Integrated site remediation cleans up for store project
A multi-disciplinary construction group has employed a range of specialist services to treat a former fuel depot which is being developed for a national store group.
The project, awarded by MJM Consulting Engineers on behalf of Asda Stores Ltd, required an integrated approach, and allowed May Gurney Technical Services to demonstrate the benefits of collaboration between its various multi-disciplinary divisions.
The group includes five operating companies, all linked to the construction industry, providing specialist services and technology that meet the requirements of modern engineering practice. The May Gurney Group's activities include building, civil engineering with particular emphasis on road and rail sectors, site investigation, piling, design services, fencing, structural steel and site remediation, road surfacing, land drainage, surface dressing and plant hire.
The site was once used as a fuel depot and, as a consequence, soils and groundwater were polluted with petroleum hydrocarbons. Before a new Asda store could be built, remediation work was required to make the site suitable for redevelopment.
Initially, existing disused buildings had to be demolished, and areas of hard standing broken out. During this time May Gurney's fencing division installed hoarding around the entire site perimeter.
The remediation scheme that was adopted concentrated on preventing the migration of pollutants within the groundwater. It involved installing a 550-metre soil-mixed reactive containment barrier - comprising some 600 columns - to an average depth of eight metres. This was the first time that May Gurney's piling division had used its new purpose-built,1.2 metre-diameter mixing auger.
Dr Chris Evans, May Gurney piling division Development Engineer on the project, explained how the mixing auger method works: "The containment barrier was installed using a modified CFA auger which mixes special cement-based grout and additives with the soil, as opposed to forming more usual concrete replacement piles. A series of soil-cement columns are produced, creating a low permeability barrier which prevents migration of polluted groundwater."
A major earthworks operation was also undertaken to remove some of the more contaminated soil, which was then replaced with both recycled materials from the site and imported clean fill. May Gurney Site Investigation characterised the fill material and carried out in situ testing after placement to make sure the specifications were met.
A pump and treat system was also installed and operated during the course of the 12-week project. Groundwater was pumped from a series of well points, situated across the site, to a treatment area. Here, a series of tanks skimmed off free-phase pollutants from the water which was then filtered through sand and activated carbon chambers to remove any residual contamination. The treated water was then tested to make sure it satisfied specified criteria before being piped back into the ground through an infiltration trench.
John Grey, Director at MJM Consulting Engineers, commented: "The site was heavily contaminated on the surface and at depth from long term spillage of fuel oil, so we required combined surface remediation and groundwater containment to be carried out to meet Asda Store's construction programme.
"May Gurney's soil mixed barrier scheme offered the best solution in terms of programme and cost, and avoided excessive disposal of contaminated spoil. We are very encouraged by the early monitoring results which suggest a substantial and continuous improvement in groundwater quality outside the containment area.
"The key to the success of the project was the close co-operation of May Gurney at every stage," added Mr Grey.