The garden outside the Darwin Centre, the latest addition to the Natural History Museum in London, is overshadowed by the building, with its instantly recognisable ‘Cocoon’ behind the glass wall. The garden missed out on the publicity associated with the opening of the museum’s new wing because it was not finished, but was brought back into the spotlight earlier this year when Prince Charles laid a wreath there at the unveiling of a new granite memorial to the British citizens who died in the Tsunami in the Indian Ocean on Boxing Day 2004. The garden includes a series of steps-cum-seats that, at first glance, appear to be concentric ellipses. But they are not. They are in fact made of a series of complex curves. NSS decided to explore the design and production of the Crosland Hill sandstone feature in more detail. The trail quickly led to stone hard landscaping specialists CED Ltd.

The Prince of Wales laid a wreath at the foot of an 11.5-tonne granite monolith at the Natural History Museum, London, on 6 July. It was a memorial to the 155 British citizens who were among the 225,000 people killed in the Sumatra-Andaman Islands Tsunami on 26 December 2004. Their names are inscribed in the paving at the foot of the memorial, which is made of the granite from Les Carrières Plo near Toulouse, France. It was designed by the architectural studio of Carmody Groarke.

It was a moving ceremony attended by the friends and families of those whose names have been cut into the paving.

The memorial is set at the opening of a garden area in front of the Darwin Centre at the museum. The Darwin Centre is the new life sciences facility at the Natural History Museum that includes an eight-storey ‘Cocoon’ providing controlled conditions for storing and studying botanical, entomological and zoological collections. It opened in September 2009. The public can visit and see scientists working behind display windows.

When the new wing opened, with its 1,000m2 of Jordans Whitbed Portland limestone paving and steps, the landscaping associated with the new wing was not finished. The garden opened unceremoniously in the winter, missing out on the publicity surrounding the launch of the Darwin Centre.

The landscaping is not prepossessing. It sits comfortably in front of the glass wall of the Darwin Centre rising above it, with a series of apparently concentric ellipses forming steps-cum-seats up to a terrace area. It seems simple… until you realise these are not ellipses and start to wonder how the complex curves, defined in Johnsons Wellfield Quarries’ Crosland Hill sandstone, were designed and laid out.

As you get closer and look at the individual stones you start to appreciate the complexity of the design and yet the economy with which it is achieved.

A bit of research reveals there are about 363 linear metres of stone in the steps / seats, about 40% of which are curved. And it turns out that the economy of the design was due to the collaboration between the designer, Philip Twaddle of the Danish-based designers C F Møller, and the stone specialists involved, CED Ltd.

Computer-aided design programs are wonderful tools, but making things to fit what the programs can draw is not always simple or economical. So the fun started when the landscapers tendering for the work started to consider the details. Gavin Berthoud of Blakedown Landscapes was the first to consult CED Ltd in January 2009, followed soon after by others, says CED Managing Director Michael Heap.

The specification was for Jura limestone and the drawing showed complex curves described as ellipses. The architect had recognised that, for the parts of the curves where the radius was large, straight sections with tapered ends could be accommodated, but there was no indication of precisely where that would be acceptable.

There is never much time for tenderers to look into every detail and there was certainly not enough time to produce accurate costings, but this is where the skill of estimating comes in.

From their experience of supplying kerbs, coping and seating units, CED were able to estimate what would be a reasonable radius at which to switch from curved to straight pieces with slightly tapered ends such that there would be no obvious kink at the junction between two adjoining pieces.

They also knew that the best way to make the curved sections of the ellipses would be with a sequence of curved pieces each of which was a part of a circle. Although each piece would be different, at least there was a tried and tested principle for their manufacture. Armed with this, they were able to obtain prices and quote to the specification. On this basis, Blakedown Landscapes were awarded the contract.

At tender stage, CED had thought to offer a Chinese granite as an alternative for cost reasons, but that was declined as an option. Believing that the issue was the source of the material, a Portuguese granite option was then offered. But Jura had been selected because the original intention had been to use Jura for the floors inside the building and the designers wanted it to flow almost seamlessly through the glass wall into the landscape, with the joints lining up. In the end, Portland was used for the interior, so the use of Jura on the outside was not important.

The severe winter of 2008/9 convinced everyone that Jura was not the right choice for the external paving. Consultant Barry Hunt (well known to readers of this magazine for his regular column in the News section) was asked for his advice. At a joint meeting of the client, architect, contractor and CED, Barry’s advice resulted in the decision to move away from Jura and to use a resilient sandstone instead.

Fortunately, Michael Heap had a large range of samples with him. There was a strong preference for a uniform beige colour. A consideration at least as important was the need for a production capability that could supply the quantity required to the highest quality, so CED recommended Johnsons Wellfield Quarries (JWQ) and their Crosland Hill stone.

The next challenge was to keep the costs within the limits set. The first task in designing the stone cutting detail was to understand the curves. Michael Heap spent much of one Sunday trying to find the mathematical formula for the ‘ellipses’ drawn by the architect.

Michael confesses his higher maths are a bit rusty and says it took him several hours to realise that the curves did not fit the algebraic formula.

An interesting debate ensued with Philip Twaddle before Michael appreciated that the curves were not true ellipses at all, but curves produced by CAD. This was a totally different mathematical problem. He resorted to simple geometry to assess the radius of each piece but realised something had to be done to simplify the exercise.

The five curves, each of which was about 75-80% of a complete ellipse, formed a nest of curves. For three of the quarters, the shapes were consistent and each curve was a scaled up version of the inner one. The fourth quarter (an incomplete quarter, like the first) was scaled up considerably and of a different family of curves, although roughly similar in form.

In the original design, each piece in all the curves was 900mm long. From a manufacturing viewpoint, this meant that each piece was unique. Not good. Michael realised that by scaling up the length of the individual pieces in each curve in proportion to the curve’s plan dimensions and by placing a piece centrally at the ends of both major and minor axes, the pieces in the larger curves could be scaled up from the inner curve and the three quarters of each curve would include three identical pieces, making production easier and less expensive.

The fourth quarter was not so simple. Each stone had to be the same length as the other stones in the rest of the same curve, so, at best, the curves could only be scaled up versions in general shape as distinct from being scaled up for each piece.

CED found that by taking the five curves (for the three quarters) as being in a series from 1-5, the curves for the fourth quarter would approximate to 11-15 in the same series. Michael now had a complete design that could be accurately costed. The design of the paving had hardly changed at all from the architects’ original lines but was hugely more efficient to manufacture and lay out for installation. Michael Heap describes it as “an exercise in common sense”.

It was only because the revisions kept so close to the original concept that C F Møller accepted them. The cost came out acceptably and the order was placed at the end of June.

Now each piece had to be detailed, which was first done by CED then independently checked by John Myers and Paul Jennings at JWQ. Production started and the team met at the quarry in the second half of July to approve both the material and production quality.

Excellent production control from JWQ and logistics management from CED completed the picture of products delivered as promised.

Blakedown Landscapes then had to set out the ‘ellipses’ in advance, knowing that the first piece had to be positioned absolutely accurately. They started from the end of one of the major axes.

Prior to this, nearly 1,000m² of paving had already been laid as a terrace in front of the Darwin Centre, with the joints in the paving all lining up with the joints in the floors inside the building. Having to withstand vehicular loadings, the exterior paving was bedded on concrete using SteinTec mortars. The joints were designed at 6mm to get the optimum result of visual regularity after allowing for the necessary tolerance on production. The joints are completely filled with mortar using a slurry system (because gravity does not fail).

This project has demonstrated the very best of UK stone production from JWQ but it has also highlighted the essential role played by CED in fostering co-operation between all parties and by finding a practical solution to the manufacture of a difficult design.

And, like all the best performances, what the public sees looks deceptively simple, showing no evidence of the complexities involved in the production.