Client The Woodland Trust, Ian Froggart
Designers Smith and Wallwork, Simon Smith and Tristan Wallwork
Cambridge University, Michael Ramage
Builders Smith and Wallwork
How do you stop a timber footbridge from decaying? Well, at Cow Hollow we have planted a bridge….our plans for a Living Willow Bridge are now reality after much thinking and doing. Using coppiced willow from the surrounding plantation we have woven a willow arch and deck that has been planted into the ditch bank.
The Woodland Trust has sponsored the project; Smith and Wallwork and Cambridge University were jointly responsible for the design and installation.
Construction began in November and the main arch and deck were completed in December, a total of some 175 hours of volunteer work were spent on site.
The bridge uses white willow and osier, there is no concrete, metal or plastic to be seen, just what was sourced from the plantation.
In the coming months we will add some handrails and come spring we hope to see some signs of growth.
In Spring 2012 the Woodland Trust approached Cambridge University Architecture Department to see if they were interested in helping develop a design for a new path and footbridge at one of their sites, Cow Hollow in Waterbeach.
On the edge of the fens, Cow Hollow is a millennium wood created on previous meadow land as part of a community involvement project. The planting comprised willow, osier, birch, alder, ash as well as some woody shrub species (http://www.woodlandtrust.org.uk/en/our-woods/Pages/wood-details.aspx?wood=5392#.UMhMg4O6c1M).
A key element to the client brief was for all building materials to be sourced from the site and to use a minimum amount of imported building materials.
Smith and Wallwork engineers were invited to join the project by Cambridge University and in June 2012 a group of students visited site and carried out a survey of the wood and drainage ditch that was to be bridged (http://www.smithandwallwork.com/archives/1246). The students came up with a number of bridge designs and in the end a meeting with a local willow expert sparked an idea for the final design when talking about large diameter willow poles being able to root and re-generate when planted.
This news unlocked the idea for a living willow structure, one that could potentially resist the onset of decay. The scheme that developed saw a woven willow arch that propped a woven willow deck.
The woven willow arch was formed by a series of willow poles planted on opposite sides of the ditch which were than bent down to meet and woven together. The weave had to be especially tight in order to stop the opposing poles sliding against each other…if they did then arching action would not be achieved. Some field trials in summer proved the concept.
The woven willow deck was formed in an upright position, two 2m high panels were formed and then stitched together on a trestle and then lifted in to position and the ends were planted into the banks.
Load testing of the 4m span bridge has been carried out with the bridge performing as expected with only 10mm of settlement when a third of a ton was placed on the deck.
White willow poles of 50mm diameter were used for the arch and deck structural members and osier was the preferred weave material. Material was cut in November, all less that 100m from the bridge site. It was important to wait until the growing season was over before cutting commenced. No trees were harmed in the making of the bridge, all willow and osier was coppiced.
No plastic, metal or concrete has been used in the final bridge design.
The next stage of development for the bridge is to install handrails and then hope to see signs of re-growth in spring 2013.