WOOD IN THE FRESH WATER AND MARINE ENVIRONMENT
WOOD IN THE FRESH WATER AND MARINE ENVIRONMENT
WOOD IN WATER. The utimate challenge. By Gervais Sawyer - Wood Technology Society
You should be aware by now of the demands of using wood in Use Class 4 situations. “Make sure it's 4' is the snappy TTF slogan, and very necessary too because all too many merchants and their customers think that as long as the wood is described as pressure treated and is green, you can use it anywhere for anything. Sadly, much that is sold can only be described as 'pressure painted' because it was not intended to be sold for UC4 applications.
Putting wood in the ground is very demanding because not only do you have soil that is packed full of bacteria, fungi and animals just looking for a carbohydrate meal, but also you have lots of moisture that can leach out preservatives or the extractives that give some timbers their natural durability.
How much more demanding therefore is it to use wood in fresh water for bridges, revetments and even simply fencing? Lots of water gives maximum leaching effect. Fortunately, in the UK, bacteria and fungi are really the only challenge because there are no aquatic fresh water wood borers. So there is really only a choice between taking a treatable low durability timber and impregnating it with the specified loading and depth of penetration of preservative (not easy to achieve), or of using a timber with high natural durability.
And then, there is using wood in salt water. Almost the ultimate challenge. Maximum leaching, a wide variety of aggressive fungi and wood borers and possibly severe mechanical demands such as abrasion and shingle scour. Photo 1. Shingle scoured greenheart piles at Sheringham, Norfolk.
Where timber is permanently or partially immersed, fungi are not such a problem owing to lack of sufficient oxygen, but above water, decay can follow. You will observe that marine piling often fails at the top. Photo 2. Rotting pile top of a groyne. Where salt water wetting is intermittent by the tides, the major wood destroyer is gribble (Limnoria spp) which is most active between low and mid-tide. What they lack in size is made up for by their numbers. They rasp away the wood surface which becomes so weakened that waves wash the damaged wood away exposing fresh material. After a time piles get a narrowed waistline. Gribble is the major problem around our shores. Photo 3. A gribbled pile in Venice.
In many locations, timbers can be rapidly damaged by shipworm such as teredo (all around our shores) and Lyrodus around our southern shores. These soft bodied molluscs line their tunnels with hard calcite. Observing this inspired Marc Brunel (Isambard's father) to invent the tunnelling shield for driving the world's first underwater tunnel beneath the Thames. As estuaries have become less polluted, teredo is returning in places. In the River Thames, teredo has been observed as far up river as Woolwich. Photo 4. Teredo at the entrance to Tilbury Dock. We can be thankful that we live surrounded by relatively cool water, because in the tropics there are species that can damage even the most durable timbers. One organism, Sphaeroma, can bore into timber for a refuge such that it is has not been deterred by high loadings of CCA preservative, whilst Martesia has even been observed to tunnel through 10mm thick hard polyurethane rubber!
Since the demise of CCA, we now have no preservatives approved for use in the marine environment other than creosote but even that will not be available for much longer, so the only option is to use naturally durable species. Amongst the temperate species there is really only oak as an option. Tropical species can meet the needs, but they should be used wisely, saving the highest performance timbers that are available in long lengths for situations where nothing else will do (Eg. Greenheart). Species with similar durability are available in shorter specifications and they can be utilised for braces and decking.
These difficulties have stimulated research into the suitability of using chemically modified timbers for use in the marine environment. Lamination of suitable species into longer lengths is another option to be explored.
Timbers like greenheart are so durable that they can be salvaged when no longer needed and recycled/upcycled into new uses. As such they can be fully FSC approved but will of course probably require strength grading.
Photo 1. Shingle scoured greenheart pile G.S.Sawyer
Photo 2. Rotting pile top of a groyne Zarkowski Designs
Photo 3. Gribble damaged pile with the typical waist. G.S.Sawyer
Photo 4. Teredo damage at entrance to Tilbury docks G.S.Sawyer
