In the Merseyside region of north west England, soda ash manufacture in the late 19th century produced a waste product known locally as galligu, which contained high levels of arsenic and other heavy metals. As well as the pollution risks posed, galligu's uncontrolled disposal in areas surrounding the factories has resulted in significant ground stability problems.
Galligu is a by-product of a process that converted rock salt into sodium carbonate. It is estimated that the production process produced 2t of galligu for every 1t of soda ash and at its height in the 1870s Britain's soda output (about 200,000t annually) exceeded that of the rest of the world combined. The resulting millions of tonnes of waste galligu were either dumped into spoil heaps or spread on the ground surface to infill valleys or build embankments.
Hundreds of hectares of Widnes, Runcorn and St Helens are underlain by the waste, which places a significant constraint on redevelopment because of its unique physical properties.
Galligu is principally composed of calcium sulphate and its physical properties vary depending on its state of oxidation and water content. When dry it has a silty sand texture, but when wet the waste rapidly softens and becomes thixotropic.
These properties lead to ground instability and as a result much of the area underlain by galligu is left undeveloped.
To enable reclamation of these difficult sites, remediation design and build contractor Celtic Technologies developed a stabilisation technique. The process transforms raw galligu into a man-made secondary aggregate that can be placed and compacted to form stable development platforms.
The remedial solution provides the developer with a geotechnically improved site in which infrastructure is supported in corridors of stabilised galligu and differential settlements are mitigated by a site-wide shallow stabilisation.
The untreated soils are excavated to formation levels determined by the proposed end use – soft cover area, building plot or roadway.
Following treatment, the stabilised aggregate is placed and compacted as engineered fill. This creates a stable platform with high California Bearing Ratios – typically well in excess of 15% – to reduce road pavement thickness. The material is not frost susceptible so development costs can be reduced by using minimum road construction.
At many sites, galligu also presents a significant risk to the environment because of the potential leaching of metals contained within it to groundwater and surface waters.
Testing of the stabilised soils demonstrates that the leachability of the contaminants of concern is reduced to concentrations well below environmental quality standards. Comprehensive laboratory testing of the treated soil has also been carried out to demonstrate its long-term durability. Celtic expects to complete the project – worth about £600,000 to the contractor – by August.
This stabilisation method enables all waste galligu to be recycled on-site. This minimises costly off-site disposal, reduces wagon movements and mitigates the import of fill that would otherwise be required to reinstate remedial excavations. At the St Helens site an estimated 20,000t of waste has been diverted from landfill at treatment costs that are two-thirds to one-half of associated landfill costs.
Some 100 years ago galligu sites would have been environmental eyesores. With modern treatment processes they can be reclaimed for beneficial use – a turnaround that even the Victorians could never have envisaged.
Jon Freeman is regional director at Celtic Technologies.