Project: Coal Tar Remediation and Stabilisation
Timescale: 5 months
Extensive remediation work was required on the narrow, residential site to address near-surface soil contamination in gardens, including removal of viscous DNAPL coal tar from a below ground gas holder structure. Additionally, a stream formed one site boundary, requiring preventive works to the bank to prevent contamination migrating off-site.
It was known from historical records and photographs that there was a gasworks and gas holder in the area but the exact location in relation to the modern properties was unknown. ERS used geophysical surveying techniques to determine the position and depth of the gas holder, which in turn helped plan the working areas and sequences.
First, contaminated soil from both gardens was excavated for disposal. As the stream bank soil was also contaminated, this also needed to be removed, which risked NAPL and/or contaminated soil entering the water and moving off-site. To prevent this, the stream flow was diverted in bypass pipework. This also provided space to later construct a replacement stream bank.
The specified remediation strategy required drilling and installing 100mm diameter extraction wells in the gas holder. However, the discovery of an unexpected void beneath the holder cap caused the drilling work to be halted after only a few of the extraction wells had been installed. This threatened to compromise the efficiency of the remediation method and therefore achievement of the remediation objectives.
To overcome these issue, ERS’ engineers came up with a pioneering new application of existing technology from the geotechnical industry – an expanding geo-polymer resin normally used to fill voids was injected at designated spacings and depths, in a sequence designed to displace DNAPL coal tar within the gas holder towards the extraction wells. At the same time, the polymer would fill the void discovered beneath the holder cap and stabilise the remaining gas holder backfill.
Meanwhile, a combination of vacuum extraction and pumped extraction was used to recover DNAPL from the gas holder. The system was operated prior to and during the injection of the geopolymer. This combined system enhanced recovery of the viscous coal tar that would otherwise have remained in-situ had we just used soil stabilisation, vacuum extraction or pumped extraction alone.
The geopolymer resin had a rapid set and curing period, which allowed stabilisation of the holder cap to be achieved very quickly – 32t HGV vehicles were able to drive over the stabilised gas holder cap within 24hrs of completing the resin injection.
The solution ERS engineered was a first of its kind and was significantly quicker and more cost effective than alternative solutions. It also caused less disruption to the residents and neighbours, and allowed elimination of a number of serious health and safety risks that were associated with alternate solutions.
Once the remediation works were complete, a geotextile warning layer and a capillary break layer were then put in place, followed by a clean capping layer back to the original soil level. The stream bank was also reconstructed using stone filled gabion baskets, following which both gardens were re-landscaped with new turf, gravel and paving for the convenience of the residents.