rings within which the ends of the gondolas ride on small bogies.

The shallow, semicircular cross-section aqueduct with overhead ‘hoops’ is supported on slim figure-of-eight reinforced-concrete piers 25m apart. The wheel arms, whose hook-like design was inspired by a Celtic double-headed axe, match the shape of the aqueduct and its supports.

Prime contractor is Morrison Bachy Soletanche, with the services of architects RMJM of Edinburgh (they designed the Scottish Parliament building) and structural engineers Butterley Engineering of Ripley, Derbyshire (best known for building St Pancras Station).