Sunday, 28 February 2016

Walls in the Willows - a renewable construction material

This week I was fascinated by a lunchtime talk about a renewable construction material that grows itself: retaining walls made from willow! This has been used in several locations in Norfolk either on its own (for footpaths or river banks which aren't particularly sensitive to settlement) or to provide living scour protection for gabion basket walls (for roads or rail applications).

Willow spiling wall comprises posts and withy infill.
Image from JPR Environmental
What is willow spiling?
A willow spiling wall consists of two elements:

  • Live timber posts measuring at least 100mm in diameter (being a natural material, the size will vary somewhat) which are installed at 0.6 to 1.0m centres like a king post wall. 
  • Willow "withies" are woven between the posts to form the infill panels (as this is a fairly open weave, a layer of Terram is advised on the landward side of the wall to prevent loss of fines). Backfill to the retaining wall should comprise silt, sand or gravel.
There is a limit on the depth to which the posts can be installed while still allowing the roots to grow. This means you can achieve typical retained heights of 0.6 to 0.9m, although most willow walls are built as a series of terraces with 2-3m wide earth fill between each vertical section.

What is it useful for?
Terraced willow wall adjacent to a watercourse
(image (c) Harmageddon)

Generally it is used to prevent scour and create low walls along riverbanks or watercourses. As well as being very cheap, construction benefits for use of these walls on riverbanks include the ease of installation, as posts are just hammered in by an auger or fence post driver; the lack of a concrete base which means there is no risk of polluting adjacent watercourses and the ability to place backfill to the wall using small lightweight equipment, so it is easy to construct in places with difficult access. There is also the potential for reuse of any excavated material on site to build the terraces, especially if it is sandy rather than clayey soil.
The walls can be installed in places which flood, but it is advisable to ensure that the fill material behind the wall cannot be washed out (ie either make sure it is well vegetated or cover it with Terram to protect it).

What do you mean, it grows itself? 
Willow wall under construction with turf laid
over the terraces (image from Willowbank Services Ltd)
Amazingly, both the posts and the infill panels will root themselves within 12 months and start growing into a living structure (90% success rate, and the 10% loss is usually taken up by the adjacent posts and withies growing into the space). The reason this works is that willow has an exceptional ability to form roots and shoots from any cutting, because unlike most trees, every part of the plant has dormant root cells which can be reactivated when they come into contact with the ground (the horticultural term for this is "Latent root primordia"). As the wall grows, its strength is likely to increase with time, although it will need to be cared for over the first few years to ensure it becomes well established, and afterwards it will need the occasional trim and pruning to keep it in check as with most vegetation.
Willow also has some other properties which make it very useful in wet environments such as adjacent to rivers. The plant is able to move oxygen round better than others trees, so willow can grow roots even in very wet or saturated ground (where the roots don't get any air). It's a very thirsty tree, so it takes water out of slope as it grows which can often help slope stability in itself (although remember that more water will be absorbed in hot dry weather than in mid-winter when the tree has no leaves!) It's easy to grow the raw material (willow stakes and withies) You can also store the stakes in a cold store until you want to use them. It absorbs carbon as it grows, and provides a great habitat (water voles in particular love the terraced structure).

How can you design with it?
As a relatively new technique, the lack of published research was restricting the use of willow walls as the primary retaining solution where settlement is important, such as roads and rail embankments, as there was no method to calculate how much settlement might occur for different loading arrangements or wall heights. This problem was the reason why chartered engineer Dr Debbie Scott from GroundSolve Ltd conducted a research project with contractor Richards Laing to determine suitable design parameters.
GroundSolve's research comprised building a willow wall and then loading the embankment above using concrete weights in order to measure the vertical deflection at the top of the wall. You can obtain a copy of the paper if you're interested by contacting the author directly, but the key findings were that the wall shows three main phases in its response to loading:

  • Initial loading (0 to 5kPa)*: Approx 10mm movement at the beginning (first 5kpa), caused primarily by (a) compaction of the loose embankment material and (b) lack of good contact between the wall and the retained material. This initial movement can be reduced by using material with a reasonable angle of friction (eg well graded sand and gravel) and compacting it to medium dense to dense with lightweight plant.
  • Normal use (5 to 20kPa): Over this load range, there is relatively little movement. The research suggested a couple of reasons for this: the weight of the surcharge compacts the retained material (increasing its strength) and compresses it against the withies, while the withies also become tighter as they are now in tension.
  • Overloading leading to failure (25 to 40kPa): The wall starts to fail with excessive displacements reported.

*The force is reported as a pressure applied over a 5m wide strip immediately behind the wall. For comparison, engineers usually use 5kPa as the standard loading  applied by a crowd of pedestrians on a footpath, while vehicles typically apply 10 to 20kPa if there is a road at the top of the wall.
Further work will be needed to revisit the test bed in a few years and load again, to investigate the behaviour of the spiling as the roots develop. As always, real projects where monitoring can be installed to provide further evidence of how the walls perform over time would be excellent.

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