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Faggot Power

Faggot Power

A centuries' old technique for river bank erosion control is being used at Dartford Creek on the Thames Estuary reports Adrian Greeman.

Since Roman times tied bundles of brushwood or "fasces" have been know for their structural properties, finding their way into fortifications, dams and earth walls. They were used as a symbol of imperial power, later usurped by Mussolini for his fascist movement.

Such bundles, known in English as the faggot, have been used for centuries in this country for fences, riverbank reinforcement and ditch lining, as well as for burning witches and heretics. But they have not found a modern mainstream use. That may be changing, however, with an erosion control project under way on the Thames Estuary.

The Dartford Creek, meandering across salt marshes on the southside of the estuary, has been a problem for some decades. Through constrained within modest flood control embankments, it has had a tendency to eat away its outer bends, threatening inundations and land loss.

"Assorted sheet pile and gabion-type solutions have been used over the years to slow the erosion, but most of these were failing." says Borbala Trifunovics, project engineer for Arup which is the consultant for the £3.2M Environment Agency Project. Works affect two main bends on the channel, each about 200m long.

Various options were considered in an initial feasibility study by consultant KBR, including variations on using more hard sheet pile walling, piles tied back to the banks, gabions and doing nothing at all. But the Environment Agency, aware that the area has imported flood plain habitat, was keen to consider ecologically sound solutions, too, and eventually the brushwood method was examined.

Experience in Holland particularly was drawn on, where the technique is relatively commonplace for reinforcing dykes and drainage embankments, and a healthy support industry exists producing the timber bundles.

The method involves tapping into natural erosion and silting mechanisms, creating a mesh of timber rods along the embankments through which the daily tidal and river flows can pass. Slowed down, the water deposits silt around and through the bundles, building up the embankment rather than cutting away more of it.

"The aim is to push the water course line back towards the other side," says Trifunovics.

There are some unknowns, she says. "Durability is an issue, especially where the timber remains above the silt level." But visits to Holland have been very positive in what they show.

The technique has a major advantage for this project in containing the impact of the construction the impact of the construction works, adds Andy Martin, project manager for coastal works specialist JT Mackley, part of the Team Van Oord and May Gurney.

"We can work from a jack-up barge for laying the bundles without having to move too much land-based machinery around," he says.

Tight restrictions apply to the SSSI saltmarsh face of the embankments, which are also deemed at risk of settlement, so that little machine movement can be made on them. The contractor has bought a new barge for the job, anticipating that it will prove useful on other works, too.

There was some heavy work to do. The works are a combination of both hard and soft engineering, says Trifunovics. "There is some sheet steel piling at the back of the embankments to cut through potential slip circles where they had been failing, with the faggots at the front," she says. A final element is stone rolls, sausage-like cylindrical gabions, 300mm in diameter and 2m long, which go at the bottom of the new brushwood facings in the deepest part of the channel, to anchor the whole.

First work was removing the old piles and installing some new conventional sheet piling. For this, the back of the embankment was built up slightly to create a level platform so a piling rig could operate without being on the embankment itself. Big A226 piles were pushed through some 11m of soft alluvium and peat to gravels. They toe a few metres into the harder ground and are cut off at ground level.

On the other side follows the work with the timber bundles. These are built up in a layer up to a maximum 5m thick extending from the edge of the protected salt marsh, which is present in places even on the west side, and down to the water's edge.

During low tides the bundles are laid loose, through in a regular pattern, Martin says. They are contained by 40mm-80mm thick timber posts, driven into the soft mud far enough to remain upright. Some 1,700 stakes are needed.

"Part of the job was finding a safe way to operate," he says. "We are working over water and at height, especially on the jack-up at low tide,"

For the stakes this initially meant positioning them by hand, using someone in a man rider to hold them upright.

"But we devised a special steel cap attached for the end of the excavator arm which allows him to push them in directly." Now the poles are simply stood in the mud working from the man rider and then the excavator, a 35t long-reach Hitachi machine, pushes them down firmly. It takes some skill by the driver, who sits above on the platform, says Martin.

The faggots are brought in from Holland, from Waterline Solutions. "They have to be a certain size, which requires three-year coppicing of ash and chestnut, which we don't have to a large extent in the UK" he explains. The bundles, which are around 2.5m long, are delivered from Holland via Felixstowe and to a local yard owned by Costain. A small 100t barge then brings them to site during high tide, from which they are lifted over by the excavator for hand placing. Care has to be taken with this because of the potentially slippery surfaces.

Once built up, a wire mesh is rolled out to retain the faggot layer and this is anchored down at the front edge because the river Darent, creates a small fluvial flow along the creek even at low tide.

Work is done behind and around the platform which is then jacked down and moved onwards a length, filling in the space it has left behind and the higher part of the embankment again.

"An aditional danger is the holes left by the spud legs, so we put a steel mesh across those first of all" says Martin.

Work began in August last year, and the first section of the work is already complete with the mud building up nicely in the bundles, says Martin. The remainder is due for completion later this month.


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