Brownfield Briefing

Spoil, rubble and a soil-tar mix

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Courtesy of Brownfield Briefing

National Grid and Wales and West Utilities recently completed their first joint remediation project at the Aberaman Gasworks following the sale of the Wales and south west of England gas network in 2005. Here Scott Lewis from National Grid and Ashley Griffiths from Wales and West Utilities explain the challenges and processes.

The project was undertaken using a stabilisation and solidification (S/S) technique, which Celtic Technologies has pioneered in the UK. Stabilisation is commonly used in Europe and the US. This is the third project by National Grid to have used this technology in the UK. The environmental improvement works demonstrate commitment by National Grid and Wales and West Utilities to the principles of sustainability, corporate social responsibility and protection and enhancement of the environment.

The former gasworks site is nestled between industrial units and residential housing. Constraints included the site’s narrow access and close proximity to neighbours, together with a number of site issues including a live gas pressure reduction station and a retaining wall (as shown in the diagram). The remedial area included three infilled gasholder bases situated on the upper terrace of the former gasworks, each base being approximately 20m in diameter and 6m deep.

The bases had been backfilled with material including colliery spoil, demolition rubble and soil which had become mixed with coal tar and other gasworks contaminants including spent oxide. Perched water within the bases had also been impacted.

As part of the design of remediation works, detailed quantitative risk assessment (DQRA) was carried out for both human health receptors and controlled waters, including groundwater. The controlled water assessment was based on proven evidence that contaminated perched water was overtopping the gasholders. There was also the potential for the gasholder structures to fail or leak. A set of critical concentrations for each contaminant present were generated. These were generally driven by risks to the river approximately 50m down the hydraulic gradient of the site.

Following team discussions and recommendations from project contractors Celtic, various remedial options were analysed both qualitatively and quantitatively. S/S of the holder contents, in conjunction with multi-phase extraction, was demonstrated as the most sustainable and cost effective option.

Celtic has five proprietary stabilisation additives suitable for a range of contaminants, which chemically lock or bind contaminants within a stabilised matrix and reduce the permeability of the material. The remediation areas contained both organic and inorganic contaminants. To design the most appropriate blend and proportion of S/S additives for the specific contamination at Aberaman, a bench scale testing exercise was carried out. Representative samples of the contaminated material were recovered by drilling boreholes in each gasholder. Material recovered from each gasholder was homogenised and divided into a number of discrete samples. S/S trials were carried out on four samples from each gasholder, with a slightly different blend used for each.

The stabilised samples were then subjected to a variety of in-house bench tests that meet and exceed Environment Agency guidance. The tests allowed the long-term chemical and geotechnical integrity of the stabilised material to be assessed in line with leachate criteria generated by the DQRA. An appropriate S/S blend for the site was chosen as a result of this work. The validation approach for S/S soils was agreed at this stage with the Environment Agency Wales (EAW) and Rhondda Cynon Taff County Borough Council.

The site works were carried out during the five month period beginning December 2008. Prior to S/S treatment, water, oil/tar and hydrocarbon vapour were removed from each of the gasholder bases using a multi-phase extraction system. It was not possible to discharge treated water to the foul sewer due to network capacity problems. To avoid tankering the treated water some 31miles to the nearest trade effluent disposal point, perched water was treated to a high standard and discharged to the River Aman, following agreement with the EAW, or reused in the S/S works. Contaminant concentrations in the treated water met Environmental Quality Standards.

S/S was generally carried out by adding the required additives to treatment cells and mixing the additives through the soil using excavator-driven mixing buckets. Each gasholder base was split into eight ‘cake slice’ type segments and the segments stabilised in three lifts. S/S additives were introduced in two parts, one as a bulk powder, the other as a liquid after mixing with water, which facilitated hardening of the S/S soils. Due to constraints from live gas services on one of the holder bases, S/S additives were introduced under pressure from boreholes drilled in the area. During the works, 99.5% of excavated or contaminated materials were reused in the works. This exceeds the National Grid and Wales West Utilities re-use targets and saved around 840 lorry movements through residential areas and negated the bulk use of imported materials.

Stabilised soils from the full-scale works were validated using similar in-house tests to the bench scale testing carried out at the design stage. The testing demonstrated that the samples met the chemical targets for the site and that a significant strength gain had been achieved. The testing also demonstrated that the stabilised soils were durable. In addition to the validation of the samples, three boreholes were drilled into each gas holder base and cores recovered to demonstrate that the full depth of materials in the holder bases were stabilised.

During the works CO2 emissions, fuel usage, usage of natural resources including aggregate, water and landfill volumes were recorded. CO2 emissions were monitored via the fuel used on site, mileage travelled to and from the site and vehicle types used, to reduce emissions and fuel and natural resource use resulting from remediation activities. Assessment of the site data indicated that the S/S treatment was comparable in terms of CO2 emissions to other remediation techniques. When assessed in terms of impact on the local community, high material reuse, low material import, financial cost and vehicle movements, the option was found to be significantly more sustainable than other techniques. Close team working and early detailed consideration of design enabled the most sustainable and cost-effective option to be adopted.

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