Fig 1 Hazard zones
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Marine hazards for timber
Marine piles are subject
to three zones of durability hazard. The bottom ends are
usually embedded in mud and are free from hazard while
the parts above the high water mark have to withstand
weathering and abrasion. The greatest hazard occurs in
the zone between the mud line and the high water mark
where marine organisms are active.
In this zone, the greatest
threats are the Teredo borer or shipworm and crustaceans
(Limnoria or wood lice), particularly in warm waters. The
borers tunnel into the timber for shelter and their
damage may go unnoticed until it becomes extensive.
Crustaceans only burrow to
just beneath the timber surface. Their damaging effect
would be negligible except for the surface breakdown
caused by wave action. Additional burrowing follows
surface breakdown until an "hour glass" shape
occurs.
Protection for marine
piles or timber in marine contact is best afforded by:
- using species with
high natural resistance such as turpentine,
satinay, cypress pine or jarrah. The few species
that are naturally resistant (either because of
high silica content or naturally toxic nature)
may last up to 5 years longer if the bark is left
intact.
- using timbers
impregnated with chemical preservatives. CCA is
effective against Limnoria while creosote type
preservatives are effective against Teredo.
Double treatment (CCA followed by creosote)
extends service lives by as much as 45% and
should be considered when a high marine hazard
exists. Refer to Figure 1 for general marine Hazard
Classifications. It should be noted that
significant local variations in hazard can exist
due to differences in water temperature and
salinity.
- using mechanical or
chemical barriers. Mechanical barriers include
copper sheathing or for more permanent work,
concrete encasement. This includes poured
concrete collars or timber piles driven through
concrete or fibre cement pipes (high water to mud
line). Where piles are encased in pipes, sand is
usually placed between the timber and the pipe to
provide mechanical support to the pipe.
Chemical barriers
include sheaths impregnated with chemicals that
are toxic to marine borers. Care should be taken
with all marine piles to ensure that any damaged
sapwood, splits, knots or other imperfections are
given additional chemical or mechanical
protection. Regular inspections and scheduled
maintenance procedures are imperative for all
waterfront structures, particularly piles.
Reference: Timber Datafile
P4 – Timber – Design for Durability of the NAFI
Timber Manual
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