New guidance on the use of tropical hardwoods in marine and freshwater construction aims to encourage the specification of more sustainable species.
For many years the automatic choice for engineers designing timber dock gates, groynes and sheet piling has been greenheart, a very strong, durable species from the north of South America.
Increasing pressure on greenheart supplies triggered a partial switch to West African ekki, an equally strong and durable timber with even greater resistance to abrasion.
But there is now increasing concerns over the sustainability of ekki supplies as well.
In a bid to encourage the specification of lesser used species (LUS) the Timber Research & Development Association (Trada) has published a new Wood Information Sheet (WIS) “Specifying timber species in marine and freshwater construction”, which gives details of 18 alternative, more sustainable species of tropical hardwoods tested by the Environment Agency in a three year research programme.
“Construction costs dwarf the material cost of the timber, so there is reluctance to use LUS if they have no track record,” said research institute BM Trada senior technical consultant John Williams.
“Recent research has identified the strength properties of five alternative species which offer equivalent durability and strength to greenheart and ekki, and 13 with suitable properties for marine and/or freshwater construction that can be specified with confidence where strength is less of an issue.”
Two of the five greenheart equivalents, eveuss and tali, have been on full-scale trial in groynes at Pevensey on the East Sussex coast since 2009.
Also on trial there is sougue, a timber with high sand and shingle abrasion resistance but lower strength. The WIS recommends sougue for groyne planking only.
Other recent LUS contracts include replacement gates for Grafton Lock on the River Thames in Oxfordshire. The timber used, okan, another greenheart equivalent, was supplied by timber merchants Wijma.
Timber has been used for marine and freshwater construction for centuries.
Its resilience allows it to absorb shock loading from waves or vessel impact better than most other structural materials, Trada claims. But it can be vulnerable to fungal attack and is particularly at risk from marine borers such as gribble (Limnoria spp.) and shipworm (Teredo spp.)
Tropical hardwoods have traditionally been preferred for such applications because they offer adequate strength and resistance to fungus and marine borers and are available in large section sizes and long lengths.
In the WIS, Trada states: “The sustainable and responsible exploitation of a wider range of tropical hardwoods creates value in tropical forests and is a strong argument against the conversion of these forests to non-forest uses.”
A lack of information on the performance of LUS in structural sizes and their resistance to marine borer attack has been the major obstacle to their use, Trada states.
However, a research project carried out jointly by Trada Technology and research consultant HR Wallingford on behalf of the Environment Agency was based on extensive strength testing to the relevant British Standards and novel fast track screening for marine borer resistance, together with the exposure trials at Pevensey.
In UK waters the most common marine borer is the gribble, a minute and highly mobile creature 2mm to 4mm long usually concentrated in the intertidal zone.
Harbours and estuarine environments carry the greatest risk: full exposure to waves and abrasion significantly reduce the spread of gribble.
This is because high abrasion destroys the sub-surface galleries burrowed out by the gribble. In less aggressive environments this network of galleries softens the wood surface, encouraging it to erode away.
Shipworms are particularly hard to detect by non-destructive surveying techniques as their only contact with the outside world is a fine hole less than 2mm in diameter.
Within the timber, however, the shipworms excavate a network of discrete tunnels that weaken and eventually destroy the entire section. Although it is generally assumed that shipworm are confined to the south and southwest coasts of the UK, Trada warns that rising sea water temperatures are likely to increase the risk elsewhere.