Quantcast Sacrificial Test Blocks

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7.2.2.1 Marine Borers.  It is more difficult to inspect for marine borers due to the
presence of water. In some cases, borers are present in a salt water wedge located under incoming
fresh water.
Limnoria can generally be detected during periods of low tide. As discussed
in Chapter 4, Limnoria concentrate their attack on the external surface of piling, usually in the tidal
zone (Figure 4-30). They may be more prevalent on the shaded side of structures. Therefore, in-
spectors normally take advantage of low tides and simply wade out to check piling in shallow water
and use small boats or floats in deeper water.
Damage signs include the hourglass shape of piles in the tidal zone, numerous tiny bore holes in the
wood and a general softening in the attacked areas. Holes made by hooks and tongs become oval-
shaped as attack progresses, ends of bracing are hollowed out, and bolts and bracing are loosened.
Limnoria attack wood inside of holes, cracks, or gouges. Therefore, a flashlight or head lamp is use-
ful. A scraper or probe is used to remove fouling organisms and to check inside holes and between
adjourning members, etc.
7.2.2.2 Shipworms.  Shipworms are much more difficult to detect since they bore
small holes into the surface of the wood and concentrate their attack beneath the surface (Figure 4-
32). The tiny entrance holes are difficult to detect. Observant divers in clear water may be able to
see the nearly transparent siphons or the tiny pallets protruding from the pile surface, if the ship-
worms are alive. The extent of internal damage cannot be assessed visually.
Sounding of piling by divers can provide some information about the degree and location of borer
damage. However, the procedure is highly subjective.
Increment coring can also be used. However, with poor underwater visibility, fouling, etc., it is dif-
ficult to determine where to bore, and the odds of hitting a destroyed area are reduced, due to the
size of the bore. The bore holes must be plugged to prevent further damage.
7.2.2.3 Sacrificial Test Blocks.  The potential damage from marine borers can be
assessed by immersing "sacrificial" untreated wood samples at the site in question. The simplest
way to do this is to band or nail (use galvanized nails) lengths of untreated 2x4's, posts or similarly
sized wood pieces to vertical posts or piling already installed in the area of interest (Figure 7-9).
These samples are removed at one month intervals and examined for damage. For shipworms, the
blocks must be split open to accurately detect attack. Where freshwater flow is present, the blocks
must be submerged and held in the salt water wedge located under the freshwater.
7.2.2.4 Sonic Testing.  A sonic testing method which will locate and quantitative-
ly assess marine borer and other internal damage for in-place timber piles has been developed. The
system operates on the premise that the velocity and strength of sound waves passing through wood
vary inversely with any voids in the wood. The signal is correlated to the degree of damage and the
amount of undamaged wood remaining in the pile cross-section. In actual practice,
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