Sew it goes

A site in Glasgow once associated with the quaint image of making sewing machines may not sound like a threatening place for a local council looking to reclaim land for public use. But its chequered past caused the land to become polluted by an assortment of contaminants – all of which have had be dealt with.

This land in the city's Kilbowie area is classed as brownfield – thanks to the remnants of the earlier-demolished Singer sewing machine factory – and has for many years been the resting place for now unwanted inhabitants.

A legacy of shipbuilding on the nearby River Clyde has thrown the industry by-product asbestos into the mix. In addition, the area was subjected to significant bombing in the Second World War, while locals have long regarded the site as a convenient tipping ground for their domestic waste.

All of this – along with the appearance of a couple of strands of Japanese knotweed – faced White Young Green when it came on board with the project. The consultant began work on the scheme in mid-2007 with the brief to produce a remediation design to level the site and deal with the legacy of contaminated land.

What followed has been a multi-faceted approach to make the site ready for one of two potential end uses – either a sustainable transport scheme or open recreational space.

At the time of GE's visit, plans were still being decided on what happens after the final topsoil is laid, but West Dunbartonshire Council contaminated land officer Sarah Hamill says: "No matter what it is, it will be an improvement on what was there before."

The consultant's plan – combining a cut and fill exercise to level the steeply sloped site and installation of geotextiles to create a barrier between the contaminants and the public – has been designed to cater for either of these two end uses.

Hamill says the site was designated as contaminated land under the Environmental Protection Act 1990, partly because of what was found in the groundwater. This scheme – funded by the Scottish Government – is being done partly to reduce the contaminants reaching the groundwater.

The old waste contributed to ground conditions across the 1.35ha site that, before work started, comprised a 100mm thick layer of topsoil overlying between 4m and 6m of ashy landfill material, underlain by peat.

The sloping site was another issue for the scheme. But White Young Green principle environmental engineer Phil Amos says the team came up with a design that dealt with the problem and generated additional benefits.

Cutting into the ashy fill at strategic points, cleaning it and then reusing it on site, would both decontaminate and level the ground – the more ideal setting for the car park option. Second, it avoided the need to cart off polluted excavated material to landfill and reduced the quantity of imported clean fill material needed.

In addition, the consultant opted to model the site using Autodesk Civil 3D software. This helped with planning the cut and fill exercise, but, more importantly, guided a change in design that led to cost savings.

While creating the 3D model, Amos says it became apparent that there would be a shortfall in the volume of reclaimed soil needed to obtain the required levels for the car park's design. However, using the program, Amos was able to calculate how much soil could be reclaimed and ultimately reduced the final level of the car park.

This meant the project used 1000m3 less fill material. The one day Amos says was spent on the model saved about £25,000 (equivalent to 6% of the project budget) that would otherwise have been allocated to buying, importing and placing the 6F2 highways grade fill material.

Contractor I&H Brown carried out the work on site, starting in late January by stripping off the top soil before it could excavate (down to a maximum 750mm) and deal with the worst of the contaminants.

Site workers compacted the soft reclaimed fill material to complete the levelling work. They then installed Naue-manufactured Combigrid (a Secutex geotextile separation layer sandwiched between polypropylene reinforcing geogrids).

Amos says this geotextile serves three functions. First, it acts as a physical barrier between underlying contaminated soils and overlying clean material. Second, as a separator geotextile, it prevents stone punching into the soil below, which would otherwise mean that the soil below could migrate to the top. Finally, it provides geogrid reinforcement, which has reduced the overlying capping layer from 600mm (suggested for the recreational ground option) to 300mm over a 4000m2 area.

Although extra money was spent on the geotextile, Amos says by reducing the imported fill again, the team saved £75,000.

With the Combigrid in place, the reduced thickness capping layer above made the site ready for the 1500m3 of topsoil (mostly saved from the first part of the job) to be placed above.

Whatever the end result of the scheme, worth £430,000 for I&H Brown and White Young Green, the design will work for both options. "If it is a car park, the tarmac will provide an additional and impermeable barrier," says Amos. "However, if it ends up being recreational space the contaminated land is still separated from the public by a capping layer."

I&H Brown and White Young Green completed work on site earlier this spring.

CONTAMINATION CLEAN-UP

Japanese knotweed
Amos says the smallest of fragments of the weed can spread dramatically so a 7m stand-off zone was planned around each of the two strands on site. No vehicle could enter this zone because of the risk of it spreading further. "In the centre of the site we really are talking about [knotweed] the size of a fingernail," says Amos. "But we had to use a 20t excavator standing outside the stand-off zone to reach in and remove it."

Asbestos
The asbestos was typically found at depths between 400mm and 500mm and subcontractor Bodycote came on site to deal with it. I&H Brown site agent James Brown says one of the difficulties was trying to contain the asbestos and stop it becoming airborne – a job made more difficult by the different forms it appeared in (primarily cement bound in fibrous sheeting and chicken wire). Once removed, site workers used a controlled stockpile (with impermeable membranes above and below) to contain it on site before removal in a single operation.

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