Timber Bulkhead Repair: Borers, Rot, Hardware and Sheathing

Wood bulkheads are among the oldest and most common residential shoreline walls, built from treated sheathing boards or sheet piles, horizontal wales, round piles, and tie rods running back to anchors in the yard. Wood is forgiving to work with and relatively easy to repair in pieces. It is also a food source for organisms in the water and in the air, and it relies on metal hardware that corrodes. Repair planning starts with understanding which of those threats is at work.

For background on the material, see timber bulkheads.

Below the waterline: marine borers

The USDA Forest Products Laboratory (FPL) describes damage by marine borers in salt and brackish water as practically a worldwide problem. Along the Pacific, Gulf and South Atlantic coasts, attack is rapid, and untreated piling may be destroyed in a year or less. Along New England, cold water slows attack, but protection is still needed.

There are three main groups:

• Shipworms (Teredo and Bankia species) are mollusks that enter wood as tiny larvae and tunnel inside it. The entrance holes never grow large, so the FPL notes that the interior may be honeycombed while the surface shows only slight perforations.
• Pholads such as Martesia are clam-like borers that also enter small and grow inside the wood.
• Limnoria and Sphaeroma are small crustaceans that bore shallow tunnels near the surface. Waves erode the weakened wood, and heavily attacked piles typically wear to an hourglass shape between tide levels. The FPL states that untreated piling in heavily infested harbors can be destroyed by Limnoria within a year.

The Department of Defense waterfront maintenance manual (UFM 4-150-07) adds that Limnoria tripunctata can attack creosoted wood, and the FPL notes that even thorough creosote treatment will not always stop Martesia, Sphaeroma and especially Limnoria.

Above the waterline: decay, insects and weathering

Above the water, the UFM identifies fungi and insects as the main wood-destroying organisms. Brown rot, white rot and soft rot all need damp conditions. Cut tops of sheathing and piles are especially vulnerable because they expose untreated heartwood. Termites and wharf borer beetles also attack waterfront wood.

Hardware, connections and sheathing

Fasteners and tie rods

FPL research on fastener corrosion notes that wood preservatives, while extending the life of wood, can in some cases increase its corrosiveness toward metal fasteners. Salt water adds to that exposure. The UFM explains why this matters: when a timber connection fails, the structure is free to move, and an exposed connection can also let borers or fungi into untreated wood.

Sheathing failure and soil loss

Split, eaten or shrunken boards open gaps that let backfill wash out. The UFM describes outward wall movement from excess load behind the wall or tieback failure, and movement in the opposite direction from loss of backfill. A Virginia Institute of Marine Science (VIMS) report also documents "short sheeting," where boards were too short for site conditions and scour and flanking led to loss of backfill.

Preservative-treated lumber standards

The American Wood Protection Association (AWPA) describes its Standard U1 as the primary specification for preservative-treated wood. Use categories describe the hazard the wood will face, and the end tag should show "AWPA U1," the use category, the preservative and its retention. AWPA advises that wood without those markings probably does not meet its standards.

The categories above follow a USDA Forest Service reproduction of the AWPA table. Approved preservatives and required retentions change over time, so confirm current AWPA U1 requirements with your supplier and engineer. The FPL notes that where severe Limnoria hazard exists, dual treatment has been recommended, and that knowing which borers are present locally is important for choosing the preservative and retention.

Repair approaches

Replacing individual members

Wales, sheathing boards, piles and tie rods can often be replaced one at a time if the rest of the wall is sound. The UFM and FPL both stress protecting the treated shell. Cut surfaces, bolt holes and pile cutoffs must be field-treated before installation in accordance with AWPA U1, and the FPL advises that no cutting or injury of any kind should be permitted in the underwater part of a pile.

Pile wraps

For round piles, the UFM describes PVC or polyethylene wraps installed from at least a foot above mean high water to below the mudline. It calls wrapping widely used for preventing or arresting borer attack and more economical than encasement or replacement, but warns that punctures or tears can lead to rapid borer damage.

Tieback reinforcement

For light to moderate movement, the UFM describes adding a new wale slightly above the old one, with new tie rods and engineered deadman anchors, and protecting the rods with zinc or magnesium anodes. Its caution is specific to timber: it is unlikely that a leaning wall can be excavated and pulled back into position without high risk of failure of the timber members. See tieback and anchor repair.

Concrete cap or face

Where deterioration is too widespread for patches, the UFM describes adding a concrete cap to restore strength at the top of the wall, or a new timber sheet wall about a foot in front of the old one with concrete placed between to stop soil loss. It notes that this does not restore the wall's bending capacity and calls for annual inspection of the wood immediately under the cap. See bulkhead cap repair.

A practical checklist

1. Probe exposed wood at the cap, wale and splash zone for soft or punky areas.
2. Look for hourglassing on piles and thinning on sheathing between tide levels.
3. Check hardware for rust streaks, loose nuts, bent washers and missing bolts.
4. Walk the yard behind the wall for depressions, sinkholes or cracks parallel to the wall.
5. Schedule an underwater inspection. The UFM notes ultrasonic techniques can help where internal borer damage is suspected.

VIMS estimated that a wooden bulkhead built with quality materials and high preservative concentrations can be expected to last about twenty years, and actual service life varies widely. Our guide to how long bulkheads last explores the factors. To record what you are seeing, start with the seawall health check.