The durability of piled foundations 481Precautions against fungal attack must be commenced at the time that the timber is felled.
It should be carted away from the forest as quickly as possible and then stacked on firm,
well-drained, and elevated ground from which all surface soils which might harbour organ-
isms have been stripped. The timber should be stacked clear of the ground with spaces
between the baulks to encourage the circulation of air and the drying of the timber to the
moisture content suitable for the application of the preservative treatment. Treatability,
based on the heartwood, is an assessment of the take up of the preservative and depends on
the structure of the wood and the method of treatment.
Suitable methods of preserving timber for piling work which have been on the market for
many years involved pressure impregnation with creosote, waterborne preservatives such as
copper–chrome–arsenic (CCA) (e.g. various Celcure formulations) and copper–chrome–
boron compounds (CCB). Although these products have a good safety record, European
Directives effective in 2005/6 proscribe many of these compounds. As a result, alternative,
more benign preservatives are being developed such as copper azole compounds (e.g. tebu-
conazole), Celcure P60, based on copper and chromium with phosphoric acid and light
organic solvents as a carrier for fungicides. British Standards BS1282: 1999, BS8417: 2003,
and the publications of the Timber Research and Development Association specify the hazard
classes, service factors for 15, 30, and 60 years service life, and ‘loadings’of the preservative
solution which are adequate for British and other temperate climates for timber piles in fresh-
and salt-water. In the USA the specifications of the American Wood Preservers’Institute are
followed. Biological deterioration including termite attack is much more severe in tropical
countries and the loadings or the selection of resistance species for these conditions should be
specified in consultation with a specialist authority in the country under consideration.
Softwoods such as Scots pine and southern pine can be impregnated completely with
preservative solutions, but the harder, ‘resistant’woods, including Douglas fir, can only
be treated to a limited depth (3 to 6 mm) after the timber has first been incised and then
subjected to a long-sustained application of pressure. The ‘moderately resistant’woods are
fairly easy to treat to depths up to 18 mm. There can be a useful advantage in using round
timber for piles, in which the outer zone of sapwood can be thoroughly impregnated to a
depth which will resist fungal decay over a long period of years. For example, the sapwood
of Scots pine or Baltic redwood can be treated to a depth of 75 mm, whereas if squared
timbers are used much of the sapwood is cut away to expose the less absorptive heartwood
which cannot be impregnated properly. Because the interior of the harder timbers remains
Species Durability European Standards Classes
Durability Treatability TreatabilityDouglas fir Non-durable 4 Resistant 3
Pitch pine (Caribbean) Durable 2 Moderately resistant 2
Larch Moderately durable 3 Resistant 3
Western red cedar Durable 2 Resistant 3
Greenheart Very durable 1 Extremely resistant 4
Jarrah Very durable 1 Extremely resistant 4
Opepe Very durable 1 Moderately resistant 2
Teak Very durable 1 Extremely resistant 4
and European Standards as shown in the following table: