RemedX completed bench-scale trials, followed by excavation work and site-based stabilisation trials for a brownfield site in the Coventry area. This was to enable the development of a detailed design for full-scale stabilisation work on on-site materials to facilitate site remediation ahead of a significant infrastructure development and construction project.
The 50-ha site comprised open, grassed closed landfills, former deep sewage sludge lagoons and confining embankments, which would all require re-engineering, stabilisation and solidification to facilitate the redevelopment. The varied ground of the landfill consisted of soft sewage sludge cake, ash, construction waste, demolition waste and other cohesive and granular matrices. The sewage sludge was primarily degraded human waste sludge with a high organic content.
A previous investigation had identified a stabilisation strategy requiring conventional hydraulic binders, which the client rejected on the grounds of cost. Instead, RemedX proposed using bypass dust (BPD), a hazardous waste in its own right, which had the potential to save the client about £15 million while achieving the same geotechnical and geoenvironmental engineered outcome.
Bench-scale stabilisation trials
RemedX personnel sampled about 500 kg of sewage sludge and 400 kg of made ground from the landfill for laboratory testing. By dividing our analysis into four specialist laboratories (geochemical, pathogen, geotechnical and leachate), we demonstrated the BPD’s viability as a binder for the application, which enabled the work to progress to site-based trials.
Bulk excavation of landfill
The aim of excavating the landfill material was twofold. First, it helped all the involved parties to understand better the makeup of the landfill through the entire landfill profile. Second, we needed to retrieve landfill materials to facilitate the site-based stabilisation trials. During the seven-week programme, on-site personnel stripped the surface of capping soil, prepared material storage bunds, excavated the landfill and then backfilled and reinstated the capping soil, after which the soil was hydroseeded to promote revegetation. The deepest profile through the landfill was intentionally targeted: the full 13.7-m depth profile of the landfill was excavated. In total, 13,500 m3 of material was excavated and reinstated. In addition, 1,400 m3 of perched water was pumped from the excavation and chemically treated and filtered before being discharged to a nearby sewer.
The excavation work was undertaken using a bespoke agreement required by the Environment Agency. This necessitated extensive negotiation and a complex regime of environmental monitoring and control to be in place before and throughout the work.
Site-based stabilisation trials
Over three weeks, we completed site-based stabilisation trials. The work involved preparing treatment pads, retrieving sewage sludge from a lagoon, careful measuring and mixing of materials in prescribed quantities and forming the trial panels before sampling and testing.
We used 20-yard open skips as the primary containment when mixing the constituent materials. The landfill made ground was added first, then the sewage sludge, before mixing them using an excavator bucket. Then, we applied the BPD and left it to mellow before forming it into the trial panels. The sampling and testing involved geotechnical, leachate and pathogen testing to confirm and verify the results achieved following the bench-scale trials.
The work successfully demonstrated the viability of BPD as an alternative binder to cement and the applicability of this new method to the site.