A corner of central Manchester is now being cleansed of its industrial past to create a Europeanstyle shopping boulevard.
Manchester’s industrial heritage is still present today with renovated canalside Victorian warehouses dotted around the city. There are now plans to spruce up an old gasworks south of the centre to accommodate an 8ha retail, leisure and residential development.
From 1840 to the late 1950s, the northern part of the site was home to a large gasworks that produced town gas from coal to provide much of Manchester’s heating and lighting requirements. It was decommissioned in the 1960s when Britain started using North Sea gas instead.
Although most of the brick gasholders were knocked down by the 1980s, the site has remained steeped in hydrocarbon contaminants, of which coal tar is the worst offender, left over from the process of heating coal to produce the gas. Client Ask Developments brought specialist brownfield remediation contractor Celtic Technologies onto the project in October 2007 to come up with a plan for the clean up.
Since starting on the 3.2ha northern section in August last year, Celtic’s priority has been to remediate the contaminated soils and groundwater to protect future site users. What this translates into is a £5.4M contract processing a total of 35,000t of contaminated soils and treating 18,000t with cement stabilisation to improve the land.
In addition, the site overlies a major aquifer – at its closest point just 4m below ground level. This once fed the nearby Boddingtons brewery. Nowadays the brewery and the borehole are closed and the aquifer is not a well-used water resource from the Environment Agency’s perspective. But the Agency still has responsibility for managing possible groundwater contamination.
Celtic decided to use cement stabilisation to deal with most of the coal tar in the soil. To do this, the team is treating the soil with its Evocem product – effectively using cement with additives including pulverised fuel ash and bentonite to trap the contaminants in the ground. “We’re basically taking tar contaminated soils and stabilising them so the leachate meets levels that are the same as drinking water quality,” says Freeman.
In addition, it means the risk of contaminants leaching into the aquifer is minimised. The ground conditions are otherwise made ground at 4m to 5m depths, although this goes deeper at up to 9m at the locations of the five old gasholders on site.
Sherwood sandstone underlies this with a thread of river gravels following the channel of a long buried underground river. Celtic also decided that additional work would be needed even though it had already agreeing the £5.4M fixed contract price. This was because the coal tar that had leached into the aquifer had to be dealt with. “What we have done on site is different from what we originally wanted but we have that flexibility to adapt,” says Celtic regional operations director Jon Freeman. “What we are using is a high vacuum system that extracts liquid tars from groundwater and soil vapour.” This is also known as a multiphase extraction system.
Site workers used rotary air flush rigs to drill a series of 110mm diameter boreholes to depths of between 6m and 15m. Installed to depth in these extraction wells are sections of pipe that have openings to allow the water, vapours and coal tar in. Vacuum pumps extract this from inside the wells that are otherwise sealed with a layer of bentonite near the surface to keep out rainwater and soil. The vacuum pump deposits the contaminated liquid and vapours into a Celtic-manufactured, insitu container, which contains a variety of carbon and similar filters to trap the coal tar. Clean vapours can then be safely released into the air, while the uncontaminated water goes into nearby sewers.
“The good thing is that we measure the volume of tar [being tapped by the multiphase extraction system],” says Freeman. “Day by day we know how much contaminant we’re recovering – and it’s diminishing, meaning there’s now less and less in the groundwater.”
As well as carrying out this part of the work on site, Freeman says the other great success of the project is in managing to recycle 99% of site excavated material – again on site – negating the need for an estimated 80,000km of lorry movements. Site workers are operating under strict environmental constraints because of the city centre location, so each excavation has to be kept small and doused with an odour and dust suppressor –partly to try and temper the unpleasant natural aroma of coal tar.
Day by day we know how much contaminant we are recovering, and it’s diminishing, meaning there is less and less in the groundwater
Jon Freeman, Celtic
Between five and seven excavators of up to 35t capacity are working on site at any one time. Work also involves dealing with brick and concrete from old foundations, walls and floor slabs from the gasholders that have to be dug out of the ground. The excavators separate out this material before it is crushed and reused on site as backfill.
About 11,000m3 of uncontaminated soils are being segregated for re-use as engineered fill on site – a capping layer of 600mm is required. The Evocem-treated soil is mixed on site and placed as backfill to the excavations. Its consistency is at first like wet concrete, says Freeman, but it achieves most of its strength within 24 hours. The final result looks like “weak mudstone and is a relatively homogenous material”, he adds. But, the geotechnical target is only to obtain a shear strength of 75kPa from the stabilised soil – not enough to support future buildings for the planned development. These will be erected on piled foundations.
Among other obstructions around the site is a vast, fivestorey building that has already been renovated. This is the Grand Island building, most recently occupied by BT. Other obstacles include three gas mains running about 1.5m below ground along the site’s southern boundary. Monitors have been put in place to check that the heavy plant roaming around the site does not disturb them and so far, Freeman says there have been no problems.
This has kept the team on its toes as has the Japanese Knotweed growing in 20t of contaminated soil which had to be taken off site and 40t of material which had to be removed following the discovery of cement-bound asbestos. But Freeman says early testing of groundwater and leachate is showing good results and that all is on target for the contract completion date of 19 June. Then, Ask Developments will step up its work to turn the site into a 300m long, 18m wide European-style boulevard called First Street.