Helena Vučenović, PhD mag. ing. min.
The ability of a porous media, and also of soil, to allow the flow of a fluid through it is called permeability. Fluids passing through a porous media can be in gaseous or liquid form. The hydraulic conductivity of soil is a well-known property and it is described by an advective flow regime. However, in nature, fluid in porous media is very commonly found in the gaseous form, and thus there is a need to quantify the gas permeability of certain materials. This property is becoming increasingly interesting lately, mostly related to environmental issues, especially in environmental geotechnics. Particular attention is paid to the huge landfill biogas production, especially in the early stage of a landfill lifetime. As the main components belong to the group of greenhouse gases, it is necessary to prevent their release into the atmosphere.
The emission of landfill gases into the atmosphere, along with other factors, contributes to the greenhouse effect which causes long-term climate change and global warming, and in recent years, this has become a growing problem worldwide. Of all the complex components of landfill gases, methane (CH4) and carbon dioxide (CO2) have the biggest influence on climate change, and they are both products of the anaerobic decomposition of organic waste (Lou & Nair, 2009).
As the main components belong to the group of greenhouse gases, it is necessary to prevent their release into the atmosphere. Geosynthetic Clay Liners - GCLs have recently been installed in the final cover of landfills as a barrier layer (fig 1). GCLs are artificially manufactured products that consist of a thin layer of bentonite between two layers of geotextiles. Bentonite is a sealing component while the layers of the geotextile are the supporting reinforcing components. Earlier research has already shown that GCLs are a very efficient hydraulic barrier, but there is a growing interest in determining GCL efficiency as a gas barrier.
Fig 1 Cross-section of landfill with liner and final cover system details.
GCL gas permeability tests have been conducted in the last twenty years. The gas flow testing of GCLs at steady-state conditions using the method of constant head permeability test is traditionally the most commonly used method to determine GCL gas permeability (Bouazza et al., 2002; Bouazza and Vangpaisal, 2003; 2004; Didier et al., 2000; Vangpaisal and Bouazza, 2004).
By examining the previous tests and the obtained results, it can be concluded that water content and degree of saturation of the sample have the greatest influence on gas permeability. However, there is still a need for a more detailed analysis such as the values of water content and saturation of the sample prior to and following the test. A large number of factors can affect gas permeability test results.
With the aim of determining gas permeability parameters through media such as sealing barriers, a simple method for gas permeability testing of different soil types was developed in the Geomechanical Laboratory of the RGN faculty. Gas permeability laboratory tests were carried out in a triaxial device that is connected to a data logger system. In order to measure the gas flow through a sample the bottom base is connected to a high pressure air supply, and the top base of the sample is connected to the gas flow meter with a flow transducer. An air compressor is a constant pressure source. On the other side, air flow is measured by a very accurate flowmeter at atmospheric pressure which is joined with a data logger connected to a personal computer. The gas permeability measuring device is designed so that during the experiment, among other things, the conditions in the landfill final cover system where the GCL is exposed to the confining pressure and the impact of the cover layers can be simulated. The gas permeability test scheme is shown in Figure 2.
Fig 2 Gas permeability test scheme (Vucenovic, 2016)
Samples of various soil types and GCL samples on which laboratory gas permeability tests have been performed are shown in Figure 3.
Fig 2 Soil and GCL samples
The modification of a standard triaxial testing device resulted in the development of the new laboratory method for gas permeability determination. This method was verified by testing different types of samples. The results of the tests indicate the applicability of the developed method since the permeability values of individual porous soil samples lie within the range of earlier gas permeability tests. The permeability of a porous media in all the samples ranged from 1.45 × 10-13 to 4.71 × 10-13 m2.
Analysis of the results also indicate that the samples of bentonite clay and GCL in dry-state do not represent an efficient gas barrier whose permeability is smaller than the permeability of some other materials. It is considered that for the determination of the optimum water content of a GCL sample to be installed in a landfill final cover system, samples of a large initial water content range for the gas permeability test should be prepared in order to find the one that gives the lowest permeability. It can also be noted that the gas permeability test in a triaxial cell is very easily adopted since it is only necessary to modify an existing triaxial cell by adding flowmeters and a gas source.
References:
Bouazza A, Vangpaisal T and Rahman F (2002) Gas Migration Through Needle Punched Geosynthetic Clay Liners, In Proceed. International symposium on clay geosynthetic barriers., Nuremberg, Germany, 16-17 April 2002, Zanzinger, Koerner& Gartung (eds), Swets & Zeitlinger, Lisse, 165-176.
Bouazza A and Vangpaisal T (2003) An apparatus to measure gas permeability of geosynthetic clay liners, Geotextiles and Geomembranes 21, 85–101.
Didier G, Bouazza A and Cazaux D (2000) Gas permeability of geosynthetic clay liners, Geotextiles and Geomembranes 18, April 2000, 235-250.
Lou, X.F., Nair J. (2009): The impact of landfilling and composting on greenhouse gas emissions – A review, Bioresource Technology, Volume 100, Issue 16, August 2009, Pages 3792–3798.
Vangpaisal T and Bouazza A (2004) Gas permeability of partially hydrated geosynthetic clay liners. Journal of Geotechnical and Geoenvironmental Engineering 130(1): 93–102.
Vucenovic H (2016) The development of laboratory test method of geosynthetic clay liners gas permeability, doctoral thesis, University of Zagreb, Faculty of mining, geology and petroleum Engineering, Zagreb, 2016.
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Helena Vučenović, PhD, mag. ing. min. is a postdoctoral researcher at the Department of Mining Engineering and Geotechnics, at Faculty of Mining, Geology and Petroleum engineering, University of Zagreb. She got her PhD title in September 2016, with thesis The development of laboratory test method of geosynthetic clay liners gas permeability
ResearchGate https://www.researchgate.net/profile/Helena-Vucenovic
Google Scholar https://scholar.google.hr/citations?user=kYsmjmEAAAAJ&hl=hr
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