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Pick 'N' Mix

A project in Nottingham is using Trenchmix technology to provide flood defence with minimum disruption to the surrounding area. Natalie Hardwick reports.

Engineers are often required to deal with sensitive site conditions when working on projects to protect vulnerable areas from flooding.

Nottingham is an area prone to heavy rain, and the River Trent regularly bursts its banks at various points.

These areas are close to residential areas, and the river is also close to a railway line and several nature reserves.

To prevent future flood damage, the Environment Agency has awarded the £51M contract for the Nottingham Left Bank Flood Alleviation Scheme to Jackson Civil Engineering.

It was extremely important to prevent flood water from swamping nearby nature reserves that host various species of water fowl, flora and fauna.

Due to the highly permeable ground in the area, floodwalls alone were insufficient protection - heavy rain could have driven flood water below them towards sensitive areas behind the defences.

This meant it was necessary to build some form of underground flood barrier by drilling into the ground, but it was essential to choose a method that would have minimum impact on the surrounding area.

Jackson proposed using a technique called Trenchmix to create an underground cut off wall by mixing soil with a binding agent.

Trenchmix has already been used successfully in France and Switzerland.

Along with contractors Bachy Soletanche and Mastenbroek, Jackson developed the patented process for use in Nottingham.

“Sheet piling is quick and effective, but it is noisy. There would have been great disruption to the residents who lived in villages very close to the site.”

Phil Myles of Bachy says the initial decision to use Trenchmix meant dismissing the option of using sheet piles to create the cut-off wall.

“When it came to the Nottingham project, Bachy and our fellow contractors were faced with the dilemma of whether or not to use conventional underground sheet piles,” he says.

Sheet piling would have driven a continuous wall deep underground to create a flood barrier.

It is a process that is commonly used on sites that require a retaining wall, but it is only suitable in certain conditions.

“We found there were several problems with using sheet piling,” says Myles. “Firstly, the railway is too close to the area where the piles would be installed. The vibrations and large size of the plant required for this process make it unsafe so close to a track.

“Not only can the vibrations damage the track from afar, but Network Rail has strict rules that stipulate that there should be a certain radius between plant and the track in case equipment overturns. It insists there should be a small radius into which the equipment can fall so there will be no tumbles onto the track,” he continues.

“Also, sheet piling is quick and effective, but it is noisy. There would have been great disruption to the residents who lived in villages very close to the site. Jacksons approached us regarding using Trenchmix and we decided to develop it together to build the underground cut-off wall.”

Work began on site in Nottingham in July.

The Trenchmix technique breaks up and mixes the soil underground using a patented trencher made by Mastenbroek.

Trenchmix4

The machine has a circulating chain with breaker attachments fixed to the front and this is inserted vertically into the ground.

It breaks up the ground and then its action is reversed to facilitate the mixing of the binding agent below ground.

This means there is little or no activity above ground.
The machine moves slowly forward while the chain works underground at high speed to mix the slurry.

The trencher has a long boom, but is highly manoeuvrable and is able to work in within a narrow corridor, an advantage on the cramped Nottingham site.

Trenchmix works on this site as the only raw material required for the process is cementitious grout.

The ground at Nottingham is highly cementitious.

The grout is mixed with water extracted from a nearby lake, adding an extra environmentally-friendly layer to the process and reducing transportation costs.

This grout is injected into the soil at a controlled rate.

Since it is relatively stiff, it does not flow away once underground but is mixed with the soil by the circulating chain’s specially designed mixing teeth.

Mixing quickly produces a stiff material that is kept in shape by the undisturbed adjacent ground, setting within hours.

“The crucial factor of Trenchmix is that it mixes rather than digs soil.

Trenchmix5

The equipment mixes the slurry by pumping it out from the ground at the same time as the machine mixes,” explains Myles.

Another benefit of the process is that Trenchmix works quietly below ground and does not use steam or explosives, so noise and air pollution are reduced.

The end result has been a 400mm thick barrier which has a compressive strength greater than 0.5Mpa.

The trenching process finished on some areas of the Nottingham project last August.

“We’re still on site at Barton Lane doing some grouting around the surfaces,” says Myles. “There are other sections further up the site which have yet to be finished.”

As well as on flood defence sites, Trenchmix can be used to build foundations or for more general ground improvement works.

Myles says: “I would say that Trenchmix has transformed the area of geotechnics that is concerned with cut-off walls and ground improvement.

It has already been used on dozens of projects in Europe and is set to grow in the UK.

“Sheet piling is still a quicker method, so in general it could be said that it is less disruptive in terms of time scales, but it is expensive and noisy so is not appropriate for residential areas,” he concludes.

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