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Feature: Digging for answers

Having to deal with anything up to 100,000 tonnes of soil and rubble from street works could pose something of a quandary. But United Utilities is on top of this waste mountain, and has developed a reliable and efficient way of recycling a difficult waste stream.

United now operates the gas network in the north, Yorkshire and north Cumbria — which was done by Transco until June 2005 — on behalf of Northern Gas Networks. Excavated waste in the form of soil and rubble is the single biggest waste product generated by the company — around 100,000 tonnes last year.

To send this material to landfill would not only be expensive but highly wasteful because the trenches from which it has been excavated must be backfilled with a bought-in material, often incorporating virgin aggregate. So, keen to find a way of recycling this material, engineers developed a way of converting it into a new type of cement-bound excavated material (CBEM) for use as high-quality backfill material.

It was while the company was still part of Transco that members of staff began research and development on a process of recycling excavated material, and they have not looked back since. In fact, the successful
development of this product led United Utilities to be highly commended in the Best Industry Recycling Initiative category of 2005’s National Recycling Awards.

“We began the research and development process around 1998 when recycling was really coming to the fore,” says health, safety and environmental manager Mike Queenan. “Working towards ISO 14001 accreditation, we wanted to address the waste management aspect of the company and deal with our biggest waste product, which is excavated waste. Correspondingly, we started to do some small-scale recycling in York with basic screening machinery.”

However, one of the problems that the company came across was the high clay content of the material. Conventional recycling copes with all the granular material but not the clay, which can form up to 30% of what they are dealing with.

“We had to look at a method of stabilising the clay using cement and water mixed in with the excavated material to encapsulate the clay and give it strength so that it could be reused in the road,” says Queenan.

“We used foam concrete as our model and wanted to create a material that was equal to this but more economical.”

What followed was a small trial in Scarborough where, says Queenan, they were mixing material in
a plastic bucket. Everything the company did was in conjunction with North Yorkshire County Council. “With the reinstatement of the roads, you have to have the approval of the Highways Authority,” says Queenan.

“We involved the council at all stages so that the process was completely transparent.”

Graduating from buckets, the second trial had a mechanised mixing system and set out to examine the way in which the varied ground would affect the product. After another 40 excavations throughout North Yorkshire, CBEM became a successful and viable product.

“The next step was to take the research to a practical stage and carry out a feasibility study,” continues Queenan. The company evaluated plant and equipment and, in 2004, decided to invest in the large-scale production of CBEM at a dedicated recycling centre in Sheepscar, Leeds. United app

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