Limnoria Timber Damage

Section A-A

Figure 3-56. Shipworm (Teredo) damage of timber pile

Woodgribbles (Limnoria) are crustaceans related distantly to the crab and shrimp family. They are quite small, averaging only 1/8 to 1/4 inch in length. This tiny organism is a voracious wood chewer, with its appendages and mouth developed for rasping and biting.

At birth, the woodgribble mother retains the young within a pouch until they develop sufficiently to fend for themselves. She then releases them within her furrow and they proceed to dig side furrows. Ordinarily woodgribbles do not burrow deeply into the timber surface but limit their attack to shallow surface trenches. In timber piling, this results in a slow but continual reduction in pile diameter. Damage is most frequendy found at the waterline or mudline, where the woodgribble population is the greatest. Severe attack will produce an hourglass appearance in piling, as illustrated in Figure 3-57, reducing the outside diameter of untreated pine or Douglas fir by up to 6 inches in 1 year.

Termites are the most destructive type of insect life to attack above-water and onshore timbers. They feed on the cellulose matter contained in timber. If termite damage is suspected, an ice pick makes an ideal tool to determine their presence, as a serious attack will eat away large interior portions of the wood just below the surface of the timber. Timbers most subject to attack are curbs and blocking on bulkhead fills.

3.7.2.3 Shrinkage Damage. Drying causes timber to shrink. After installation this drying process continues, especially in hot dry climates, and the timber members split and check. This shrinkage also causes bolts to loosen in connections which, in turn, causes slippage and deflections in timber members and even distortion and weakening of the entire assembly. Ordinarily, splitting or checking is not serious and is allowed for

1/8 to 1 /4-inch long; no tubercles tail section

Limnoria

Ugnorum

(Rathke)

tail section

Limnoria

Ugnorum

(Rathke)

f/8 to-1/4-Inch long; 4 tubercles

1/4 to 1/2-inch deep burrows

Umnorta] Quadripunctata f HoKhuis tail section tail section

Umnorta] Quadripunctata f HoKhuis

Limnoria

Tripuncata

Menzies tail section tail section

Limnoria

Tripuncata

Menzies

Mudline

2 to 2-1/2 inches long; 1-inch diameter

Martesia

Striata

Linne

Martesia

Striata

Linne

Adults can grow 1 to 2 feet long; 1/2 inch in diameter

Teredo Navalis Linne

Shells at head

Adults can grow 1 to 2 feet long; 1/2 inch in diameter

Bank/a Setacea Try on)

Bankia Pallet

(Featherlike)

Teredo Pallet (Spadelike)

Worm like body

Worm like body

Adults can grow 5 to 6 feet long; 7/8 inch diameter

Bank/a Setacea Try on)

Bankia Pallet

(Featherlike)

Teredo Pallet (Spadelike)

Adults can grow 5 to 6 feet long; 7/8 inch diameter

Figure 3-57. Three groups of marine borers.

in standard timber design specifications. However, splits and checks in excess of those allowed by the standard grading rules are potential troublemakers and should be closed.

Splitting and checking create an opening in the timber face or end that is an ideal means of access for insects and borers. These openings also tend to accumulate moisture and dirt, which can also easily lead to decay and rot. Should excessive moisture freeze, the split or check will widen.

3.7.2.4 Overloading. Axial and bending overloading of piles may be due to a continuous source of loading or to an infrequent type of loading. Materials being stored in a warehouse on a pier is a form of continuous loading. Short-time loading is exemplified by the impact of a vessel striking a pier or heavy vehicles passing over the deck of the pier. Timber fender faces are particularly subject to bending overloading during ship impact. Failure of one pile requires the adjacent piling to carry not only its own but also part of the damaged pile's load. Continual overloading will cause failure of the adjacent piling, leading to the eventual collapse of the entire structure. Figures 3-58 and 3-59 illustrate the effect of compression and bending overloading, respectively.

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