by Chris Kelsey
Waste management regulations, landfill construction techniques and approaches to daily operations and final cell closure, vary wildly around the world. But on the whole, the waste management industry is in better shape than most, if not all, areas of infrastructure. This has a great deal to do with the roles played by geosynthetics and their affiliated quality control measures in modern cell construction and monitoring.
Even in places such as Hong Kong where there are not, strictly speaking, landfill regulations, geosynthetic barriers and drainage systems are being used to manage cell liners and caps. The specifications for Hong Kong’s limited active waste facilities have drawn upon European and North American practices. This is fairly similar to what is being done by other nations which do not have the stringent codes common in fully industrialized countries.
Ultimately, this has prepared the waste management field to anticipate future containment and environmental protection needs. This vital control is part of a landfill’s life, from initial permitting to final cover, and even site redevelopment.
No single approach to quality assurance – or design and material selection for that matter – can possibly serve the global waste management community. That more than anything is encouragement for discussion.
Proper function
Barrier geosynthetics are the most common geosynthetics used in waste management. Their function in landfill design and the way they are vetted in construction quality assurance (CQA) offers some fascinating differences.
A bit about the materials themselves: geosynthetics are polymeric materials used as barriers to, or containment for, liquids, gases and soils. This is the standard function for geomembranes and geosynthetic clay liners (GCLs) or barriers (GCBs) in landfill cells.
Other functions of geosynthetic materials, include separation, drainage, erosion control, reinforcement and filtration. You’ll find various classes of geosynthetics performing these functions in and around waste management operations – for leachate control, improvement of access road performance and service life, prevention of rainwater incursion and other applications.
Regarding the materials used for the primary barrier function, geomembranes are impermeable, polymeric sheets. They can be textured or smooth. They are manufactured with a range of resin formulations such as high-density polyethylene (HDPE) and polypropylene (PP). Those formulations give the membrane sheet particular qualities such as chemical resistance, flexibility, durability, etc.
GCLs (or GCBs) are made of a thin layer of clay – generally bentonite – bonded to a geomembrane or sandwiched between geotextile sheets. Clays such as bentonite have excellent swelling properties, such that a puncture might seal itself. That certainly is not the only feature of GCLs, but it is one unique to these materials. They are used in most of the same installation situations as geomembranes.
Both classes of barrier materials are highly engineered. But all the quality that goes into their construction is lost if they are not managed properly on-site.
The crews installing them, either as landfill bottom liners or landfill caps, must treat them with care so that the geosynthetics can perform their designed role.
CQA and liner/barrier integrity
Two distinct pictures emerge here in regards to where one might concentrate construction quality assurance (CQA) efforts. These are:
- with liner integrity checks during bottom liner construction
- with cap integrity checks during landfill cell closure.
Electrical resistivity testing is the primary means for verifying the integrity of an installed geomembrane or GCL in a landfill bottom liner or capping system. This electrical leak location measure is also referred to as a liner integrity survey (LIS).Computer-monitored and permanently installed wires and electrodes may be used, as may more mobile methods that involve the use of roving, hand-held probes.Although performing a leak survey properly is not easy, the guiding premise behind them is simple. If one can map accurately the flow of an applied potential from one side of a liner to another and through the barrier material, then a leak exists. In general, gauging this requires:
- a conductive medium either over the liner or in contact with the liner
- a conductive medium through any holes
- a conductive medium immediately underneath the liner so that, when you put the electrode in the soil above, the liner current will flow through the liner to the conductive medium below
- no contact around the edges with the medium above/below the liner so the electrical test cannot circumvent the process.
Depending on the operator and the method used for the integrity test, accuracy of detection can be incredibly precise – down to a pinhole.
The USA and Germany provide interestingly disparate approaches here. Both have thorough regulatory systems for all phases of waste management but, in regards to liner integrity survey systems, one focuses primarily on the bottom liner quality check and the other on the cap.
The view from New York
‘The New York State Department of Environmental Conservation (NYSDEC) is developing new landfill liner standards that will require electrical integrity surveys on both primary and secondary geomembrane liners at the state’s municipal solid waste landfills,’ reports Dr Robert Phaneuf. He is the acting director of the Bureau of Hazardous Waste and Radiation Management, Division of Solid and Hazardous Materials – part of the nationally influential NYSDEC. The rules and regulations adopted in New York are often adopted by other states. New York, with its large urban population, is usually by necessity at the forefront of regulation.
 Engineers, contractors and regulators learning how to perform liner integrity surveys in Austin, Texas, at the TRI-CORP Liner Integrity Center (T-CLIC) |
‘Surveys are already routinely being performed on primary geomembrane liners at most of New York state’s double-lined municipal solid waste landfills,’ Phaneuf says. What’s interesting here is how the state has used voluntary programmes to collect the appropriate evidence and hold the right discussions before issuing more liner CQA regulations.
Based on information gained from the voluntary performance of liner integrity surveys on primary liner systems, a marked improvement is noted in way of fewer construction-related defects being found when the liner integrity survey forms an integral part of the construction specifications.
Seeing this benefit, and when one considers the importance of construction quality on the overall performance of the liner system (coupled with the long-term uncertainties associated with liner systems needing to function for an undefined period of time), a requirement for surveys for both lower and upper liner systems makes sense. ‘It’s all about construction quality and acceptable long-term operational performance,’ says Phaneuf.
Leak location testing then serves to verify that the approved barrier materials have been installed properly.
Phaneuf also notes that the state’s proposed regulations do not impose liner integrity surveys on final cover systems because final covers are easily accessible in the event of a problem being detected. The approach in New York is that repairs can be made or a liner integrity survey could be conducted at anytime deemed necessary during the post-closure care period of the landfill.
The view from Germany
In Germany, liner integrity testing is used for CQA on landfill caps – not on bottom liner systems. ‘The approach at the base of the landfill is to use a composite liner: a clay liner and a high-density polyethylene (HDPE) geomembrane,’ says Dr Werner Müller, head of the Working Group on Plastics in Geotechnical and Geoenvironmental Engineering, for Germany’s Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung; BAM). The idea is that, when these composite systems are installed, there is enough redundancy for assurance. Small leaks in a geomembrane are not necessarily relevant because of the clay liner beneath it.
German regulation focuses more on permanent monitoring methods. ‘The problem is that if you have a permanent leak detection system at the base,’ Müller says, ‘the question is always, what would you do if you found some leakage?’
This does not suggest a lack of concern in Germany for the bottom liner systems. There are stringent regulations governing material quality, selection and installation. Thick, certified geomembranes must be used. Smart welding must be used. The presence of quality materials, experience and proven practices produces a safe site.
‘Originally, we were set to make the same rule on the cap as at the base – a composite liner system,’ says Müller. ‘But then people found a lot of problems with compacted clay liners in a capping system.’ Major problems included desiccation and root penetration.
The discussion then went away from compacted clay and into other capping system combinations. ‘For example, there are bentonite mats and polymer-amended sand/bentonite mixtures to combine with a geomembrane,’ says Müller.
‘But the other idea was to use a leak detection system in combination with a geomembrane. In capping, leak detection can now be used as an alternative to other liner components,’ he continued. Leak detection performs the function in the cover that clay does in the base.
This long-term monitoring of landfill cap quality requires approval. To date, only two companies have qualified to perform the cap integrity monitoring – Progeo GmbH and Sensor Dichtungs-Kontroll-Systeme GmbH.
Approval of capping systems and capping system integrity monitoring requires many things – such as the ability to properly perform and understand material selection and testing, long-term behaviour of the geomembrane, stress-crack resistance if using polyethylene, resistance against oxidative degradation of the leak detection system cables, etc. To obtain certification to perform these services for a site requires passing a model installation in which the system and the accuracy of the integrity monitoring are vetted by third party controls.
‘There are requirements for lightning protection and documentation and quality control and so on,’ says Müller. ‘If in one case the system was properly installed and it’s been shown that there’s enough resolution, and the material properties of the cables and sensors and electrodes are okay, then they can get certification.’
Beyond that, when used on a landfill site, there must always be testing even though an approved system is being used. Artificial holes, for example, are placed in the liner system by a third party controller.
A note on CQA education
As global waste management practices continue to modernize, greater industry efforts are being directed at education and certification for construction professionals using geosynthetics. A few of these are worth noting.
The International Association of Geosynthetic Installers (IAGI) has performed extremely admirable work with the establishment of its Certified Welding Technician (CWT) Programme. It’s improving the quality of installations by giving project teams a way to specify experience. More than 40 companies now employ CWT-qualified welders. The welding or seaming of geomembranes is critical on-site as most geomembrane installation sites require multiple panels of geomembranes. While some panel types can be fabricated together in the controlled environment of a factory, a good number must still be joined on-site. This is not an easy process. If poorly conducted, minor breaks in a seam could lead to leakage, and the leakage to larger failures, litigation and a need to redo (or remediate) the site. IAGI has also launched an Approved Installation Contractor (AIC) programme that is helping to further raise the bar of quality by vetting the business strength of geosynthetic installation companies, including issues such as bonding readiness.
The Geosynthetic Institute (GSI) continues to improve site inspection with its Construction Quality Assurance Inspector Certification Programme (CQA-ICP), which focuses on both geosynthetic and compacted clay liner construction quality assurance (CQA). GSI White Paper No. 8 addresses the CQA-ICP and cites a dramatic figure from research conducted by Forget, Jacquelin and Rollin in 2005: covered geomembranes were found to have an average of 6.5 leaks per acre (2.6 hectares) when the project did not use CQA, but only 0.2 leaks per acre when using CQA. Clearly, experience is a driving factor in significantly reducing long-term risk.
The advent of T-CLIC’s training sessions have been another major development in preserving the quality of installations. TRI/Environmental Inc. (one of the most experienced testing laboratories in the USA) and I-CORP International Inc. (an engineering company with more than two decades of geosynthetic liner integrity testing experience) have jointly developed the TRI-CORP Liner Integrity Centre (T-CLIC). T-CLIC has spent the last few years training contractors and engineers how to correctly perform liner integrity surveys. LIS training, like the CWT programme and the CQA-ICP certification, is putting more qualified professionals in the field. T-CLIC’s combined approach of short-course classroom education, hands-on field training and follow-up oversight, has resonated. Although the course is generally taught in Austin, Texas, (sometimes combined with CQA-ICP training) a session was held in New York State in summer 2007 at the invitation and arrangement of the regulators there.
Information and quality
Although the approaches in the USA and Germany seem to be opposite to one another, they both work incredibly well for the waste management needs particular to each country. As the world adapts to the sudden growth of other nations (e.g. Brazil, Russia, India and China) and the waste management needs of their rapidly growing middle classes, more approaches will be tested and found to be right.
Geosynthetics are going to be part of that future infrastructure security. And liner integrity surveys – be they on bottom liners or landfill caps – seem poised to play an ever more important role.
Chris Kelsey is an editorial consultant with geosynthetica.net, an online source for news, technical information and resources in the geosynthetics industry.
e-mail: chris@geosynthetica.net
The author would like to thank Drs Werner Müller, Bob Phaneuf and Ian Peggs of I-CORP International for sharing their time and experience during the writing of this article.
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