During MacArthur’s liberation of the South Pacific, combat engineers joked that the timetable for his island-hopping campaign was governed by the vigour of the local marine borer populations rather than Japanese resistance. Harbours and port facilities were either heavily defended or mined. Therefore, temporary jetties and wharves – vital for maintaining supplies to the armies in the field – were constructed from various local timber species. However, the vigour of marine borers combined with variable timber ‘durability’, meant that these essential structures, even if they remained free from Japanese bombardment, had a service life of only a few months.
Fortunately our cooler, temperate waters do not support such aggressive populations, but nevertheless, marine borers can be a significant threat to timber structures in the marine environment.
Every year £500m is invested in flood and coastal defence engineering in England and Wales, of which in England alone £10m per year is spent on timber structures. Allowing for expenditure in other parts of the UK and including the ports and navigation sector, a conservative estimate of the expenditure on timber structures in marine engineering may be in excess of £20m per year.
Timber from tropical forests has been used for marine engineering in the UK for centuries. This environment is challenging for all construction materials, but timber suffers much less from the effects of the salt content of seawater than, for example, concrete and steel. In addition, timber’s resilience, favourable strength-to-weight ratio and the relative ease of fabrication and repair, make it an attractive construction material. There is a demand for strong, durable, cost-effective and environmentally acceptable construction materials. However, engineers must balance technical requirements with environmental and cost considerations.
The material cost of timber in marine construction schemes is invariably dwarfed by the actual construction costs. Consequently, this sector of the construction industry is conservative and there is a reluctance to specify timber species without a proven track record. Engineers are often reluctant to specify lesser-known species because their technical properties are not fully appreciated. Two of the principal obstacles in using lesser-known species are that, either little is known about their resistance to marine borer attack, or there is limited confidence in the pedigree of the technical information that does exist. It should also be borne in mind that high natural durability in terrestrial conditions does not necessarily guarantee robust marine performance.
Key attributes
A recent survey of marine engineers carried out by TRADA identified that, in their opinion, the key material attribute for timber was resistance to marine borers. The two principal invertebrate groups that attack timber within the tidal range in UK waters are the teredinids and limnoriids, more commonly known as shipworm and gribble respectively. EN 275 Wood preservatives – determination of the protective effectiveness against marine borers specifies a five-year test period – too long a period for these screening tests to be economically viable. Our own initial screening trials to determine gribble resistance take a mere four weeks!
Recent ‘fast track’ laboratory screening trials of South American and Ghanaian timber species identified a number of lesser-known hardwoods that may be suitable for marine engineering where there is a high risk of marine borer activity. From a total of approximately 60 species, 18 demonstrated good resistance to gribble (Limnoria quadripunctata) in laboratory screening trials. These species were then selected for comparative marine exposure trials.
The hypothesis behind the laboratory screening trial was “if it fails in the lab – it will fail in the sea!” In this study, comparative resistance to shipworm and gribble was assessed by comparing the rate of deterioration observed in candidate species of lesser-known hardwoods against that for greenheart, a species with a proven track record for marine construction. Some of the species tested actually outperformed greenheart: afina and ananta, both from Ghana; angelim vermelho, cumaru, cupuiba, favinha prunhela, lourou gamelha, piquia and uchi torrado all from Brazil.
The test site was located in Olhao harbour on the Ria Formosa lagoon. Although the lagoon is on an Atlantic coast, the climatic conditions are essentially Mediterranean, with hot dry summers and warm wet winters: the average temperatures range from 12OC to 28OC. Previous research trials indicated that Scots pine sapwood samples were heavily colonised by both gribble and shipworm within three months.
The exposure trial was carried out over a two-year period with assessments made at 6, 12 and 24-month intervals. With the benefit of 20:20 hindsight vision, we were able to observe that good, indicative results could be obtained over a six-month period. However, as we were in uncharted waters we ran the test for 24 months to validate our initial findings.
While data indicated that six months in such an aggressive environment was adequate exposure, we concluded that the minimum ideal test period was 12 months. And not only did the additional six months serve as a ‘fail safe’, it also allowed TRADA staff to see some winter sun (mud and diesel thrown in) and research a number of excellent seafood restaurants.
Gribble attacks the timber surface and causes erosion of the timber in the inter-tidal zone, consequently its activities are easily visible. In contrast, assessing for shipworm presented the researchers with a significant challenge. Problem – how could we assess shipworm activity without destroying the blocks? Solution – test racks were wrapped in plastic to prevent drying out and transported to a local
X-ray clinic. One test rack was equivalent to one lower abdomen and three thorax X-rays at €20 apiece. The look on the faces of plaster-encased patients when watching corpse-sized, plastic-wrapped racks dripping sea water being carried into the clinic was priceless!
Species identified
In summary, we have identified a number of timber species with, potentially, good resistance to marine borers. The fast track laboratory test is an effective screening method, although it will require some fine-tuning.
The test site in Olhao harbour provides an excellent, sheltered research station. Not only are test racks protected from the full force of weather and stormy seas, but the presence of the Portuguese Navy (see photograph) also protects them from vandalism. The test racks were immersed along the mooring points of the local patrol boats. Ideally, comparative marine exposure trials should run for at least 12 months. This is still a huge advantage over the five years recommended in EN 275.
In addition to determining marine borer resistance, we have been carrying out work to develop a fast track abrasion resistance trial to support this research and this will be completed by December.
As far as possible the test samples used mirrored the variability you would find in commercial timber supplies, although we were dependent to some extent on the goodwill of timber suppliers willing to condemn perfectly good stock (and profit) to take its chances on the seabed. As a result of the research, there is a valuable market opportunity for suppliers of lesser-known hardwoods.
