Pile Design and Construction Practice, Fifth edition

482 The durability of piled foundations
untreated, careful attention should be paid to bolt holes. When these are drilled after the
main impregnation treatment, preservative should be poured into the holes. Incisions made
by lifting hooks, dogs, or slings should be painted with the solution.
Similar attention should be given to the end grain after trimming the tip to receive the
shoe or preparing the butt for the driving cap or ring (Figure 2.2). The exposed end grain
should be given two heavy coats of the preservative.
Some hardwoods, for example, ekki, greenheart, jarrah, okan and opepe can be used without
preservative treatment, but in these cases it is essential to specify that no sapwood is left
on the prepared timber. It is difficult to distinguish between sapwood and heartwood in greenheart
and either expert advice should be sought to ensure exclusion of the former or a preservative
should be used to treat the sapwood as a precautionary measure. Timber used for piling
is normally required to have large cross-sectional dimensions making it impracticable to
remove the sapwood. BS 8004 strongly recommends using round logs when the preservativetreated
sapwood provides a deep uniformly treated protective zone around the pile.
The adoption of preservative treatment by using creosote or some other solution does not
give indefinite life to the timber above groundwater level, and it may be preferable to adopt
a form of composite pile having a concrete upper section and timber below the water line,
as shown in Figure 2.1a.
10.2.2 Timber piles in river and marine structures
The moisture and oxygen in the atmospheric zone of timber marine piles above the water line
creates a favourable environment for fungal growth, which usually starts in the centre portion
where preservatives have not penetrated. Fungal activity occurs in the splash zone but is limited
due to poor oxygen supply. Marine borers do not attack wood in these zones. Brown rot
decay is the most common type of fungal decay in coniferous wood species, and in the early
stages of attack the wood will have lost weight and, while visually appearing sound, will have
suffered considerable loss of elasticity. Fungal attack does not occur below a maintained
water table and immersion in salt-water protects against fungal decay.
The most destructive agency which can occur in piles fully immersed in brackish or saline
waters in estuaries or in the sea is attack by molluscan or crustacean borers. Conditions in
the tidal zone are also likely to be favourable for attack by borers where adequate oxygen
and salt-water are present, but crustacean borers can often attack near an exposed mud line.
Below the mud line, adequate oxygen is not available for the survival of marine borers.
These organisms burrow into the timber, forming networks of holes that eventually result in
the complete destruction of the piles. Timber jetties in tropical waters have been destroyed
in this way in a matter of months.
The main types of marine boring organisms are
Molluscan borers Teredo (‘shipworm’)
Bankia
Martesia (in tropical waters only)
Xylophaga dorsalis
Crustacean borers Limnoria (‘gribble’ or ‘sealouse’)
Cheluria
Sphaeroma