Overhead, the boats sail by

In spring this year, the œ95 million Cardiff Bay Barrage project came within an arm's length of catastrophe. But this is the hi-tech 1990s: an arm's length was just what the designers had planned.

A freak tide raised the waters of the bay higher than expected, bringing the sea level terrifyingly close to the rim of the colossal sand-bund cofferdam, within which all the costly hydro-mechanical element of the barrage is being constructed.

With the sea-level just 750mm from breaching the oval-shaped cofferdam, a few more knots of windspeed, a few more centimetres, a few large waves, and the project's half-constructed locks and sluices - not to mention the contractor's entire rationale at Cardiff - would have been destroyed.

It did not happen - just as the tide forcasters, statisticians and the probability boffins had said it would not.

Paul Neal, project manager of the Balfour Beatty-Costain (BB-C) joint venture at Cardiff, has no grey hairs, though whether the rest of the team survived similarly unblanched seems unlikely.

high tide

"You can design these things to cope with a one-in-10,000 year event," says Neal, "but would that be efficient? The dam here is designed to cope with an unexpectedly high tide, and that's just what it did."

The cofferdam is an amazing structure, roughly as big as the barrage will be when completed.

Oval in plan, 350 metres long, 200m wide, and high enough to keep out the 14m tidal range of the Taff Estuary, the the cofferdam is so massive that it is difficult to remember that it is merely temporary works. It effectively extends the shore out into the estuary, and it could contain two Cardiff Arms Parks with ease.

Yet it was not part of the original design by consultant Sir Alexander Gibb & Partners. BB-C believes that its inspired plan to construct the cofferdam was instrumental to winning the contract. Indeed, it must have taken a rare combination of nerve and lateral thinking for BB-C's tender team to suggest such a radical way of handling the project.

That was back in 1993. After pre-qualifying alone, Balfour Beatty teamed up with Costain to win the project. Work commenced on site in May 1994 after BB-C beat off competition from Laing, Tarmac, Bouyges as well as a Kier, Christiani & Nielsen and Hochtief group, a team of Taylor Woodrow and Robert McAlpine, and a pairing of Trafalgar House with Ballast Nedam.

The 1.1km long barrage is designed to isolate the estuary reaches of the rivers Taff and Ely from the tides and the sea. It will create a permanent 500-acre, development-friendly, freshwater lake, where now there are only tidal mud flats.

The barrage will run from Cardiff Docks on the East of the bay to the residential area of Penarth to the West. The Penarth side the barrage will feature locks, sluices, and the largest fish pass in Europe. The creation of a new harbour to protect boats waiting to enter the lake via the locks is also part of the contract.

island

It was originally envisaged that the locks and other major structures would be constructed with the help of much smaller "island" type cofferdams, sheet-piled into the estuary. If just one structure was to be built, then this would have been virtually the only method available.

But BB-C took the view that since there were several, large, structures it was worth considering the more radical option. The structures are three 10.5m by 40m navigation locks, the huge fish pass, five massive sluice gates, and three hydraulically operated bascule bridges to span the locks.

But there were at least three persuasive arguments against the creation of the vast, all-encompassing cofferdam.

How, for example, would it be possible to create the encircling bund in an estuary whose tides are so high, and so destructive? And having constructed such a dam, would it not be in the way of the final barrage? The counter argument was that the thing would be so huge as to be uneconomical.

option

But having done its sums, BB-C decided it was by far the best option, offering a number of important advantages. Most obvious, it enabled the structures to be constructed in the dry, making that a faster, quicker process.

A single cofferdam bund would also protect this element of the project from the risk of bad weather. This is an important point as the heavily-amended ICE 6th Conditions of Contract used on the project do not give the joint venture contractor extension of time due to delays caused by unworkable marine weather.

Another plus point for the bund was that, although the effort involved in its construction - calling for colossal quantities of sand, gravel and rock - much of the material could be re-used to form the barrage proper after the cofferdam had served its purpose.

Recalling the early days of the project, Neal says: "The dredging went on 24 hours-a-day, seven days a week. We removed almost two million cubic metres of alluvium."

To construct the cofferdam, dredgers bottom-dumped vast quantities of sand won from the nearby Bristol Deeps. This filled the dam's dredged foundation channel and then began to build up to form the embankment.

Dredging activity was intense, as tidal flow naturally tended to flatten the rising underwater embankment.

To help win the battle with nature, BB-C's subcontractor, Dredging International, used one of the largest split-hopper dredgers in the world, able to dump its shipments of 3,500 m3 in just 30 seconds.

"Licences for winning sand and dumping alluvium were secured by the employer before the tender stage," says Neal. "It removed a lot of uncertainty and risk, which would otherwise have been reflected in the tender price."

BARGES

As the embankment rose in height, the barges could no longer float above it, even at high tide. So sand was placed by pipeline pumping and by "rainbowing" - where a sand and water mix is pumped out over the sea in a well-aimed arc. A gap was left in the dam between two relatively small sheet-piled structures and the sea flooded and drained from the dam with every tide.

"The sand's natural angle of repose was a one in six gradient," he says. "We needed it to be much steeper, one in three, inside the dam. So at every low tide 18 machines headed into the cofferdam from the shore and dozed up the sand. It was a race against time - and the elements."

The cofferdam was also armoured with rock on the external, estuary side, and to make it more impermeable a vib wall - effectively a membrane of cement-bentonite has been inserted vertically from the top of the bund.

"This involved vibrating an 800mm I-beam down through the sand and injecting cement bentonite slurry through two pipes connected to the beam," says Neal. "You form a contiguous and more or less impermeable wall this way.

"Perhaps it wasn't strictly necessary: the barrage itself won't have one. But we felt it would pay for itself in lowering our dewatering and maintenance costs."

Work on the wall commenced in January 1996 and was completed in April, allowing the cofferdam to be closed in May last year. The dam was then further excavated so that piling for the major structures could begin.

Examining these now half-built locks and sluices from within the cofferdam is a strange experience. It is possible to forget that they are founded beneath sea level, except, as Neal says: "When you see a boat sail past above your head - that's weird."'

Three tower cranes and four crawler cranes are servicing the creation of the concrete structures. The massive fish pass and five controllable sluices are growing, plant-like, from a mix of steel and timber formwork. The lock gates are not yet constructed, but each leaf will weigh 44 tonnes.

Neal points towards a row of dewatering wells sunk into the sand around the periphery of the site. Not surprisingly, no effort is spared in this work. The computer-monitored outflow from the wells, currently around 110 litres per second, is pumped up and over the cofferdam. Markers in the sand walls are surveyed daily for trend analyses.

emergency

Neal adds: "Emergency electrical back up is on hand to keep the pumps working. If they didn't, and water table rose beyond certain limits, the integrity of the dam would be doubt. We have a full-time geotechnical engineer worrying about this."

But should the worst happen, and the cofferdam become unstable, an evacuation procedure has been well rehearsed. "We can get everyone off site within five minutes," says Neal. Much of the wider project's design and programming has a similar, bomb-proof, feel to it. It has also been heavily shaped by environmental considerations.

The most important of these is a restriction on the movement of construction plant through Penarth. It has meant that virtually all materials have to come in by sea, or via the docks on the other side of the estuary. BB-C tackled this by constructing the first few hundred metres of barrage out from the dock side of the bay, at the same time as constructing the cofferdam. A temporary steel bridge was then constructed off the barrage to the dam.

So far, BB-C has poured 65,000 tonnes of concrete - the remaining 40,000 tonnes, and associated works to the structures, are scheduled to be complete by the end of this year.

This activity is on the critical path of the project. The dam will remain in place until the structures are complete. It is set to be removed in early 1998.

"The sand will be re-used, much of it pumped across to form the riverside flank on the dock side of the barrage," says Neal. "Once we remove the sand the vib wall will disintegrate - so there are no particular removal costs associated with that."

Of the dramatic and extraordinary cofferdam, there will be no trace.n