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Smooth sailing in South Wales

Ports and harbours - Burry Contractor: Dean & Dyball is coming to the end of a project to transform a crumbling Welsh coal wharf into a vibrant yachting marina. David Taylor reports.

Of all the places the Queen might want to visit on 12 June - 50 years to the day since her coronation - Neil Beresford's construction site in South Wales might not seem the obvious choice. Beresford is project manager for contractor Dean & Dyball on the £4.2M redevelopment of Burry Port Harbour, an old coal wharf on the South Wales coast.

As civil engineering projects go, Burry Port Harbour is fairly modest, but it is the final part of a much bigger scheme: the £29.9M Millennium Coastal Park that stretches 21km along the coast west of Llanelli. It is this that the Queen will come to inspect in June.

But all through this spring, the pomp and ceremony that will accompany the Queen's visit has been the last thing on Beresford's mind. He has had a piling programme to focus on.

Beresford's task at Burry Port is to turn the town's old coal wharf into a floating marina for pleasure craft. The old Grade II listed harbour has walls built of stone blockwork with sand infill.

Tidal range at this part of the coast is around 8m, which means that, traditionally, the harbour empties at low tide, leaving any vessels within it sitting on the mud. What the Millennium Coastal Park wants is a marina - a harbour with floating pontoons where yachts can moor without ever running aground.

The project involves restoring the crumbling harbour walls and constructing an impounding gate to trap water in the harbour as the tide recedes. Dean & Dyball is an old hand at this type of work, having completed a similar project last year at Watchet, on the other side of the Bristol Channel. Here, the job involved construction of a precast concrete wall across the middle of the harbour, with a self-activating impounding gate located at the eastern end. Burry Port Harbour is different in several respects.

The first task was to narrow the 140m wide harbour mouth by extending the two block-faced revetments with a new wall consisting of a double line of 20m long steel sheet piles, backfilled with stone. The 14m wide impounding gate is roughly in the middle of this new wall and alongside it is an 8m wide weir set at the maximum impounding level. This weir is designed to handle the effects of extreme tidal conditions which might overwhelm the narrow harbour entrance.

'Technically, the wall has been very difficult to design and construct, ' says Beresford.

Working around the 8m tidal range has required meticulous planning and very careful design to ensure the structure was not undermined or swept away by the incoming tide.

The new wall has been built in a series of 10m by 7m 'cells', each formed within a sheet piled cofferdam. The 20m long Larssen LX25 sheet piles are driven into the silt and gravel with a vibrating hammer, then toed into the bedrock with an impact hammer.

'Once we've done that, we've basically got a box full of silt and mud, ' says Beresford. 'Unfortunately, we couldn't leave the silt inside because it has no strength.

We've had to dig it out with a long reach excavator to a depth of up to 9m in places.

'Without the silt, we would have had a big hole which would have collapsed under the pressure of the incoming tide. We had to let the water back into the cofferdam to equalise the pressure, and continue to dig out the silt underwater, ' adds Beresford.

Hydraulic shoring rams were also deployed, which helped to prevent the cofferdams collapsing.

Once the cofferdam was excavated down to firm gravel, Dean & Dyball started filling the hole with Type 6A granular fill from a local limestone quarry. The problem was reversed and steel tiebars had to be installed to counteract the outward forces of the stone.

Once the cofferdams were filled to the level of the surrounding silt bed, Dean & Dyball switched to Type 1A fill, which was compacted inside the cofferdams to finish the impounding wall.

The gate is a 30t steel barrier, hinged at its base like a castle drawbridge. As with the wall cells, this has been constructed within a sheet piled cofferdam but, instead of Type 6A fill, a 1.5m deep insitu concrete plug was cast in the bottom to support the piles and provide a foundation for the gate structure and operating mechanism.

As with its counterpart at Watchet, the Burry gate was designed by Bournemouthbased consulting engineer Kenneth Grubb Associates and built and installed by Cardiff-based specialist Taylor & Sons. But whereas the Watchet gate used a floating displacer to activate it according to the tide, Burry Port Harbour's gate is operated by a single hydraulic ram.

'The reason for this is that the client wants to be able to operate the gate at any time, which allows them to impound water at any level, ' says Beresford.

'Being able to impound at a lower level gives you a bigger window of operation - you can leave the gate open for longer.'

High winds, heavy seas, bureaucratic planning requirements and even one or two seismic tremors have conspired to hamper progress on the project since Beresford's team started on site in October. But throughout the contract, Dean & Dyball has shared offices on site with the client's project manager, Arup, and solutions have been engineered to keep the job on programme. 'Our contract is the NEC Option C (Target Sum) - very much a partnering contract, ' says Beresford: 'Everything's open-book.'

As the work progresses, Beresford's mind has become more concentrated on the 12 June deadline. He knows what is expected of him up until then:

but what will he be required to do on the day? 'To be honest, I haven't even thought about that bit yet, ' he admits.

Site team Client: Millennium Coastal Park Commission Client's project manager: Arup Design and build contractor:

Dean & Dyball Contractor's consulting engineer: Babtie Group Piling: Steel Pile Installations Plant: Parker Plant Hire Impounding gate design:

Kenneth Grubb Associates Impounding gate fabrication and installation: Taylor & Sons

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