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Warming to limpets Repairing sheet piling in busy ports and harbours is never easy. A British-developed technique is being used in the Caribbean to deal with major corrosion problems. Report and photo

Nutmegs and cruise liner passengers are the economic mainstays of the Caribbean spice island of Grenada. Nutmegs by the million are exported through the port of St Georges, cruise line passengers by the thousand come ashore there for a few hours of sightseeing and shopping. Small and crowded it may be, and badly in need of extension and refurbishment, but St Georges is the only deepwater port on the island and must be kept open at all costs.

So when the Grenada Ports Authority decided on a 4M scheme to extend the existing deepwater quay and reclaim a small boat dock behind it, the top priority was to minimise disruption to the shipping using the harbour. But one part of the project had the potential to be extremely disruptive refurbishment of the existing sheet piles which made up the 244m long deepwater quay.

The sheet piles date from 1959, and replaced a wooden structure that was destroyed by Hurricane Janet in 1955, the only hurricane to hit Grenada in recorded history, explains John Martin Construction project manager Adrian MacDonald. After nearly 40 years they were badly corroded, especially behind a concrete capping and facing that was added about 15 years ago.

Most of the damage has been caused by conventional corrosion. But much to our surprise we also discovered accelerated low water corrosion, adds MacDonald. ALWC had previously only been found in temperate zones (see box overleaf).

Canadian main consultant Novaport Vaughans first thought was that the piles would have to be cut back to well below water level and a new concrete quay wall poured on top. Then, while on a visit to London, one of the companys engineers was introduced to the limpet dam concept by JMC (see box right). The result was a radical rethink, and the final decision to repair rather than replace.

A 900,000 contract was signed in September 1997 and six weeks later eight containers arrived at St Georges from the UK. Inside were two specially- built limpet dams and their support equipment, plus all the steel plate and steel reinforcement needed to complete the project.

MacDonald comments: These are quite small dams by European standards, as tidal range here isnt much over 1m. Total depth is only 5m. In Europe 9m is more typical.

By the time JMC had assembled the equipment, main contractor Trinidad Contractors had cut away the existing concrete capping and facing. JMCs first set-up, however, was on the other face of the 107m long finger jetty which formed the outer end of the main quay. This enclosed the shallow dock which was to be backfilled to create more space in the port, and JMCs task was to reinforce the ends of the tie rods which passed through the finger jetty and retained the outer face. After 50m was strengthened, Trinidad Contractors could begin the backfill and JMC could start on the main part of the contract.

A single 35t tracked crane serves both dams. Each dam is fitted with two 8,000 litres/min hydraulically-powered submersible pumps fed by dedicated mobile power units on the dockside. A 50kVA generator, a compressor and a 700bar pressure washer complete the kit.

Transporting the 5t dam is made easier by keeping it under water as the crane tracks along the quay from one set up to the next.

As the dam is offered up to the sheet piling the pumps start up and suck it into position.

The idea is to seal on to outpans four piles apart, explains MacDonald. Our seals can accommodate irregularities up to 50mm, but in fact theres surprisingly little marine growth here, just corrosion products.

Safety chains are fixed to guard against the dam slipping upwards as its buoyancy increases. As the water drops below the bottom work platform the wetman goes down but not without a careful visual inspection. We once discovered a massive moray eel in the bottom, MacDonald reports. Fortunately it was sucked into the pumps soon afterwards.

First operation is to pressure wash the steel. Then thickness readings are taken with an ultrasonic meter to determine where strengthening is most needed. Even when the pile is unperforated it can be as little as 1mm thick, MacDonald says.

Two welders then swing into action. Three soffit plates are welded into the inpans, followed by hooped rebars. Faceplates measuring 2.44m deep by 760mm wide, already fitted with rebars, are lowered on to the soffit plates and welded into place. Projecting edges are burnt off, and the welders come out.

Then we fill behind the plates with normal site-mixed concrete, MacDonald goes on.

Once work there is complete, the crane hooks on and the pumps shut down. Caribbean water begins to flood into the dam, and as levels equalise the crane jibs out and eases the dam away from the wall.

Each cycle takes around eight hours, unless we have to pull out earlier to let a ship come in, says MacDonald.

It is on the outer face that the limpet dam concept has really came into its own. Cruise liners up to 250m long dock there every other day, alternating with container vessels, visiting warships and tramp steamers.

JMCs ability to set up or strike down at short notice to accommodate the needs of the port authorities has impressed.

GPA port manager Ian Evans reports that operations at the port have so far been completely free from disruption. MacDonald adds that the option of nightworking has not yet been needed.

JMC will need to complete 115 cycles on the outer face and 50 on the inner by early April. Work on concreting a new capping beam has begun, and soon JMC will begin digging down behind the sheet piles to check the condition of the tie rods.

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