Permeability clay liners

Understanding the basic hydraulic mechanisms for synthetic liners and clay liners is very important in appreciating the advantages of a composite liner. Clay liners are controlled by Darcy s law (Q = kiA). In clay liners, the factors that most influence liner performance are hydraulic head and soil permeability. Clay liners have a higher hydraulic conductivity and thickness than do synthetic liners. Additionally, leachate leaking through a clay liner will undergo chemical reactions that reduce the concentration of contaminants in the leachate. 

The primary characteristic necessary for a liner, cover, or cutoff wall is low permeability, which essentially enables them to slow down the seepage or diffusion of chemicals. Clay is therefore the main material used to construct these containment systems. The thickness and chemical compatibility of containment systems are of concern in assessing the performance of a system. For example, clay liners are constructed as a simple liner that is 2 to 5 ft thick. In composite and double liners, the compacted clay layers are usually between 2 and 5 ft thick, depending on the characteristics of the underlying geology and the type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 1.2 in./yr. However, the effectiveness of clay liners can be reduced by fractures induced by freeze-thaw cycles, drying out, and the presence of some chemicals. 

The design of cover systems is site-specific and depends on the intended function of the final cover—components can range from a single-layer system to a complex multilayer system. To minimize percolation, conventional cover systems use low-permeability barrier layers. These barrier layers are often constructed of compacted clay, geomembranes, geosynthetic clay liners, or combinations of these materials. 

The U.S. EPA guidance6 discusses three types of liners FMLs, compacted clay liners (CCLs), and composite liner systems (an FML overlying a compacted low-permeability soil layer). Material specifications in the guidance for FMLs and CCLs are briefly reviewed below, along with regulations regarding all three liner systems. 

For compacted, low-permeability soil liners, the U.S. EPA draft guidance recommends natural soil materials, such as clays and silts. However, soils amended or blended with different additives (e.g., lime, cement, bentonite clays, and borrow clays) may also meet the current selection criteria of low hydraulic conductivity, or permeability, and sufficient thickness to prevent hazardous constituent migration out of the landfill unit. Therefore, U.S. EPA does not exclude compacted soil liners that contain these amendments. Additional factors affecting the design and construction of CCLs include plasticity index (PI), Atterburg limits, grain sizes, clay mineralogy, and attenuation properties. 

FML placed on a bed of sand, geotextiles, or other highly permeable materials would allow liquid to move through the defect in the FML, spread over the whole area of the clay liner, and percolate down as if the FML was not there. With clay liner soils that contain some rock, it is sometimes proposed that a woven geotextile be placed on top of the soil liner under the FML to prevent the puncture of rocks through the FML. A woven geotextile between the FML and the clay, however, creates a highly transmissive zone between the FML and the clay. The surface of the soil liner instead should be compacted and the stones removed so that the FML can be placed directly on top of the clay. 

Operating Limitations. The landfill (or treatment) site should be lined with a very low permeability clay or a synthetic liner to prevent migration of oil or leachates covered to prevent any nuisance such as blowing sand and designed so that any leachate, contaminated surface water, or groundwater can be contained and treated before release. 

Barrier Compacted clay Geomembrane Geosynthetic clay liner Recycled or reused waste (low permeability) Asphalt Sand or gravel capillary barrier... 

General requirement for clay liners The permeability of a re-molded clay is influenced by a number of factors, the key ones being plasticity, density, moisture content during compaction and method of compaction. Although the detailed requirements for compacted clay liners (CCLs) vary, the following parameters usually apply ... 

Compared with other clay minerals, kaolinite has a lower affinity for water, has a lower dispersivity, and does not achieve as low a permeability upon compaction. On the other hand, because it is not as electrochemically active, its behaviour may be less affected by chemicals than other clay minerals. Thus, a kaolinitic clay liner may have a higher permeability than liners composed of other clays, but the permeability of a kaolinitic clay liner may not be as sensitive to changes in moisture content or to chemical attack. 

Kalkana, E. and Akbulut, S. (2004). The Positive Effects of Silica Fume on the Permeability, Swelling Pressure and Compressive Strength of Natural Clay Liners. Eng. Geol, 73 145. 

Natural clay barriers appear to be the best approach to engineering landBll sites, assuming that the natural clays have appropriate permeabilities and if they are reworked natural clays, they have to be reworked within a strict range of moisture content to achieve the necessary permeability. Other factors which help improve the performance of clay liners with time include the replacement of calcium and magnesium absorbed on most natural clays by the sodium in landfill... 

Data about the imperviousness of a faultless liner system under defined boundary conditions for as large a number of pollutants and soil materials as possible form an important component of the characterisation of the efficacy of this liner for instance in comparison to equivalent alternative liner systems. Therefore, in the following, a parameterization will be discussed for the permeation rate (or the permeability) and the induction time for diffusive mass transport in the composite liner consisting of a geomembrane and a compacted clay liner (or more generally a porous mineral material). Quantities, which refer to the geomembranes, will be denoted with index 1, such as thickness d and diffusion coefficient D, and quantities referring to the mineral liner will have index 2 such as thickness d2 and effective diffusion coefficient D2. The porosity of the water-saturated mineral liner is denoted with 0 as above. 

Farquhar Parker (1989) reported examples of 4-15 year long field monitoring studies carried out in the USA and Canada. In these cases, municipal solid waste leachate did not affect appreciably the permeability of clay liners composed of mixtures of minerals such as illite, chlorite, quartz, feldspar, carbonates and smectite. Sodium and chloride proved to be the most mobile ions. All heavy metals were quite efficiently removed by the top 15-20 cm of the liner. 

Fang, H. Y. Evans, J. C. 1988. Long term permeability tests using leachate on a compacted clay liner material. Ground Water Contamination Field Methods, ASTM STP 963, 397-404. 

Nasiatka, D. M., Shepherd, T. A. Nelson, J. D. 1981. Clay liner permeability in low pH envi ronments. In Proceedings of the Symposium on Uranium Mill Tailings Management, Colorado State University, Fort Collins, Colorado. 627-645. 

Peterson, S. R. Gee, G. W. 1986. Interactions between acidic solutions and clay liners permeability and neutralisation. In Johnson, A. I., Frobel, R. K., Cavalli, N. J. Pettersson, C. B. (eds) Hydraulic Barriers in Soil and Rock. ASTM STP 874, American Society for Testing and Materials, Philadelphia, 229-245. 

GCL2 Permeability of Geosynthetic Clay Liners D6766... 

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

The soil used in the experiment was a low plasticity sandy material with a PI of about 11%. The variations in hydraulic conductivity probably reflected zones of material that contained more sand in some places and more clay in others. Tests have been performed on a couple of liners in the field where liquid flowing into the soil liners has been dyed and traced by cutting a cross section or trench through the liner. The result seems to indicate that dyed liquid finds a defect in the top lift, moves down and spreads along a more permeable zone between lifts finds another defect, moves downward, spreads finds another defect and so forth. 

Conversely, clay-rich soils are most effectively recycled into a low-permeability liner. Soil or rock aggregate can be used to supplement soils as needed. All information is from the vendor and has not been independently verified. 

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