Sub-Wharf Structures and Durability

Most people experience a wharf from the top: the deck, the bollards, the view. The more interesting part is underneath. Every wharf on Wellington’s waterfront stands on a forest of piles and beams that spend their lives in one of the harshest environments any structure faces. Understanding how that hidden structure ages helps explain why wharf buildings need the kind of maintenance they do, and why repairs below the deck are slow and costly.

Timber and concrete

Many of Wellington’s early wharves were timber, built on hardwood piles driven into the harbour bed. Timber was available, workable and could be replaced pile by pile. Its weakness in seawater was marine borers, small organisms that tunnel into submerged wood and can hollow a pile from the inside while it still looks sound at the surface.

Reinforced concrete arrived in the early twentieth century. Clyde Quay Wharf, completed by the Wellington Harbour Board in 1910, is often described as the first wharf in the city built of reinforced concrete rather than timber. Concrete promised a longer life and no food for borers, but it brought its own problem, which engineers have been managing ever since.

The wharf is a good example of how these structures layer up over time. Greater Wellington’s 2020 heritage addendum report describes three phases: the original reinforced concrete piles, timber piles added when the wharf was widened and lengthened in 1962-64 for the Overseas Passenger Terminal, and major strengthening during the apartment construction that followed.

How seawater attacks concrete

Plain concrete is quite durable in seawater. The trouble is the steel inside it. Concrete is naturally alkaline, which protects reinforcing bars from rust. Over time, chlorides from seawater work their way in through the concrete cover. Once enough chloride reaches the steel, the protection breaks down and the bars start to corrode.

Rust takes up more space than the steel it came from. As it forms, it pushes outward and cracks the concrete around it. That lets in more water and salt, which speeds things up. Eventually pieces of concrete break away, a process known as spalling, exposing the corroded bars beneath.

The most vulnerable part of a pile is not the bit that stays permanently underwater, where oxygen is limited, nor the dry deck above. It is the tidal and splash zone, where the concrete is wetted and dried over and over with every tide and wave. That cycling pumps salt into the concrete and keeps oxygen supplied to the steel. On many older wharves, the band just above and below the tide line is where the worst damage is found.

The deck and its underside

Wharf decks face a different mix of loads. The top surface carries foot traffic, vehicles and, on working wharves, heavy equipment. The underside sits in constant salt-laden humidity with little sunlight and no rain to rinse it. Beams and the soffit of the deck can corrode quietly for years before anyone notices.

Where a wharf carries a building, the connections between the building and the wharf structure matter too. Joints, bearings and drainage paths all need to keep working. Water that ponds on a deck or leaks through joints into the structure below will find any weakness.

What the Building Code says about durability

New Zealand’s Building Code sets durability requirements in clause B2 Durability. Building elements must, with normal maintenance, continue to meet the Code for set periods from the issue of the code compliance certificate. In broad terms:

• 50 years, or the life of the building if less, for the structure and for elements that are difficult to access or replace, or whose failure would go undetected during normal use and maintenance.
• 15 years for elements that are moderately difficult to access or replace, or whose failure would only be noticed during normal maintenance. Most claddings sit here.
• 5 years for elements that are easy to access and replace, and whose failure would be obvious in normal use.

Wharf piles and beams supporting a building are structural, so the 50-year requirement is the relevant one. In practice, designers of marine structures often aim for much longer service lives through generous concrete cover, high-quality dense concrete, protective coatings and, in some cases, cathodic protection systems that use a small electrical current to slow corrosion of the steel. The words “with normal maintenance” in B2 are important. Durability assumes someone is looking after the structure.

Inspection and repair

Because so much of a wharf is out of sight, inspection is a specialised job. Typical approaches include visual surveys from boats at low tide, underwater inspections by divers, and testing of concrete for chloride levels and cover depth. Repairs can range from patching spalled areas to jacketing damaged piles in new concrete or replacing them entirely.

Working under a wharf is high-risk. It combines water, tides, confined spaces, heavy lifting and often diving. WorkSafe sets out the general duties that apply to anyone managing that work, and wharf owners rely on specialist marine contractors for it.

Why this matters above the deck

For anyone living or working on a converted wharf, the structure below is mostly someone else’s problem until it is not. For a unit title development, the condition of the wharf structure belongs in the long-term maintenance plan, with realistic budgets for inspection and periodic repair. These are not glamorous items, and they rarely come cheap, but deferring them tends to make them more expensive.

The good news is that well-designed and properly maintained wharves can last a very long time. Parts of the 1910 concrete piling under Clyde Quay Wharf are still in use more than a century later, which says something about both its builders and those who have looked after it since. For more on planning ahead in shared buildings, see our notes on body corporate long-term maintenance.