Home » Projects » Enhanced DNAPL coal tar extraction using geopolymer material


DNAPL extraction

Injection tubes through gas tank holder

Geobear recently worked on an innovative scheme to use our expanding geopolymer resin to enhance, pump and treat the removal of DNAPL coal tar from a sub-surface gas holder, whilst simultaneously stabilising the holder backfill and encapsulating the residual coal tar remaining in-situ.

The approach, designed and implemented by Environmental Reclamation Services as Principal Contractor with Geobear Ltd as subcontractor, resulted in the enhanced removal of the DNAPL coal tar, whilst achieving significant cost and programme savings, as well as significantly less ‘local’ environmental disruption to the surrounding environment and residents.


The Geobear approach

Visual of injections

CAD illustration of material in gas holder

The innovation uses Geobear’s injected expanding geopolymer resin to stabilise the holder contents, whilst simultaneously displacing coal tar towards the existing extraction wells and encapsulating any coal tar left-in-situ. The expansion of the resin would either displace trapped coal tar or encapsulate it, and once cured the resin would provide geotechnical stability to the holder fill and reduce future environmental risk.  Careful design of the injection sequence allows the coal tar to be displaced towards the existing extraction wells, from where it would be removed by suction for disposal without impacting on the surrounding environment.


The benefit of this approach were the significant time and cost and time savings against a full excavation solution. The site comprised two private neighbouring dwellings, which had been constructed on the site of a former gasworks.  Investigation by others on behalf of the local authority, identified an unacceptable risk to human health via shallow soils and the presence of DNAPL coal tar within a subsurface gas holder base in the garden, and partially beneath one of the properties, was also identified. With the obvious risk to human health, the main driver was to remediate the problem, but also with minimal disruption to the dwellings and their occupants with them remaining within their homes.

The outcome

DNALP site

Site after works completed

Approximately 24,600 litres of coal tar impacted water and 18,400 litres of DNAPL coal tar were removed from the gas holder.  Pore volume within the holder was estimated as 109,000 litres, thus extraction of the liquid from the holder has reduced the mass of pollution burden by just under 40%.  The closely spaced, intensive injection of expanding geopolymer encapsulates any remaining liquids and seals potential flow pathways within the holder backfill. In essence, the pollution burden associated with the gas holder was effectively eliminated by the combination of techniques, and all without the need to relocate the on-site/neighbouring dwellings or add risk to those undertaking the work.

The ability to assist in the remediation of land, especially contaminated land situated in residential areas, without impacting on its occupants through having to relocate them or interrupting their day-to-day activities.

Additionally, the Geobear solution is a no compromise, fast, efficient and cost-effective solution to alternative methods often deployed for such projects


The project was considered a great success and the process is being developed for use across a wider range of sites. The project is currently shortlisted for the Sustainability project of the year at 2018 Ground Engineering Awards.

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