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Barrier systems need to be designed for longer life says Rankine lecturer


A LEADING geotechnical researcher has called for better design and construction of containment barrier systems to reduce leachate leakage from waste.

Professor Kerry Rowe said engineers should refi e the way barrier systems are designed, built and maintained to avoid the problems that caused them to fail after only a short time.

Rowe, vice principal (research) at Queen's University, Kingston, Ontario in Canada, gave the British Geotechnical Association's 45th Rankine Lecture, Long term performance of contaminant barrier systems, on 23 March at Imperial College in London.

Barrier systems for waste containment facilities typically included both permeable drainage and low permeability liner components and included a combination of clay and geosynthetics, Rowe explained.

'Long term performance depends on the selection of suitable liner material, appropriate protection of the geomembrane, the chemistry of fluid in contact with the liner and its temperature, ' he said.

A variety of problems could affect the liner and performance of individual elements could impact on the entire system.

Design and maintenance of leachate collection systems should prevent clogging. 'Clogging in eases the head of leachate in the landfill and that increases leakage.' Bacteria fermenting volatile fatty acids in the waste could lead to preci tation of calcium carbonate, creating a 'weak concrete' cementing gravel in the drainage blanket and build-up in drainage pipes.

This could be reduced by using uniform coarse gravel, reducing flow in the critical zone, keeping close control of the sequencing of waste deposition and pipe cleaning, Rowe said.

Clogging and increased leakage in turn caused temperature rises in the lining, which had a 'profound effect' on performance, he added.

'Design of landfill systems has traditionally been based on an average temperature of 20¦C, ' he said. 'This is OK for six to eight years but after this, the actual temperature is between 30¦C and 40¦C, even in well operated systems.' A typical municipal waste liner could be expected to have a life span of 150 years at 35¦C and 700 years at less than 20¦C, he said, noting that leakage could raise temperatures to nearer 50¦C.

Increased temperature could cause desiccation of clay liners beneath geomembranes, forming pathways for leachate spread. 'Desiccation can be avoided by appropriate design and construction, ' Rowe said.

Liner holes could also affect leakage rates. Conventional design, installation and placement of drainage blanket assumed between two and a half and 12 holes per hectare, he said.

'But data would indicate that there would need to be 60 to 70 holes per hectare to explain the typical ranges of observed leakage.' Increased leakage could be explained by wrinkles in the geosynthetic - up to 1,200 wrinkles per hectare had been found - which created pathways to allow spread of leachate and if holed could increase leakage.

But help is at hand for engineers, Rowe said. Clogging could be modelled, liner systems could be designed to extend service life, and liner temperature could be reduced by better design and construction and proper consideration of operational issues.

l The 46th Rankine lecturer will be Robert Mair from Cambridge University.

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