The young molluscan borers enter the timber through minute holes in the surface or
through incisions. They then grow to a considerable size (bankiacan grow to a diameter of
25 mm and to nearly 2 m long) and destroy the wood as they grow (Figure 10.2a). The crus-
taceans work on the surface of the timber, forming a network of branching and interlacing
holes (Figure 10.2b). Their activity depends on factors such as the salinity, temperature,
pollution level, dissolved oxygen content and current velocity of the water. A salinity of
more than 15 parts per 1000 (the normal salinity of seawater is between 30 and 35 parts
per 1000) is necessary for the survival of most species of borer, but sphaeromahave
been found in nearly fresh tropical waters in South America, South Africa, India, Srilanka
(formerly Ceylon), New Zealand and Australia. Attack by cheluriais usually dependent on
the presence of limnoria. Limnoriacannot survive in fresh water.
Chellis(10.3)states that teredoand limnoriado not attack in current velocities higher than
0.7 m/s (1.4 knots) and 0.9 m/s (1.8 knots) respectively. Although activity from some species
may be marked in tropical waters, borers have been found above the Arctic Circle. They
show cyclic activity rising to a peak in some years, and not infrequently dying away
completely. Conversely, previously trouble-free areas can become infested with borers
brought in by ships or driftwood.
It was stated in the twenty-first report of the Sea Action Committee of the Institution of
Civil Engineers(10.4)that no species of timber is absolutely free from borer attack, but certain
species are highly resistant and in many conditions of exposure they may be considered to
have practical immunity. The report lists the more-resistant species (the ‘very durable’
class), greenheart, pynkadou, turpentine, totara and jarrah, as being suitable for conditions
of heavy attack by limnoriaand teredoin temperate and topical waters. ‘Moderately
durable’woods will resist moderate attack by limnoria.In commenting on the suitability of
various types of preservative, the report concludes that ordinary coal–tar creosote is the
most satisfactory, and states that in British waters any timber which is efficientlyimpregnated
with creosote should be practically immune to borer attack.
A comparative study was made by the Sea Action Committee of the relative effectiveness
as preservatives of creosote, Celcure (copper sulphate–potassium dichromate), and creosote
with the addition of copper napthenate. The latter material was tried as there was evidence
that some copper salts were poisonous to borers. The treated specimens consisted of
Douglas fir and they were exposed at Singapore, where the borer attack was mainly by
The durability of piled foundations 483Figure 10.2Attack on timber piles by marine borers (Crown copyright reproduced with permission
of BRE) (a) Attack by Teredo(b) Attack by limnoria.
(a)