Materials / Joint Specifications
Geosynthetic Clay Liners

ASTM's definition of a GCL is: "a manufactured hydraulic barrier consisting of clay bonded to a layer or layers of geosynthetics" (ASTM)

ISO/CEN's provisional definition of a GCL is: "A factory assembled structure of geosynthetic materials and low hydraulic conductivity clay materials (e.g. bentonite), in the form of a sheet, used in contact with soil and/or other materials in geotechnical and civil engineering applications."

Geosynthetic Clay Liners (GCLs) consist of a layer of bentonite carried on one or between two geosynthetics. The different variations are generally as follows, shown in more detail in Figure 1:

1. Bentonite adhered to a geomembrane (Gundseal)
   
2. Bentonite between two geotextiles (Claymax, )
   
3. Prehydrated bentonite between two geotextiles (Rawmat)
   
4. Bentonite between two geotextiles needlepunched together (Bentomat,
   Bentofix)
   
5. As 4 with one surface heated to fuse needled fibers to geotextile
   (Bentofix)
   
6. As 4 with bentonite powder in nonwoven geotextile (Bentofix)
   
7. As 4 with thin geomembrane on one side (Bentomat, Lintobent)
   
8. Bentonite within nonwoven carrier needled and thermally bonded to
   geotextiles (Lintobent)
   

New products are being developed that contain rigid spacers between the geotextiles or between a geomembrane and a geotextile that provide added compressive strength and restrictions to sideways migration of the bentonite.

Bentonite should be natural sodium bentonite rather than calcium bentonite that has been activated to become sodium bentonite.

Therefore, GCLs like geomembrane materials have significant differences. While specifications can be written for a GCL (or geomembrane) to perform as a hydraulic barrier, it also becomes necessary to contemplate preparing specifications for the specific GCL selected for the project, just as it would be for PE, PP, or PVC geomembranes.

Unreinforced GCLs, those without connections between the two geotextiles, will typically be used on flat surfaces, while reinforced GCLs can be used on slopes. In either case, to generate the required low hydraulic conductivity, there must be adequate confining pressure on the GCL to prevent bentonite expansion during hydration - typically no less than 18 in. (450 mm) of soil.

A generic specification for a GCL to perform as a barrier is presented together with three typical product specifications - a German specification for Bentofix-type product, a US specification for a Claymax-type product, and a US specification for Gundseal. Those parameters that should be modified for each specific application are identified.

Generic Barrier Specification

Typical German Specification

Typical US Specifications

^{(1) Zero moisture content}

^{(2) Index flux and permeability testing with deaired distilled/deionized water at 80 psi (551 kPa) cell pressure, 77 psi      (531 kPa) headwater pressure and 75 psi (517 kPa) tailwater pressure.}

^{(3) Peak value measured at 200 psf (30 kPa) normal stress.}

Gundseal Specification

GCL Joint Specifications

Joints are just overlapped, therefore there are no mechanical peel and shear
strength specifications.

• Edge-roll overlaps should be a minimum of 6 in. (150 mm).
  
• End-roll overlaps should be a minimum of 24 in. (600 mm).
  
• All overlaps shall contain a continuous strip of uniformly distributed granular bentonite at a distribution rate of 0.25 lb/ft (0.4 kg/m)

Edge-roll overlaps for Gundseal should be a minimum of 12 in. (300 mm) but no additional granular bentonite is required.