Project director Marcus Foweather describes The Avenue as a patchwork of a project.
"It's the most intricate job I've been involved with," he says.
Intricate is an understatement. The site covers 98 hectares and contains some of the most contaminated ground in Europe. Yet every single cubic metre of earth is being tracked and can be accounted for, in some parts of the site, to a depth of 10m.
The site of The Avenue, near Chesterfield, used to be of great economic significance to the area. It housed, among other things, a power station and coking works, as well as a waste tip licensed to hold asbestos.
Now it is hoped that the area can flourish again, eventually accommodating a mixed-use development of housing, business units and recreational facilities nestling in an undulating green landscape.
But before the people of Chesterfield can enjoy the football pitches and wildlife areas envisaged in the master plan, work needs to be done. As well as an extensive cleanup involving bioremediation, thermal desorption, and painstaking hand sorting and separation of materials, several civil engineering projects are under way, including the diversion of the river Rother, the construction of a culvert and dam, a major strategic gas and electricity diversion, and the modification and rerouting of the sewer network, as well as the demolition of a bridge and the rebuilding of another.
A view from the air indicates the scale of work underway; huge lagoons are being excavated, and trucks rumble along 5km of haul road, shifting and depositing the muck in neat piles for testing, treatment, or reuse elsewhere on site.
"It all looks very regimental," says Foweather with some satisfaction. This is a remarkable achievement considering that the programme is changing on a daily or even hourly basis depending on what is discovered in each new bucketful of muck.
The Avenue project has had a long gestation. Joint venture consortium VSD Avenue tendered the job in 2005, but with rising fuel and gas prices, the cost of remediating the soil was escalating and it was necessary to demonstrate to the client (at the time, the East Midlands Development Agency) as well as central government, that the energy-hungry cleanup was worth the money.
Reviews followed in which other techniques such as cement stabilisation and in situ bioremediation were considered. Eventually, after extensive on site trials and laboratory testing, it was decided that the original strategy of ex situ bioremediation, thermal desorption and water treatment was still the best.
It may not be the cheapest option, but it provides the most peace of mind for the client. Out of the 2.1 million cubic metres of muck that will moved in the prime muck shift, the bulk will be safe enough to be reused on site. Only materials such as rags, plastic and metals, separated out from the waste tip, will be taken away.
Although the thermal desorption plant consumes roughly £5,000 of gas every 24 hours, Foweather stresses that this is a highly sustainable strategy. "We commissioned a carbon footprint study. We discovered that, over a 25 year period, it is actually more sustainable to treat the material on site and to use the gas over a shorter period than if we left the material where it was and is covered it over.”
The consortium began preparatory work in 2009 with the construction of the haul roads and concrete slabs. It was also necessary to construct the bespoke £6.5m thermal desorption unit (TDU), one of the largest of its type, designed to process an estimated 300,000m3 (half a million tonnes) of grossly contaminated material.
"The components came from Holland and getting them to site in the order that they were needed was a complex logistical challenge. The actual kiln was 23m long and weighed 65t. It was the largest size that we could get onto UK roads," says Foweather.
Only in autumn 2010, when the TDU started operating, could remediation begin in earnest. The bulk of the work is focusing on the removal of sediment from four lagoons, and the excavation of a 300,000m3 waste tip.
The cleanup of the four lagoons is one of the major drivers for the clean-up: three are unlined and leaking contaminants such as benzene, phenols and naphthalene into the river Rother.
At present a 300m coffer dam, with sheet piles sunk 6m below the river level, separates the lagoons from the river. Eventually, the river will be diverted onto the other side of the coffer dam as part of flood alleviation scheme to protect Chesterfield.
"The removal of the sediment from the lagoons is on our critical path," says Foweather. "We cannot divert the river onto the other side of the coffer dam until lagoons one, two and three are out of the way."
Much of the sediment is so grossly contaminated with tars and heavy metals that it would damage the offsite laboratory facilities, run by AlControl in Flintshire, Wales, were it to be sent for testing. Analysts working on the site can tell by sight and smell alone that it needs to be sent to the TDU, which will heat the soil in temperatures of up to 600°C.
The TDU is operating 24 hours a day, processing some 20t of material an hour, which consists of roughly 9t of sediment mixed in with granular material (such as crushed concrete). The amount of material and moisture that needs to be mixed with the sediment needs to be constantly adjusted and monitored: if the calorific content is too high it would damage the plant.
This challenge is further complicated by the fact that it is virtually impossible to predict in advance what exactly is being dug up: “Even in the lagoons, the calorific values and moisture content of the sediment varies on a day to day basis within the same pond. It’s totally variable,” says Foweather.
As a result, he says, all organisations within the project team must be integrated, from Sita, the Dutch company that is running the TDU, to VolkerStevin, predominantly involved with the civil engineering work, and Belgian company Deme Environmental Contractors (DEC) which is overseeing much of the remediation. All must coordinate their activities with on-site analysts and technicians from Environmental Services Group Ltd (ESGL).
The rate at which the TDU can process the sediment is setting the pace for the entire project.
"It has taken longer than we were expecting to treat the sediment, it is a more difficult material than we anticipated," says Foweather. This has put back the remediation programme from an initial completion target of April 2014 to early or mid 2015.
A veteran of muck shifting, Foweather is used to working on projects where speed is a priority. The Avenue is different: "We don't need to dig any faster than we are currently digging because our entire operation is throttled by the amount of sediment that can go through the thermal desorption plant," he says.
Nevertheless, the site looks busy with 17 backacters, including a 22m longreach Komatsu PC350 for excavating lagoon sediments, a fleet of ten 25t Volvo dump trucks and three loading shovels, as well as a pair of Cat D5 and D6 bulldozers. At peak the number of workers on site reaches 120.
Much of the equipment is kept busy moving the soil into different piles, according to the level of contamination and the cleaning required. Less contaminated material is sent for bioremediation, where it is aerated using an excavator with an Allu screening bucket. This blends and mixes the soil, increasing the oxygen content and reducing the temperature. The soil rests in piles for 12 weeks before it is clean enough for reuse.
Foweather estimates that, with all the repositioning and double and treble handling of the soil, roughly 4 million cubic metres of muck will be shifted over the course of the whole project.
“We have to work with absolute precision here, it's not something you usually need to think about in terms of earthmoving. But this project has to be very coordinated, very efficient and you really have to trust your team," says Foweather.
Client: The Homes & Communities Agency
Total project value: £172m
Contract for ground remediation and landscaping: £86m
Contract type: NEC2 option C
Project completion date: 2015
Main contractor: VSD Avenue, a consortium of VolkerStevin, DEME Environmental Contractors, Sita Remediation
Consultant: Amec
Air quality consultant: Bureau Veritas
Plant hire: Hather Plant Hire, Yorkshire
Airlines, boots and sniffers
The dangers of working on a site as grossly contaminated as The Avenue were all too apparent from the first shift on site. The alarm was raised after standard-issue steel-capped Wellington boots were found to be under attack from the toxic ground. Special chemically resistant boots were immediately ordered.
Any worker in close proximity to the excavation of the lagoons must also wear respiratory protective equipment (RPE). Special measures have also been taken for the plant operators: air filtration boxes have been retrofitted to the top of several excavators and dump trucks, while photo ionization detectors constantly monitor the atmosphere inside the cabs.
During the construction of the coffer dam, workers lowered into the excavation by crane were also kitted out with protective masks and airlines attached to a compressor 300m away. Large fans were installed along the length of the excavation to create air flow.
The old waste tip, which was licensed to take asbestos, contains further hazards. "We excavate very carefully at the face of the waste tip and our workers are fitted out with all the PPE and RPE. The operators are trained in identifying various types of asbestos. They are being watched all the time and if asbestos is identified we contact a licensed specialist contractor to remove it for us," says project director Marcus Foweather.
The 12-man team in the sorting station that separates the plastic, timber, asbestos and other elements from the waste tip must also work in full protective gear attached to airlines.
“All employees who work on the dirty side of the site undergo significant health screenings, at least quarterly," says Foweather. “Every activity is identified and risk assessed separately by health professionals, who advise us on how frequently we should be screening the workers. This is something that we are doing ourselves as contractors. Health and safety is paramount here.”
Odour suppressants and water sprinklers have been liberally installed around the site to prevent unpleasant smells reaching local villages. As part of efforts to maintain neighbourly relations, the consortium has trained some local residents and workers to be ‘sniffers’, who report to the site team, describing and grading any unpleasant smells that they detect in their area.
"A sniffer will contact us immediately if they smell something particularly bad. We then send out one of the team to investigate," says Foweather." Historically we have never had to stop operations because of the smell, because of the number of mitigation measures that we have put in."
This has all helped community relations. VSD Avenue was awarded a Considerate Constructor’s Silver Award in 2011 and scooped a Gold award this year.
Keeping track of the programme
To demonstrate that the cleanup process really will make the ground safe for future generations, every single cubic metre of muck that is being shifted on The Avenue is recorded and tracked. The entire site has been broken up into grid squares, and eventually a 3D model will be available for the client, developers and interested parties to scrutinise in the future.
"We have to produce a validation report for every load of muck. We have to know where it came from, what we did with it, where we sent it for treatment and where it was finally placed," says Foweather.
This complex task is being overseen by two specialists, Steve Dobson and Mick McNiffe, earthmovers with whom Foweather has worked for years and recruited to the team.
“This is very intricate, but I knew that they were competent and could do this kind of work,” says Foweather. “We call them sentencing managers, because they literally sentence the soil to its fate.”
The project has been intricately programmed, with 6,000 separate lines of work. However, because the ground contains so many surprises, the programme is constantly changing, and full-time planner Neil Kendrew keeps track of operations.
Because it is impossible to ascertain the toxicity levels, and subsequent treatment needs of each patch of ground or bucket full of sediment, the team will often have to stop work and move to different areas while waiting for test results, which take five or six days to come back.
On other areas of the site, structures below the ground are being uncovered; some have been anticipated, others are a surprise. All this calls for further programme revisions.
“We found a prototype coking works in ground that we thought was going to be virgin dig,” says Foweather. “That’s typical of this job. We can’t go to work each day with the full knowledge of what we will be doing or where we’ll be working, because what we find in the ground is often different to what we were anticipating.”
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