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No more dig-and-dump

19 Aug 13 A novel method of recycling excavated soil for re-use as backfill is catching on…but not necessarily in the way its inventor had hoped. David Taylor reports.

There’s hardly a single aspect of modern life where we aren’t exhorted to save energy, reduce carbon emissions and, basically, save the planet. Even the mundane job of digging up the roads is environmentally unsustainable according to the prevailing orthodoxy.

In fact, digging up the roads is indeed a particularly wasteful and inefficient operation under current highways specifications: most of the time the soil removed from trenches dug to lay or repair pipes, cables and other utility services has to be got rid of somehow, the irony being that once dug out it is no longer suitable for putting back in the hole after the work is finished.

Current streetworks regulations require that holes and trenches dug for utility maintenance and installation works are back-filled with Type 1 aggregate, which is granular and compacts firmly. Back-filling with cohesive clays, silts and other materials is not allowed because this cannot be properly compacted. The result is settlement and the appearance of potholes – the bane of motorists and subject of endless column-inches in the local newspaper.

The usual practice, therefore, is to dig the trench and remove the excavated soil for disposal – usually in a landfill site somewhere – and then import graded Type 1 aggregate, freshly quarried or recycled from crushed concrete, to backfill the trench.

Clearly this is inefficient. It is also expensive. Quite apart from the cost of transporting soil to landfill, buying new aggregate and trucking it to site, there is also the rising cost of the landfill tax and, in the case of quarried aggregate, the Aggregates Levy on top.

From a purely environmental point of view, the diesel burned while transporting all this material enlarges the carbon footprint, to say nothing of the fuel consumed quarrying, crushing and grading the aggregate. Quarrying itself despoils the land as does the expansion of landfill sites.

The solution is pretty obvious: find some way of treating the excavated material as close to site as possible and throw it back into the hole again when the job is finished. The trick is finding a way to convert a poor quality cohesive soil into a granular material that meets the rigorous specification for Type 1 aggregate.

About 13 years ago a north London demolition contractor rose to the challenge and found a solution. Keanes, a family-owned business based in Kilburn, already crushed and processed demolition waste for re-use as aggregate, but it also provided muckaway services to the capital’s utility contractors and knew just how much soil went to landfill every year.

Managing director David Keane realised that milling the material and mixing in certain additives could transform a clay-based soil into the desired granular fill. After a couple of years of research, which included bringing in soil specialists and chemists to refine the process, Keanes launched its new product, which it dubbed Trenchmod.

And it wasn’t just the product that emerged but also a purpose-designed machine, the Keanes mixer. The thoroughness of the R&D process was remarkable for such a small company, especially as it had no background in materials science or machinery design. Both Trenchmod and the Keanes mixer were duly registered and worldwide patents applied for and granted.

“(Engineering consultant) Scott Wilson conducted a two-year independent study of Trenchmod with tests carried out at regular intervals to test the durability of the product,” comments Keane.

Developing and producing Trenchmod was a costly and time-consuming venture; but a far greater challenge was the task of convincing clients, engineers, contractors and the Highways Agency that Trenchmod really could do the same job as expensive quarried aggregate. The civil engineering industry is notoriously conservative and radical new ideas are invariably met with scepticism.

“We approached a number of major civil engineering contractors and materials suppliers who initially showed a lot of interest,” says Keane. However, potential partnerships failed to materialise. Keanes continued to develop the product independently, eventually winning technical approvals from several local authority highways inspectors in London and the south east.

The big breakthrough came when Trenchmod was approved as an alternative reinstatement method by nearly all of the London boroughs, meaning that the material could be used for streetworks reinstatement throughout the capital. Shortly afterwards, Trenchmod was approved by Thames Water Utilities for use on its district mains replacement (DMR) project, which entails the replacement of miles of life-expired water main – including customer connections – throughout the Thames Water region.

Starting in 2008, this project consumed between 600 tonnes and 1,500 tonnes of Trenchmod every week. With trenches typically 2.5m deep and 1m wide, each excavation yielded approximately 2.5 tonnes of soil per linear metre. This was taken to a Keanes’ processing depot at Jenkins Lane, near Beckton sewage treatment works, where the Keanes mixer processed the material and stockpiled the resulting Trenchmod ready for delivery back to site.

Within the first year of the DMR programme, streetworks contractor J Browne Construction processed more than 60,000 tonnes of soil at an estimated cost saving of £20 per tonne – a total cost saving of roughly £1.2 million.

As well as cutting costs, Trenchmod diverted 60,000 tonnes of waste away from landfill and saved 60,000 tonnes of virgin quarried stone in the first year of the project. Carbon emission savings from the reduced vehicle movements were calculated to be in the region of 760 tonnes. “But actual CO2 savings were even greater due to the elimination of quarrying and landfill and the emissions from vehicles associated with these activities” says Keane.

Having proved itself as a low-cost, sustainable method of trench reinstatement, Trenchmod was soon extended to other projects where Type 1 aggregate was required, such as converting excavated soil into sub-base for a car-park project.

The product was then adopted by the South East Electricity Substation Alliance (SEESA), a joint venture of Skanska, Areva and Mott MacDonald formed to deliver a major investment programme to update and develop the high-voltage transmission network for client National Grid.

Over the winter of 2009/10 the SEESA joint venture converted more than 5,000 tonnes of clay soil into Trenchmod as sub-base for a 400m-long access road into the new Kemsley substation at Sittingbourne in Kent. Keanes project manager Andy Goodson estimates that Trenchmod saved Skanska almost £9 per tonne on this project.

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“Muckaway locally was around £10 per tonne and imported Type 1 granite cost £16 per tonne so the total combined would have been £26 per tonne” he says. “Trenchmod cost only £17.25 per tonne. And of course there were the green issues such as no landfill, no quarrying, no haulage and a reduced carbon footprint – approximately 300kg of CO2 saving per tonne of Trenchmod, according to WRAP”.

Despite the early success of Trenchmod and its proven capabilities, David Keane admits to frustration that the method has not become more widely used. Local authority highways departments in Essex, Hertfordshire, Kent and throughout Greater London have given Trenchmod their blessing. Despite this, many contractors still quote for the traditional “digand- dump” method when they could be saving their clients money and reducing their carbon emissions.

More frustrating still is the fact that some contractors do see the benefits of the system but also see an additional advantage in cutting out the middle man and developing their own product. David Keane believes his company pioneered the concept of off-site recycling of trench arisings, but he has since seen several contractors – including a good many of those who showed an early interest in Trenchmod –produce something similar themselves.

“In the very early days we went round every major contractor doing presentations and demos and a lot of them were very enthusiastic. But when it came to making a commitment, they seemed to go off the boil,” he says.

Keane estimates there are over a dozen copycat systems on the market. Some, he readily admits, might have been developed from scratch and owe nothing to the Trenchmod idea – after all, the principle of

modifying soils with the addition of cement or lime is well known and the in-situ stabilisation of subsoils is a familiar process in civil engineering.

But what concerns David Keane is the apparent disregard being shown for his company’s intellectual property. “We have patents – granted, not pending – relating to both the Keanes Mixer and Trenchmod itself. But we have good reason to believe that several companies, ranging from small to very large, are infringing those patents, quite possibly inadvertently,” he says.

Nevertheless a patent in itself offers no protection until tested in court and litigation is a topic Keane is reluctant even to acknowledge at this time.

Several years ago Keane rejected a bid by a major materials supplier to acquire a controlling stake in Trenchmod. “Given what’s happened since I sometimes wonder if I made the right decision,” he declares. But Trenchmod was his idea and his project and he is determined to see the technology win the recognition it deserves.

“We’re still here and we know Trenchmod is the best quality, most consistent and most sustainable solution to streetworks reinstatement,” he says. Meanwhile, the company is now investigating the potential of establishing a permanent Trenchmod depot near its head office in north London.

Filling a hole

WRAP, the Waste & Resource Action Programme, estimates that trench arisings from gas and water asset renewals are in the order of 4.8 million tonnes per year.

Recycling and re-using this material not only reduces the amount of waste sent to landfill but also reduces the demand for primary aggregates.

WRAP has identified 12 key benefits from the recycling and re-use of trench arisings:

Environmental benefits

  • Conservation of finite resources
  • Reduction in material going to landfill
  • Potential for reduced energy consumption
  • Potential for reduced vehicle emissions

Social benefits

  • Increased local employment
  • Business development opportunities
  • Minimal disruption from fewer haulage movements

Economic benefits

  • Recycled aggregates cost less
  • Reduced waste disposal costs
  • Recycling as a new revenue system
  • Reduced haulage costs
  • Value of social and community gains

Recycled material for trench reinstatement can be broadly divided into two categories: unbound materials and hydraulically bound materials. Unbound materials include selected trench arisings suitable for re-use; process or treated trench arisings made suitable for re-use; and aggregate derived from processing inert material previously used in construction.

“Hydraulically bound materials” covers a wide range of products including stabilized and cement-bound materials such as are used in-situ for soil improvement.

Stabilised materials are improved soils treated with the addition of hydraulic binders such as lime. The effect is to dry out the material through absorption and evaporation, modify the structure through a reaction between the lime and clay minerals, and stabilise the material through ongoing long-term hydraulic reaction.

In addition, foamed concrete, either alone or in combination with recycled aggregate, is also used as trench fill. This can be used solely to provide a stable surround for pipework or underground apparatus or as the entire layer in varying combination with other backfill materials.

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