Timber from tropical forests has been used for marine engineering in the UK for centuries. The marine environment is a tough test for all construction materials, but timber suffers much less from the effects of the salt content of seawater than, for example, concrete and steel. Marine timbers are at risk, however, from marine borers, which can cause severe damage in a relatively short time. Good natural durability in terrestrial conditions does not necessarily guarantee robust marine performance.
In the UK and other temperate waters, the two most problematic borers are the shipworm mollusc (Teredo spp) and the gribble crustacean (Limnoria spp).
Previous research carried out by TRADA in the 1960s indicated that gribble is capable of existing in one species form or another around much of the UK coastline. Shipworm tends to be limited to our southern coastline, although it has been reported in more northerly waters.
And numbers are growing rapidly. Over the last 20 years, stringent environmental legislation has led to a vast and continuing improvement in water quality, which in turn has enabled marine borer populations to flourish in harbour areas where pollution had previously excluded them. In addition, rising water temperatures have increased the vigour of these populations.
The material cost of timber in marine construction schemes is often dwarfed by the actual construction costs – within a coastal protection scheme the timber cost can be as low as 10-15% of the total. Premature failure because of poor durability must therefore be avoided and, as a result, this industry sector is conservative.
Specifiers are reluctant to select lesser-known species – particularly timbers whose resistance to marine borers is unknown – and will almost always choose a dense, naturally durable species of timber with proven marine performance, such as greenheart or ekki. A recent TRADA survey of marine engineers and specifiers identified that the key obstacles preventing specifiers choosing lesser-known timber species was a lack of technical data and proven track record.
The major disadvantage of this conservative approach is that commercial exploitation of such a narrow range of species can accelerate the depletion of timbers extracted from tropical forests – as well as inflating the price. Taking a holistic view of the timber industry, this makes profitable forestry and sustained yield management increasingly difficult to achieve.
Without the necessary test and technical evidence, however, specifiers will not take a new species on trust. EN 275 Wood preservatives – determination of the protective effectiveness against marine borers specifies a five-year test period – too long for durability screening tests to be economically viable, but too short to demonstrate a proven track record. Another catch-22 for the timber trade.
A solution, however, is in sight. For the past two years, TRADA Technology has been working with the University of Portsmouth to develop a fast-track screening programme to assess the resistance of selected timber species to gribble. The theory behind the programme is that if a species fails a gribble resistance test in the laboratory, there is little point selecting the timber for further field (marine) testing.
The comparative resistance of timber species was assessed by comparing the number of faecal pellets produced by gribble fed on TRADA’s test species against those produced by gribble fed on industry stalwarts – greenheart or ekki. A brief trip to Southsea ensured that sufficient gribble were captured for test purposes. The gribble were given one sample of timber as their only food source and low faecal pellet production was deemed an indication of resistance to gribble attack, as was death in service – some of our test species proved to be particularly toxic to gribble.
The laboratory screening test condenses a potential five-year test programme into 12 weeks. Timber species that pass the lab test are then exposed to a hazardous marine environment and tested for resistance against both gribble and shipworm. As there was no suitable site in the UK we set up a test site on the Algarve which offers an aggressive marine environment.
Sapwood samples of European redwood have been devastated within six months of exposure, which demonstrates the vigour of the marine borers at our test site. This in turn validates our findings and confirms the suitability of the Algarve site for test purposes. We expect to generate additional meaningful test data within the next 6-12 months as exposure trials continue.
The ongoing research programme has attracted the Timber Industry Development Division of the Ghana Forestry Commission, HR Wallingford and Ecotimber Ltd as commercial partners who are providing financial and material assistance.
The current test methods cannot demonstrate a successful track record, of course, but this new approach to testing may form part of a tiered approach to the evaluation of resistance to marine borers. The expense of long-term field trials as described in EN 275 is reduced by the elimination of less promising options at the laboratory screening stage.
Some promising species have been identified and we have attracted a number of enquiries from the timber trade. By working closely with our project partners, our long-term goal is to help timber exporters unlock greater value from their forests by finding a valuable use for lesser-known timber species. Increasing the inventory of timber species for export may help tilt the scales in favour of sustainable forest management rather than designating the land for other purposes.
Not bad for boring research based on faeces!
