Soil Treatment Project Case Studies from Celtic Technologies
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Case Studies

Technical article: Solidification and Stabilisation (S/S), A Long Term Solution to Contaminated Land ?

Summary –Solidification and Stabilisation is gaining popularity as a treatment solution for contaminated soils as design and feasibility are better understood, costs of application reduce and regulators become more familiar with the technology.  However, relatively few sites have been subject to long term durability testing to verify the performance of the treated material over time.  Celtic is able to report

Aldershot. Former Gas Works

Commercial development had taken place over the infrastructure remaining from a former gas works site, remnant coal tar contamination was present in underground gas holder bases. Extensive stakeholder engagement established project parameters and constraints, including the requirement to maintain safe access at all times. The remediation design incorporated de-watering of gas holder bases with waters treated via a sludge reactor to reduce ammonia concentrations. NAPLs were recovered by multi-p

Midlands. Former Gas Works

Coal tars from a former gas works had the potential to migrate across a site boundary. The source area could not be addressed as it lay below a centre controlling utilities distribution across the Midlands. Based on a sophisticated understanding of contaminant distribution and hydrogeology, Celtic designed a pragmatic solution to intercept mobile NAPL through a hydraulic containment system which has operated successfully since 2009.

Woolston Canal

Celtic was contracted to undertake remediation works of this former canal to enable development of an Urban Ecology Park. Historic industry along the canal banks had resulted in metal and hydrocarbon contamination of the silts. Works included: clearance, drainage and dredging of the canal; stabilisation / solidification of over 5,000m3 of dredged, contaminated silts; canal re-construction using gabion walls; a cover sequence in accordance with WRAP protocols and emplacement of landscaping soils.

Windsor Street

Winner of the Sustainability Award, Celtic shared the podium with runner up the Olympic Delivery Authority at the Brownfield Briefing ceremony. Historic gas holder bases at the site contained significant volumes of coal tars and liquors presenting a risk to the underlying regional aquifer. The sustainability of the project was maximised through a wide range of approaches, beginning with the design process. Works were carried out with the gas holders still covered to minimise impact to neighbours


The licensed waste lagoon at the former Albion chemical works was filled with an alkali sludge with mercury contamination. In order to surrender the Waste Management Licence and mitigate risk to Controlled Waters, Celtic applied stabilisation / solidification to some 17,000m3 of the sludges. Robust feasibility and validation testing confirmed mitigation of risk and enabled surrender of the licence. Close out was achieved by demonstrating that solidified / stabilised material met leachate targ

Raynesway Resource Park

The Raynesway Resource Park is a 35 hectare site bounded on three sides by the River Derwent. The southern half of a former chemicals plant, soils and groundwaters were impacted by chlorinated solvents, acid tars, hydrocarbons and ammoniacal sludges. Celtic designed a sophisticated groundwater treatment system, integrating air sparging, multi-phase extraction and chemical oxidation to address groundwater impacted by chlorinated solvents, which ranks as one of the largest chemical oxidation proje

New Romney

A cul-de-sac of 19 residential properties in New Romney had been built over a former gas works site. Removal and replacement of shallow soils in garden areas was required after coal tar contaminants were identified presenting a potential risk to human health. Resident support for the remediation process had to be retained during the remediation works. Working closely with Shepway District Council and consultants RPS, Celtic maximised communication with local residents regarding processes during


The Environment Agency had determined the Neston site as Contaminated Land under Part 2A of the Environmental Protection Act 1990. Former use as a tank washing facility had resulted in chlorinated solvents (primarily cis 1,2 dichloroethene and vinyl chloride) and hydrocarbons (primarily benzene, toluene, ethylbenzene and xylenes - BTEX) impacting the underlying Sherwood Sandstone, a major regional aquifer. Celtic carried out a detailed assessment of the sandstone bedrock to identify free and res

Morecambe Bus Depot

A former bus depot, soils and groundwater were impacted by refuelling and maintenance activities. Works involved the removal of below ground tanks, fuel lines and drainage. Hydrocarbon hot spots were identified and 1,500m3 of soils were remediated by bio-physical treatment. Free phase hydrocarbons were recovered by skimming. Japanese knotweed was eradicated by excavation and disposal, and the development further protected by the installation of root barriers.


The legacy of the coal gasification processes at the former Thwaites gas works site on the eastern edge of Keighley was a wide range of contaminants presenting a potential risk to human health and controlled waters. As part of a long term relationship with the site owner National Grid, Celtic developed a detailed remediation design including extensive site turnover to address shallow contamination, followed by groundwater remediation to address deeper, mobile coal tars. Contract value was in exc

Golden Hill Lane

Rubber processing had resulted in elevated metal and hydrocarbon concentrations in soils presenting a potential risk to an adjacent water course. Made ground was excavated, with hand picking to remove deleterious materials, asbestos and waste rubber. Stockpiles were then tested against remedial criteria. Solidification / stabilisation was applied to 6,500m3 of failing materials to reduce contaminant leachability. Over 50 tonnes of soils with >0.1% asbestos were disposed off site, and over 660


A former hydrocarbon refinery had disposed of acid tars and oily wastes to an in-house landfill located close to the River Mersey, erosion into the landfill resulted in the escape of free product hydrocarbons to the river. Celtic installed one of the UK’s first multi-phase extraction systems to recover some 19 tonnes of free product, achieving Regulatory close out through demonstration of asymptotic mass recovery.


The former marine diesel storage facility is located adjacent to the Freeport Marina at Fleetwood. Historic loss of diesel had resulted in impact to shallow soils, with free product identified adjacent to buried foundations. Works included: bio-physical treatment of ~2,500m3 of soils; excavation and break out of over 10,000m3 of foundations; recovery of some 50,000 litres of diesel via multi-phase extraction system.

First Street

A major scheme in Manchester city centre, the new development will combine retail and office space over a 17 acre site. Former industry included the Gaythorne gas works, whose legacy was a wide range of contaminants related to the coal gasification process. Chemical stabilisation was applied to contaminated materials excavated from four gas holder bases, with in-situ stabilisation applied to materials in a fifth gas holder base and impacted soils in other areas of the site. Multi-phase extractio

Ebbw Vale

Regeneration of the former Ebbw Vale steel works by Blaenau Gwent County Borough Council included construction of the Aneurin Bevan hospital. Former industry resulted in extensive areas of hydrocarbon impact and hence uncertainty over the volume of soils requiring treatment. In order to meet conservative remedial targets, Celtic employed a proprietary approach of ex-situ bio-physical remediation which provided certainty over outcomes, and flexibility in accommodating additional volumes while min

Coventry Engineering Works

The site was impacted by metal, hydrocarbon and chlorinated solvent contaminants presenting a risk to Controlled Waters. Contaminants were in a range of settings including: from former site use as a metal plating works; historic drainage; and the backfill of a rail underpass. Works included the stabilisation / solidification of 5,400m3 of contaminated soils, cut and fill to create a development platform and installation of a capping layer. Works were completed to a tight programme to meet the re

Coventry Council Depot

A former landfill subsequently used as a Council depot, fill materials were impacted by hydrocarbons, metals and asbestos; with the fill and underlying alluvium also having low geotechnical strengths. Works included: slab and foundation break out; site wide turnover; excavation, segregation and re-cycling to ensure leachate targets were met; and 10,000m3 cut and fill to deliver a warranted development platform for subsequent residential development.

Cornmill Crossing

Former industries on the 3Ha site included a gas works, corn mill and coal yard. Celtic was awarded the contract on a fixed price, with a commitment to a tight programme driven by proposed retail end use of the site. Site sensitivities included proximity to Furness Docks, and adjacent industrial units. Works included removal of historic structures, processing of 15,000m3 of soils, and bio-physical treatment of 4,000m3 of hydrocarbon contaminated soils.

Coed Darcy NAPL

This is one of the most ambitious regeneration projects in the UK. The vision is to remediate and then develop the former oil refinery site to provide homes, employment areas and schools. Development is led by Celtic’s client and the landowner, St. Modwen. Celtic has recovered over 600,000 litres of free phase product and remediated over 50,000m3 of oily sludges from lagoons while ensuring the adjacent Site of Special Scientific Interest is protected. Contract value is in excess of £2 million

Castlegate, Caerphilly

An ideal residential development site on the southern edge of Caerphilly was formerly a waste dump for the South Wales chemical industry. The site included a barrel dump, and soils impacted by contaminants including a wide range of volatile, semi-volatile and chlorinated solvent compounds presenting a risk to human health and controlled waters. A detailed design included a sophisticated water treatment system employed during excavation of the barrel dump, and a vapour barrier whose design was a

Aylestone Park

Improvement of the leisure facilities at Aylestone Hill by Herefordshire Council included development of community football and cricket pitches, a bowling green, and access to parking. The Herefordshire and Gloucestershire canal runs along the * border of the site and had become backfilled with silts contaminated by hydrocarbons and metals. Celtic was contracted to provide a sustainable remediation solution to the issue, provided by stabilisation / solidification of the dredged sediments to both


Remediation of contaminated soils in 3 gasholder bases using multiphase extraction works followed by stabilisation/solidification of soils. This combination of remedial techniques enabled 5,500 m3 of soil to be treated, then retained on site and the site to be beneficially reused. The technologies used minimised the impact of the works on a constrained site and the local highway network. Contaminated water from the works was treated to enable subsequent discharge to the river adjacent