TERRAFIX Thermal Lock Geosynthetic Clay Liners


Bentofix® Thermal Lock Geosynthetic Clay Liners (GCLs) are produced by distributing a uniform layer of sodium bentonite between two geotextiles. Fibers from the non-woven geotextile cover are then needle-punched through the layer of bentonite and thermally locked into the lower geotextile (either a woven or a non-woven). This process results in a strong mechanical bond between the fabrics. A proprietary heat treating process – the Thermal Lock process – permanently lock the needle-punched fibers into place. Properties of this process include increased internal shear resistance, long term creep resistance, and a much lower bulk void ratio.

Superior GCL Performance

Since the late 1980s, Bentofix® GCLs have been specified and used as an alternative to compacted clay barriers in various applications. Bentofix® GCLs are very effective as hydraulic barriers even under high gradient conditions; they are easy to install; more robust against installation stresses and can withstand elongation as well as settlement stresses without significant impact on hydraulic performance.

Gesosynthetics such as Geosynthetic Clay Liners (GCLs)

Geosynthetics such as Geosynthetic Clay Liners (GCLs) and geomembranes can play a very important role in providing cost effective barriers for environmental protection for a wide range of applications. However, like all engineering materials they need to be carefully selected to meet the design criteria, placed in accordance with best construction practice, and protected from damage after placement.

The method of GCL manufacture can impact on engineering properties ranging from shear strength to diffusion coefficient and hydraulic conductivity, and the susceptibility of the GCL to internal erosion. Factors affecting these characteristics include the type and mass of bentonite, the initial water content of the bentonite, the nature of the cover and carrier Geosynthetics and whether or not a nonwoven carrier geotextile is scrim reinforced, whether the GCL is needle-punched (and if so the amount of needle-punching), and whether the GCL has been heat burnished or not.

The engineering characteristics such as the hydraulic conductivity and diffusion coefficient can be both directly related to the bulk void ratio of the GCL. Research done at Queen’s University over the last two decades has showed that needle-punching has a significant effect on the bulk void ratio by comparing results on heat burnished GCL and comparing them to GCLs that have not been heat burnished. The effect of heat burnishing is greatest at low confining stress when the combination of needle-punching and thermal treatment (Thermal Lock™) reduce the amount of swelling and hence the bulk void ratio. The lower the bulk void ratio the lower the hydraulic conductivity (permeability).

Effect of GCL Structure on Bulk Void Ratio (Confined Swell Test). Values listed are Final Bulk Void Ratio

Needle-punching No(1) Yes Yes
Thermally-treated No(1) No(1) Yes(2)
Confining Stress (psi) Final Bulk Void Ratio eB (Confined Swell Test)
0.9 7.6 5.1 4.0
3.6 4.0 3.2 3.0
14.5 2.6 2.3 2.2
29 2.0 1.7 1.7

(1) Petrov et al. (1997) - (2) Lake and Rowe (2000)

Research done at Queen’s University shows that GCL Thermally Locked provides a built-in confining stress which yields a lower permeability than GCLs that are not thermally locked.


Table presents results for tests where the stress is applied prior to hydration. It shows that the effect of thermal treatment is even more significant for cases where the sample is allowed to hydrate at a low confining stress (e.g. 0.9 psi) and then, after hydration, the stress is applied. This is a likely situation in many environmental applications where a GCL below a GM (HDPE) hydrates by taking up moisture from the underlying soil with only the leachate collection system in place and the waste is not placed until after hydration has occurred. In this case the effect of heat burnishing is manifest throughout the stress history because the fibers thermally locked to the carrier geotextile are far more resistant to pull out of the carrier geotextile during hydration at low stress than when they are simply held by needle-punching.


Bentofix® Thermal Lock GCLs represent a cost effective solution, which is both simple and economical. To install, a core bar is inserted through the core, and the roll is suspended from a spreader bar. Ease of installation using a spreader bar allows the contractor to roll out the Bentofix® rolls with a minimal amount of labor.

One truckload of GCL can carry over 40,000 ft2, which is roughly equivalent to 100 truckloads of compacted clay liner material.

GCL’s vs Compacted Clay Liners

  • Greater durability, consistency and reliability
  • Reduced risk of groundwater contamination
  • Ensured compliance with regulations
  • Increased landfill capacity; optimized land use
  • Easier and quicker to install
  • Lower installed cost
  • Reduced impact to borrow areas and plant operations

Simple, cost-effective installation techniques make Bentofix® Thermal Lock GCL a practical alternative to a compacted clay liner or other lining systems.

Bentofix features and benefits for Coal Combustion Residuals (CCR) facilities include:

  • Thermally Locked Geosynthetic Clay Liner providing a lower bulk void
  • Scrim-Reinforced Geosynthetic Clay Liner to avoid shrinkage issues
  • Scrim-Reinforced Geosynthetic Clay Liner to avoid internal erosion issues
  • Enhanced polymer bentonite mixture to protect the GCL in CCR facilities
  • Friction angle tested to ensure slope stability
  • Our plant laboratory is an accredited GAI-LAP facility
  • Bentonite compatibility testing and proven with CCR
  • Polypropylene textured coated GCLs available for extreme site requirements
  • Wyoming based bentonite along with proven and tested polymers for CCR facilities