Articles & Case Studies

Waste not, want not

Posted: Wednesday 16th July 2008

When Site Waste Management Plans (SWMPs) took effect in April it signalled a further crackdown on the construction industry’s poor record on waste management.

Meeting the new requirements of SWMP’s will not have been too much of a shock for the many companies who have been steadily improving their waste management practices under the considerate contractor scheme launched 12 years ago.

The fact that 15,000 sites had already registered to this voluntary scheme provided a good indication of the level of interest in site waste management well before SWMP’s kicked in.

But for the unprepared, SWMPs will mean a very different way of doing business.

SWMPs set out how building materials and waste are to be managed during the life of a project. The main aim is to ensure building materials are managed efficiently and waste is disposed of legally with material recycled, reused or recovered as efficiently as possible.

The regulations apply to construction projects where the tender price or estimated cost is more than £300,000 excluding VAT.

Any firm caught falling short of the new regulations face fines of up to £50,000 and only time will tell just how many companies were really prepared.

But for those already practicing good waste management methods, this is a good time to be demonstrating innovative ways of dealing with waste.

Award-winning developer Urban Splash is one company doing exactly that and its recent development of the former Imperial Tobacco site in Bristol is a prime example of how site waste can be put to something really useful.

Called Lakeshore, the site will provide 400 apartments in a project which is setting high environmental standards including the use of ground source heat pump technology to provide most of the heating and hot water.

The site spans a small 10,500m2 lake located in a steep sided valley. Plans include restoring the lake as a central visual feature on the site but before work could begin Urban Splash had to address the significant quantities of silt which had built up over the past 25 years, forming small islands and reedbeds.

Urban Splash called in environmental practice Aardvark EM to come up with a solution to the problem.

Aardvark’s answer was to design GTubes, a new dual-purpose desilting system built specifically to effectively recover silt and slurry from large volumes of water.

Aardvark EM director Mark Clayton and chief designer of the GTube system, is upbeat about the system’s potential: “GTube is an exciting step forward in providing low cost contained dewatering and treatment systems.

“It’s particularly effective in urban redevelopments where space and time are limited, historic contamination is a problem and now the Landfill Regulations require pre-treatment and dewatering prior to disposal.”

The first stage at Lakeshore was to discover how much sediment needed treatment. Aardvark estimated around 15,000m³ of silt would need removing for the lake to be restored.

A boat-based sampling rig was used to extract core samples and complete a bathymetry survey of the lake bed to find out the silt volume and operation depths.

Once clear about the extent of desilting required, the Aardvark team then set about designing a remediation system to allow the silt to be recovered, treated on site and water from the pumping works to drain back into the lake.

One of the hurdles to be crossed was the limited space at the site. A key feature of the system, therefore, needed to be its flexibility. The second challenge was recent legislative changes.

Mark Clayton says: “The other issue we had to address was meeting the new rules governing the landfilling of non-hazardous waste which came into force on 30 October 2007.

“The rules are part of the final changes under the Landfill Directive 2002 which sets demanding targets to reduce the amount of biodegradable waste and which must be fully implemented by July 2009.”

The Landfill Regulations, which apply to businesses in England and Wales, now contain pre-treatment requirements which mean that no waste is able to be disposed of to landfill unless it can be demonstrated to have undergone a treatment process. The rules also ban liquid waste from any landfill

The solution to meeting both the challenge of space and tougher legislation was a series of geotextile closed tubes, GTubes, which not only dewater but are also designed to provide containment for slurry treatment.

A series of trials were set up to test the stability and effectiveness of the treatment at the Lake Shore site before developing the large scale system.

The GTubes are designed for a 1:0.8 volume to surface area ratio and engineered to withstand pressure at the base of greater than 6.69 tonnes/m³ to allow for stacking.

The fabric chosen is determined by the dewatering application but in the majority of projects, including the Lake Shore site, is a close weave monofilament fabric which holds the sediments in but allows the water to seep out.

The tubes can be made to almost any size, depending on where and how they are to be used. The six tubes on site at Lake Shore measure 7m wide 30m long and are filled to a height of 2m with silt and water slurry mix.

These are the same dimensions as Aardvark’s standard GTubes range which, when full, can hold up to 300m³ of sediment.

The GTubes can be stacked one on top of the other, once the bottom bag has been filled. This, combined with the fact that the only plant used is the pumping equipment and traditional 360º excavators, means the tubes are able to work very effectively on even confined sites.

Silt is moved from the lake bed by excavator and placed onto a specially-built screened chamber which screens out the gross solids before pumping the sediment into the GTubes.

This pumping line can run up to 2000m, pumping objects the size of a cricket ball through the system.

Silt is pumped from the lake via dorsal sleeves in the top of each tube which allow a 450mm diameter pipe to be inserted and then clamped securely for filling to occur.

Working at full capacity the tubes fill at a rate of up to 1,000m³ per hour and can accommodate a variety of slurry consistencies from 10% to 70% solids.

Pumping continues until the GTubes become rigid and the flow is then disconnected, the pipe removed and the sleeve sealed off. It is now that the dewatering process begins as the clear water is drawn through the geotextile fabric and into channels running along the side of each bag and back into the lake.

If the slurry being pumped requires processes other than dewatering, then the GTubes also provide a confined space in which to mix any additional treatments such as microbes (Bioremediation), mixing, agitation or aeration.

The desilting at Lake Shore will be completed later this year and Urban Splash intends using all the recovered sediments for landscaping on the site.

James Howard, associate director at Urban Splash (South West) is clearly delighted with the outcome: “Lake Shore is an exciting development and one of the most environmentally friendly projects of its type and size anywhere in the region.

“The lake is a key part of the design. Using the GTube system has helped speed lake restoration and, because it allows us to re-use the treated silt, cuts down on vehicle movements to and from the site.”

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