Specifying timber species in marine and freshwater construction

Brackish water may be present in estuarine conditions, which may support attack by marine organisms.

The only biological hazard to timber in fresh water in Europe is fungal decay. Unless specifically referred to, the heartwood of the LUS detailed in this WIS is considered to be naturally durable or very durable in the context of attack by decay fungi. If timber is used externally in contact with fresh water, moisture content will rise and decay is certain to follow unless naturally durable or preservative-treated timber is employed.

An important qualification is that timber permanently and completely immersed in fresh water will not deteriorate due to the action of higher fungi (for example wet rot type fungi) as there is insufficient oxygen available for these types of decay fungi to carry out the process of biological degradation. However, soft rot decay can occur on the surface of timber completely immersed in fresh water and salt water, but its rate of progress is so slow that, on timber of any large dimension, its effect may be considered negligible unless the timber is in an abrasive environment.

At and above the water line, conditions may be suitable for terrestrial decay fungi and to avoid this it is advisable to specify a naturally durable timber.

Timber that is exposed in the marine environment below the high tide mark is subject to attack by marine bacteria, fungi and wood-boring animals. Of the three risks, marine bacteria and fungi have a comparatively minor role in the bio-deterioration of timber below the high tide mark. However, within the intertidal zone and above the high water mark, the risks associated with fungal decay in the marine environment are similar to those in fresh water, and to avoid these risks it is also necessary to specify a naturally durable timber.

Marine borers may cause severe damage to a timber structure over a comparatively short space of time.

According to research that TRADA undertook in the 1960s there are two main groups of marine borers in UK waters – shipworm (Teredo spp.) and gribble (Limnoria spp.). Gribble are ubiquitous, whereas shipworms are limited to the south coast and isolated estuarine areas along the west coast of the UK.

Records show that shipworms tend to be found along the south and south-west coast of the UK. However, no similar survey has been carried out since the 1960s and information on the current distribution of shipworms is scant. Given the improvements in water quality and sea water temperature rises that have occurred, it is reasonable to surmise that the risks associated with marine borers are likely to increase in the future.

Figure 2 Electron micrograph of Limnoria spp. Photo: L. Cookson, CSIRO

The wood boring crustaceans found around the UK coastline are members of the Limnoriidae more commonly known as gribble (Figure 2). Unlike shipworms, adult gribble inhabit the surface and they are fully mobile. Although attack by this animal is superficial, persistent attack can result in failure of structural timbers. It can burrow an extensive network of galleries at or just below the wood surface which results in softening of the wood surface.

Gribble are small animals, just about visible to the human eye when timber is inspected in situ. Attack is distinguished by holes in the surface of the timber, with underlying galleries. Gribble are generally 2–4mm in length and are pale grey in colour. Ecological factors such as sea temperature and salinity can affect the reproductive and feeding activity of gribble. They tend to concentrate at the intertidal zone.

Gribble tend to be sensitive to their environment. They can cause significant damage in harbours and estuarine environments, where there is less risk of abrasion. However, where structures are exposed to the full force of the sea and where there is a high risk of mechanical abrasion, gribble find it difficult to establish large populations. This is because the abrasive nature of the environment destroys the galleries.

Figure 3 Example of Teredo spp. extracted from Barmouth viaduct Photo: TRADA Technology

Shipworms found around the UK coastline are generally teredinidae (a family of molluscs). They have a soft worm like body (Figure 3) with two shells or valves at the anterior end of the animal which enable it to bore into timber.

The animal remains in the same tunnel throughout its life. Each tunnel is discrete and the animals avoid intruding into neighbouring tunnels as they grow and excavate into the timber. Even in timbers of limited volume, the shipworms will not emerge from the timber and will continue to bore alongside its neighbours until the timber is more or less destroyed and breaks apart. Usually, shipworms line their excavations with a secretion of calcium carbonate for protection. Growth rates can vary according to species and environmental conditions, particularly temperature. In warm waters some animals can grow in excess of two metres in length.

The posterior part of the animal maintains contact with the external seawater environment via a fine hole about 1–2mm in diameter. This hole is the only external sign that shipworms have colonised a timber component which makes surveying for shipworm infestation, using non-destructive techniques, extremely difficult in situ.

Tables 1 and 2 summarise the performance of greenheart and ekki and a range of LUS when benchmarked against greenheart and ekki.

The guidance in this WIS specifically covers timber for construction in Use Classes 3, 4 and 5 as defined in BS EN 335 (Ref. 4) where Use Class 5 is the most hazardous.

Use Class 3 is defined by this standard as: “A situation in which the timber is not under cover and not in contact with the ground. It is either continually exposed to the weather or is protected from the weather but subject to wetting.”

The timber is exposed to decay fungi that can cause pockets of rot.

Examples include: decking (including pontoons), parts of a jetty structure above the Mean High Water Springs (MHWS) level or 1–2 year water level for rivers.

Use Class 4 is defined by this standard as: “A situation in which the timber is in contact with the ground or fresh water and is permanently exposed to wetting.”

The timber is exposed to decay fungi and soft rot fungi which exacerbate erosion.

Examples include: moorings, navigation structures, parts of landings, piers, jetties and stages that are immersed, fenders, lock gates and slipways.

Use Class 5 is defined by this standard as: "A situation in which the wood or wood-based product is permanently exposed to salt water."

The timber is exposed to decay fungi, that can cause internal pockets of decayed timber, soft rot fungi and attack by marine borers.

Examples include: groynes, slipways (incl. decking), parts of landings, piers, jetties and stages that are immersed, fenders and lock gates.

Five LUS have been allocated to strength classes, as detailed in Table 1, on the basis of test results for bending strength, bending stiffness and density. The strength classes provide values that may be used in design using Eurocode 5 (Ref. 5).

These timbers (angelim vermelho, cupiuba, eveuss, okan and tali) are illustrated in Figure 6. They may be used as alternatives to greenheart and ekki, provided their resistance to marine borer attack and abrasion, where relevant, and their strength properties meet project requirements.

The strength class allocations for these five LUS were derived from timber tested at a moisture content close to that likely to be found in service for Service Class 3 (SC3) where the average moisture content is greater than 20% (timber immersed in the marine and freshwater environment will have a moisture content in excess of 20%). No correction was made for this high moisture content during the strength testing programme. Therefore, structural engineers need not apply any of the strength reduction for SC3 in their designs when specifying angelim vermelho, cupiuba, eveuss, okan and tali. However, this must not be taken as advice that the strength values may be increased if the expected Service Class is SC1 or SC2.

If strength is not critical, a longer list of 13 LUS (Table 2) may be suitable for marine and freshwater construction applications. These timbers exhibit varying degrees of resistance to marine borers and abrasion. Their suitability will need to be assessed on a project-specific basis.