FAQs About Geosynthetic Clay Liners (GCL)

  • We are currently using a compacted clay liner. Can we save money by switching to a GCL?
    Yes, unless clay is close to the site, GCLs are usually less expensive.
  • What should I specify for certifiable properties of a GCL?
    CETCO certifies to the bentonite and GCL properties found in ASTM D5889 for manufacturing quality control of GCLs. Separate certifiable properties are provided for all CETCO GCL products.
  • What is the steepest slope on which BENTOMAT may be used?
    Reinforced BENTOMAT GCLs have been successfully used on slopes 3H:1V and steeper. These designs were based on internal and interface direct shear testing. Refer to the historical direct shear data found in TR-114bm.
  • When is a GCL "too wet" and should be replaced?
    A GCL only serves its function of low permeability after it is hydrated. The key is that cover material should be placed over the GCL prior to significant hydration. Based upon experience and field testing, a reinforced GCL can usually be hydrated without requiring replacement. Refer to TR-312 for specific guidelines.
  • I want to line a pond; what GCL should I use?
    For high head applications, such as recreation ponds and wastewater treatment lagoons, CETCO recommends BENTOMAT CL.
  • The contractor has requested permission to use cover material that contains limestone or dolomite. Should I be concerned?
    Calcium and magnesium can leach from cover material and exchange with the sodium ions in the sodium bentonite used in GCLs thus reducing its efficiency. As a general guideline, CETCO does not recommend calcium-rich cover materials.
  • What is the difference between flux and permeability?
    Flux is the actual rate of flow through the GCL and is expressed in units of volume per unit area per unit time (m3/m2/sec). Permeability or hydraulic conductivity is a mathematical constant derived from Darcy’s Law and describes a soil’s ability to convey flow under known hydraulic forces. The benefit of using flux is that this value can be used for a direct comparison to any maximum leakage specifications or requirements for a given project. TR-317 provides a complete description of these concepts.
  • Is a GCL compatible with organic chemicals?
    In general, yes. Small concentrations of common organic groundwater contaminants, for example, have no effect on the hydration and function of a GCL. Even concentrated solutions have little or no effect provided that the GCL is prehydrated with water prior to exposure.
  • What is the lifetime of a GCL?
    To the best of our knowledge, the natural and synthetic components of a GCL will uphold far into the future, likely over 100 years under normal conditions of soil burial. While conclusive data is impossible to provide, the bentonite in GCLs is already extremely old (millions of years), and the synthetic geotextiles are made of chemically stable polymers which resist long-term degradation.
  • Will CETCO make rolls longer than 45.72 meters?
    Longer rolls are extremely difficult to lift and handle, and CETCO actively discourages the use of rolls exceeding 45.72 meters. However, in certain applications where large areas must be covered by experienced installers, CETCO has provided rolls up to 76.2 meters in length.
  • What equipment do I need to unload the delivery vehicles?
    When GCL rolls arrive via flatbed trucks, the trucks are best unloaded using the slings included with each roll. Other alternatives include using an articulating lift truck equipped with a “stinger” which is pushed through the roll core, or a steel core pipe can be inserted manually and the rolls lifted using an excavator or crane. CETCO provides both stinger assemblies and lifting cores as accessory items.
  • Are GCLs affected by freeze/thaw?
    Geosynthetic clay liners are not adversely affected by freeze/thaw cycles. Laboratory testing has shown that the hydraulic conductivity of GCLs remains unaffected after many freeze/thaw cycles. TR-104 provides actual laboratory testing data.
  • Are GCLs compatible with landfill leachate?
    Many laboratory hydraulic conductivity tests have been performed on specimens of CETCO GCLs with leachate as the permeant. This testing has shown that hydraulic conductivity generally remains unchanged. TR-101A is a test report showing laboratory testing data of BENTOMAT permeated with landfill leachate. TR-254 summarizes laboratory testing results of BENTOMAT permeated with multivalent salt solutions representative of landfill leachate.
  • How long do GCLs take to hydrate?
    When a geosynthetic clay liner is placed on a soil, it will hydrate through absorption of moisture contained within the adjacent soil. Testing results provided in TR-222 shows that the GCL will even hydrate when placed on a soil with a moisture content well below optimum.
  • What is the typical moisture content of the bentonite in Bentomat GCLs?
    The moisture content of raw bentonite, before it is incorporated into the GCL, is less than 12%. During manufacturing of the GCL, water is added to the bentonite to assist GCL needle-punching. The resulting bentonite moisture content in BENTOMAT leaving our manufacturing facilities is between 25% and 40%, with typical values of about 30%. These values are well below that of fully hydrated bentonite, and will have no negative effect on performance. In fact, bentonite with 25% to 40% moisture has the appearance, consistency, and strength of a granular soil.
  • Should I be concerned about GCL panel separation in the field?
    Separation of overlapped GCL panels may be a concern in "exposed" lining systems - situations where a GCL is left covered only by a geomembrane with no confining cover soil for extended periods of time (months or years). Under these conditions, there have been some documented instances where wide temperature fluctuations in the exposed geomembrane compromised the underlying GCL seams, resulting in gaps between the panels. However, reasonable measures can be implemented to prevent panel separation. This topic is discussed in detail in TR-338.
  • Will BENTOMAT GCLs shrink in the field upon deployment?
    As shown in TR-339, both laboratory and field data have demonstrated that BENTOMAT deployed at its as-manufactured moisture content (25% to 40%) will not shrink upon drying. Thiel (2005) found that GCLs can experience shrinkage when subjected to repeated cycles of wetting and drying (full hydration followed by complete desiccation). However, as discussed in TR-338, reasonable measures can be implemented to prevent this from occurring.
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