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Kingsnorth remediation: Mixing session

Cleaning up a development site with an unusual wartime legacy has involved some heavy duty specialist equipment, as Margo Cole reports.

During World War II, many areas of north Kent were given over to military facilities built to defend the UK against a German invasion. Among those installations was an airfield at Kingsnorth on the edge of the Medway estuary.

The area has been settled since the Iron Age, and has a long history of industrial activity, culminating in the power stations for which it is perhaps now best known. But it is that 20th century wartime legacy that is of specific interest to developer Goodman in the final stages of a project to open up a 50ha portion of the site for commercial development.

Clean up challenge

The ground on the site has avoided contamination by materials typically related to a long industrial heritage, but it has its own particular clean-up challenge: 50,000 barrels of tar buried a few metres down.

“The common story is that they were used for lighting up roadways and runways during the war,” explains Rab McLeod, project manager for contractor Volker Fitzpatrick, which has a £16.5M contract to improve access to the site and build infrastructure for the new commercial development.

“There used to be a whole network of rail lines coming into this area, and it looks like the barrels were brought in by rail and dropped into a big hole,”
he adds.

Goodman has owned the land since 2007, and has carried out a number of site investigations since then, before deciding to go ahead with the development in 2010. The investigations identified the area where the barrels would be found – Plot 2A of the development plan - enabling the client and its design and environmental consultants to come up with a strategy for dealing with this unusual contaminant.

“It looks like the barrels were brought in by rail and dropped into a big hole”

Rab McLeod, Volker Fitzpatrick

Capping layer

“In the early days, our consideration was to leave the tar barrels in place and put a capping layer in to stop water running off,” explains Wellstone Project Management director David Rothwell, who is acting as the client’s representative on the project. “We decided that would leave too much of an unknown for anyone buying or leasing that building, so we took the decision to take the material out, treat it and leave it on site.”

McLeod adds: “We took samples and looked at various ways to stabilise or remediate the soil until we came up with a mix design.”

The solution was to treat the contaminated ground insitu, with a combination of pulverised fuel ash (PFA) and cement, and then move it all into a landscaping bund on the northern edge of the site.

Initially the bund was designed to contain 23,000m3 of the treated material, but as work progressed it became clear that a redesign would be needed to accommodate a higher volume.

“Fairly soon after we started on site, we had an inkling there was going to be more than we originally thought, so we started remodelling the bund to take more material,” explains Rothwell. The finished bund is likely to contain up to 35,000m3 of the treated material.

“The fact that the barrels were there wasn’t a surprise, but the extent of it was,” says McLeod.

“In plan area it extended beyond what we thought, but the biggest surprise was the density,” he says. It was thought that there would only be couple of barrels per cubic metre, but in fact the ground was packed with them.

The firm doing the digging is demolition and remediation specialist DSM, which has come up with a range of innovative methods and technologies to cope with the changing circumstances on the site.

The first stage in the remediation is an overburden shift to remove all the material above the barrels. “The overburden consists of crushed concrete, hogging material and PFA,” explains DSM project manager Colin Burford, who says that much of the PFA – a by product of years of power generation on the site – can be reused for the stabilsation.

Stripped overburden

Once the overburden has been stripped to reveal a thin layer of clay/brick capping over the barrels, trial holes are dug every 20m to determine the barrel depth and to work out how much PFA must be added to soak up the tar, at a ratio of around 700mm thickness of loose PFA or 600mm of insitu PFA to every 1m depth of barrels.

“Every single hole I dug had barrels in,” says Burford. “It was just a carpet of barrels.”

Adding the PFA makes the ground suitable for a machine to run on it, so the next task is to use an excavator to dig right down to the underside of the barrels in 5m wide strips. The barrels are removed and placed on the side of the excavation to allow the tar to harden, while the remaining materials (mainly soil and PFA) are placed back into the excavation.

The tar is then removed from the barrels by shredding them with an Allu bucket on the end of an excavator arm if they are dry, or using excavators with grabs if they are wet. This material also goes back into the excavation.

The remediation plan includes incorporating the barrels themselves into the mix, and DSM assumed it would be fairly easy to break them up into small pieces using an excavator bucket. While some are timber, the majority are steel, which has not corroded as much as might be anticipated, making them very tough to break up. As a result, the contractor has brought in a Doppstard 2560 single shaft slow speed shredder - a heavy duty shredder usually seen at waste transfer facilities.

“The machine was chosen as it is tough enough to eat the odd bit of concrete, wood and tyres whilst shredding the barrels to sub 300mm,” explains Burford.

The excavated barrels are mixed with PFA at a 9:1 ratio and fed into the shredder. Oversize shards are recovered by the machine’s magnet belt, while the chopped up barrels and PFA get reintroduced into the pre-dug strips.

The final stage of remediation is to add cement to the mix to stabilise the tar within the PFA/soil matrix. DSM opted for an Allu PMX 5m mixing arm mounted on a Doosan DX340 excavator for this process, choosing an extra high torque variation of the PMX to cope with the presence of barrel shards and the PFA shear strength.

“Initially the soil wasn’t soft enough for the mixer to penetrate, because the PFA has too much shear strength,” explains Burford.

The kit

DSM is using an Allu PMX 500 to introduce the cement into the tar/soil/PFA mix. This excavator-mounted hydraulic mixing tool penetrates the ground, and has two drums at the end of the arm that rotate simultaneously in three ways. Cement is fed by compressed air into the ground through a tube, emerging near the drums. On the Kingsnorth project the 2.4t PMX 500 mixer is mounted on a 44t Doosan DX340 tracked excavator.

“The wetter the material the better it worked, so we had to incorporate a new step in the process and wet up the pre-dug strip with water to make it into a slurry consistency.”

Once this is done, cement can be added at a rate of between 2% and 2.5% by volume.

“The wetter the material, the better it worked, so we had to incorporate a new step to wet up the pre-dug strip with water”

Colin Burford, DSM

After mixing, the material is left in place for 24 hours and transported to the north end of the site to be placed in the screening bund.

This will eventually be covered with a 600mm layer of topsoil and planting.

“The bund has been designed to be pretty much waterproof,” explains Burford. “It is effectively a bituminous material bound up with PFA and cement to stabilise it, so the water should just run off.”

DSM expects to finish the remediation at the end of this month, and the entire job should be completed in mid-March.

At that stage, all site access roads will be built, and level platforms created for the individual development plots.

Project team

Client Goodman
Design/build contractor Volker Fitzpatrick
Contract value £16.5M
Contractor’s designer THDA
Environmental consultant ERM
Remediation contractor DSM
Remediation contract value £1.5M
Contract period May 2010-March 2012

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