A former gasworks in Aldershot, surrounded by residential property and businesses, presented a tricky clean up challenge for Celtic Technologies. GE reports.
Poisons and people do not mix together well.
So dealing with an old gasworks site in Aldershot, Hampshire, was a bigger challenge than usual for Celtic Technologies, a Cardiff-based specialist in remediation and ground cleaning work.
“The area is on the edge of the town, but is still surrounded by residential properties and some commercial and industrial use,” says Celtic senior engineer and health and safety manager Trevor Bamber. “On top of that there are a few active businesses within the site boundary.”
Though the Victorian gasworks itself is long gone, and three gasholders that remained there afterwards have also been taken away, there is still an active gas maintenance depot on the site for service and emergency vans and crews.
A call centre, with 80 or so staff also lies within the site boundaries and a vehicle hire centre on the south side and other development have arisen around the site.
But clean-up work was deemed necessary by client National Grid’s property services division which wanted the site remediated as part of an overall environmental improvement programme of its portfolio.
As work has progressed, pedestrian and vehicle access to carparks and between buildings has had to be preserved.
On top of that, the maintenance vehicles need 24-hour access because they are constantly on standby for emergency call outs.
Within the overall area of about 1ha there is a legacy of organics and tars in the ground.
These date to the mid-19th century when a “town gas” plant was established along with various gasholders and an initial distribution to 2,000 properties, which rapidly increased in subsequent years.
A century of operations followed before the advent of North Sea gas and conversion of the site for its distribution with depressurising equipment.
“The closed in nature of the site meant that conventional soil removal and replacement would have meant a lot of inconvenient truck movement and traffic”
The still important gasholders were eventually demolished, but the site is still used for gas distribution and for the maintenance depot.
“Large gas mains, one intermediate-high and one low pressure, run across the site and these have to be avoided,” says Bamber.
The 450mm diameter pipes have a 3m “stand-off” around them.
Celtic was brought in initially in its consultant and ground investigation capacity, under two earlier contracts, in 1996 and 2007.
Not unexpectedly it identified a variety of typical gasworks contaminants, in solids and in water.
These included polyaromatic hydrocarbons, cyanide, sulphur, sulphates, ammonium, metals and volatile organic compounds including benzene, and oil hydrocarbons.
Coal tar and hydrocarbons were present beneath the sites of the old gasholders and in the river terrace deposits under the site, says Bamber “and there were typical dissolved phase contaminants in the groundwater”.
The real problem areas were underneath the remaining bases of three large gasholders, around oil and liquor tanks and purifiers and at a few other locations.
The holders had sunken bases 6.5m into the ground “so that when they were not full they would have almost disappeared,” says Bamber.
Celtic proposed that using insitu methods would be the main strategy for the clean up.
“The closed in nature of the site meant that conventional soil removal and replacement would have meant a lot of inconvenient truck movement and traffic,” says Bamber. “The insitu treatment methods also have a smaller carbon impact.” Celtic was awarded the £1.2M contract in 2009.
A certain amount of removal and replacement was needed - just over 1000m3 from an area near an office building.
Here there were traces of asbestos and a “spent oxide” dump.
Lime oxides were used as part of the gas filtering for coal gas production and were dumped once exhausted.
It was easiest to haul these to a designated disposal area.
For ground that could be treated insitu there was a two-pronged strategy with ground stabilisation and multiphase extraction of liquids and gases.
The primary areas to be treated were underneath the sites of the circular gasholders.
“There was one large unit 23m across and two of 19m diameter,” says Bamber.
Soil from each of these was to be mixed with a stabiliser “basically forming a solid block in the ground which locks in the contaminants”, he says.
Celtic has its own trademarked admixture, a cement base with various other additives tailored to the particular contaminants.
The precise formulation for the job was tested in conjunction with Cardiff University near the firm’s headquarters, using site samples, to make sure it worked well and that leachability was within bounds.
“We worked to a ‘pie’ strategy, dividing the circular gasholder sites into a number of layered slices,” explains Bamber. “For each of those we dug the ground out in layers of about 2m to 3m depth.”
“our working space was quite tight and the logistics were quite critical, particularly organising deliveries”
The bottom layer was left intact and the additive mixed in using an Allu bucket, a hydraulically powered mechanical agitator within a bucket that is fixed onto the arm of a 30t excavator.
The mixer bucket is made in Finland.
Once the bottom layer was mixed, the one above it was replaced and then the top.
“It cures overnight and in the morning is sufficiently rigid to excavate the next slice of pie without the adjacent face falling in,” says Bamber.
Each of the holders was done separately within a small fenced off enclosure, so that as much of the site could be left free for local activity, as possible.
A total 3,300m2 of ground was treated.
“Actually our working space was quite tight and the logistics were quite critical, particularly organising deliveries,” says Bamber. “We had to ensure pedestrian passage across the site at all times, for example.”
He says a five-phase site layout was used during the work as the team moved from one section to another “though we kept the area near the main gasholder as a working space”.
While work at the gasholder sites was under way a series of 29 boreholes was used at critical points across the site for dealing with perched water near the gasholders and oils, contaminated water and vapours elsewhere.
Multiphase extraction was also used at the gasholders in advance of the stabilisation.
“We deal with those using multiphase extraction, like a big vacuum cleaner really feeding into a system of filters and oil traps,” explains Bamber.
Carbon filtration was mostly used to bring the separated water to a decontaminated level sufficient to comply with local water authority Thames Water’s standards.
It then went into the foul water system for further treatment.
A particular additional process was needed at this site because of fairly high ammoniacal nitrogen content in the water.
“We developed a small, on-site activated sludge treatment plant with a couple of small tanks and a recirculating system, which was inoculated with sludge supplied by Thames Water,” says Bamber.
The process was devised in-house by one of Celtic’s researchers working in conjunction with Cardiff University.
Celtic’s site used five full-time workers, staff and a local graduate hired in to help with monitoring.
“The active work areas were separated off by a decontamination changing room so that site workers could come out for lunch and so on into a clean area,” says Bamber.
Bamber says the project involved some complex thought and planning over its five phases, to avoid disrupting the area and its residential and working population.
The team also carried out a series of local meetings to keep people informed.