Opened on 24th May 2008, Kew’s Rhizotron and Treetop Walkway takes you under-ground to explore the mysteries of a tree’s root system and then up into the air for a birds’ eye view of Kew and the opportunity to discover the rich biodiversity of a temperate woodland canopy. There were 9000 visitors on that first day, proving once again the public’s insatiable desire to see the world from unusual vantage points.
Kew’s plan was straightforward: to build a walkway, some 18 metres high, that would weave its way through a section of mature woodland containing trees planted during the time of Capability Brown’s renovations in the 1770s. Our response was equally simple: to provide a series of bridges — preferably of identical length and design to keep costs down — supported by platforms at each end raised on tall pylons. We proposed 12 bridges, each 12 metres long, linked by 10 small round platforms, with one main ‘access’ pylon at the start supporting stairs and a lift, and a ‘classroom’ platform halfway round for use during school trips or where visitors can just stop and enjoy the view.
From the outset, our aim was to keep the Walkway as visually unobtrusive as possible, producing a discreet presence at ease with its natural surroundings. The choice of materials was therefore important, with the added challenge that they should require very little maintenance. Working with structural engineer Jane Wernick we quickly homed in on weathering steel, which gains its strength and longevity, somewhat unexpectedly, by developing a thin layer of rust on its surface. Colour and texture were a perfect match in this context, the only slight downside being that the material was only available in flat sheets. This, however, was quickly turned to our advantage, when Jane proposed fabricating the pylons as long tapering triangles that would split near the top to form three triangular ‘branches’ to support each platform.
We decided to build the platforms and bridges in the same material, but wanted to give them a more organic form. We turned for inspration to the Fibonacci sequence and, to our delight, found that if it was used to determine the distance between intermediate support points in a truss, it matched exactly the forces requiring support. Using the bridge balustrades as the trusses, it was possible to design these symettrically around the midpoint, considerably simplifying the design and fabrication process. Read more
Another major challenge was to strike a balance between enabling visitors to get as close as possible to the trees, while minimising damage to their complex root systems. This was resolved using radar surveys of each pylon location, which enabled us to strategically position the concrete piles between major roots. A bespoke pilecap — in lattice steel to allow water to penetrate to the roots below — was then designed to allow each pylon to be positioned exactly. To save time and minimise disruption and on-site welding, the entire structure was prefabricated off-site by W S Britland & Co.
The walkway opened in May 2008 as part of the celbrations for Kew’s ‘Year of the Tree’ and has proved a considerable success, with over 500,000 visitors in the first year alone. It has also won numerous awards, including an RIBA Award, the Stuctural Steel Design Award and the Civic Trust Award, all in 2009.