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KNOT A PROBLEM

CONTAMINATED LAND

Wembley Stadium might be making headlines on a 'will it, won't it-' basis for its completion date, but one thing that will not be holding up its opening is an invasive weed on the site next door.

Here, an access bridge to the north London stadium is being built over a railway line and surrounding land that had been virtually taken over by Japanese knotweed, which can cause structural damage.

The 5ha site belongs to Quintain Estates, but the London Development Agency (LDA) compulsorily purchased enough land for the White Horse Bridge project - named after the police horse used to restore order following a pitch invasion in the 1923 FA Cup Final. And this is where Thurlow Countryside Management (TCM) has been battling the tenacious invader.

The bridge was due to be finished this month but the works, which included deep piling, could not begin before the problem of Japanese knotweed was addressed.

As a noti' able species under the 1981 Wildlife Act, the LDA was legally obliged to tackle the issue.

Leaving even a fragment of the plant root in the soil could have severe consequences in later years with the possibility of regrowth causing structural damage to the bridge.

An initial strategy devised by a team of environmental consultants called for dig and dump - but at an estimated cost of £3.2M, this was deemed an expensive, inflexible and disruptive solution, involving removing soil up to 7m around the perimeter of the knotweed and to a depth of 3m.

A contractor has been putting in crunching tackles on Japanese opposition at England's new home of football.

The spoil would then have had to be treated as controlled waste under the Environmental Protection Act 1990, which only a few landfill sites are able to take under the new licensing regulations.

TCM came up with a better answer. Part of its remit was to advise bridge contractor Edmund Nuttall, to ensure none of its work compounded the knotweed problem. The company also liaised extensively with bridge consultant Halcrow and Wembley Infrastructure Technical Team.

Using an insitu treatment programme, TCM excavated the affected areas and relocated the spoil to a non-essential area of the site, allowing construction works to begin on time. Once relocated, the stockpile was graded to remove the knotweed roots and general debris.

When the root content was analysed it amounted to about 2% of the soil volume, a sharp contrast to the amount which would have been excavated and transported under the dig and dump method. As it was, TCM excavated, relocated, graded and treated about 4000t of soil.

The cleaned soil was treated with a residual pesticide and placed in a new stockpile, which is being monitored and treated as required, before it is reintroduced to site.

To keep the cleaned part of the site sterile, workers used root barriers on the boundaries to prevent knotweed growing outside the work area from infecting it.

The contract is due for completion in July and TCM says signs are encouraging. Root material put into incubators is not showing signs of regrowth and is gradually rotting, an indication that it is not dormant. This has always been the major concern in Japanese knotweed verification.

At a cost of £300,000 for the eradication programme, TCM says the insitu method treatment has proved a great deal more costeffective than dig and dump and has saved the neighbourhood from the inevitable noise and congestion caused by lorries making their way to and from land'll sites.

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