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High tech machine helps build flood defences

Environment Agency midlands director Mark Sitton-Kent visited Attenborough Site of Special Scientific Interest (SSSI) yesterday to inspect progress on the £51M Nottingham Left Bank Flood Alleviation Scheme.

He also observed in operation the Trenchmix machine, which has just arrived on site from projects in France and Poland.

Trenchmix looks like a gigantic chainsaw. It is normally used for the installation of land drains but, at Attenborough SSSI, it offers an innovative solution to stop water seeping beneath flood defences. This is believed to be probably the first time it has been used for flood defence works in the UK.

“Trenchmix offers significant benefits. It is faster than conventional sheet piling machines, and has lower noise and vibration levels.”

Innes Thomson

It is important to prevent seepage at Attenborough because the ground is highly permeable. This means that, unless measures are taken to prevent it, the weight of floodwater could drive it underneath the flood wall towards sensitive areas behind the defences.

A ‘seepage cut-off’, an impermeable barrier below ground level, must be created to prevent this from happening. The chosen method had to:

  • Minimise the amount of land used
  • Reduce disturbance to both people and wildlife
  • Be able to work alongside high voltage electric cables and a major railway line to the satisfaction of Central Networks and Network Rail

Initially, because of the ground conditions, the only possible solution seemed to be conventional hammer driven sheet piles.

Trenchmix offered a new approach. It breaks up and mixes the soil with a cement grout without the need to excavate and remove it.

The resulting mix of soil and grout solidifies to form a continuous underground barrier, preventing high flood levels driving water through the ground beneath the flood wall, and protecting the areas behind the defences.

Reduced disturbance

Area flood risk manager Innes Thomson said: “Trenchmix offers significant benefits. It is faster than conventional sheet piling machines, and its lower noise and vibration levels will reduce disturbance to wildlife, visitors and local residents.

“We will also need to use less land to build the defence, and any earth excavated can be recycled on site for landscaping work. This will result in less traffic on local roads.”

The Nottingham Left Bank Flood Alleviation Scheme will reduce flood risk for 16,000 homes and businesses.

Readers' comments (2)

  • It would be interesting to know how economical it is to mobilise this impressive bit of kit. I imagine there is a minimum length of seepage cut off required to make it viable to use this method, or was the decision to use it simply based on the constraints listed? Another thought - were there any sensitive areas around trees? Did they require a different method? All the same an impressive method and good to see some innovation.

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  • Whenever a new cut off of whatever type in what is highly permeable ground is installed there can be a rise in long term groundwater levels (with potential waterlogging) in the protected area. On this site was there a need for a ground water drain behind the cut off line?

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