The Woodland Trust's living bridge at Cow Hollow
Engineers team up with Cambridge architecture students to design and build a coppiced willow bridge
After 175 volunteer hours, Cambridge students have completed a living willow bridge for the The Woodland Trust at its Cow Hollow Wood site in Waterbeach, approximately 7 miles north-east of the university. All materials were sourced from the site, and the 4-metre span bridge includes no steel or plastic elements.
The project initiated last spring when the Woodland Trust approached Cambridge University Architecture Department seeking a design for a bridge and footpath at Cow Hollow. The architecture department in turn invited structural engineers Smith and Wallwork to join the project. Use of materials from the site was a key element of the brief from the outset.
Located on the edge of the fens, Cow Hollow is a millennium wood created on previous meadow land, planted with willow, osier, birch, alder, and ash. A meeting with a local willow expert sparked the idea for the final design because large diameter willow poles are able to root and regenerate when planted. This suggested the idea of a living willow structure, which is hoped will be resistant to decay.
The final design is comprised of a willow arch that supports a woven willow deck. The arch was formed from a series of willow poles planted on opposite sides of the ditch which were bent to meet and woven together. The weave had to be very tight to stop the poles from slipping as if they did the arching action would not perform. Load testing demonstrated that the bridge met performance requirements and only 10mm of settlement occurred when a load of a third of a ton was placed on the deck.
The deck consists of two 2m high panels which were stitched together on a trestle and lifted into position. The ends were planted into the banks. White willow poles of 50mm diameter were used for the arch and deck structural members with osier for the weave material. All materials were sourced less than 100m from the site of the bridge.
Construction began in November after the growing season so that no trees were harmed during the building process. All willow and osier was coppiced.
Installation of handrails is planned soon, and the living bridge awaits signs of re-growth in spring 2013.