The ability to perform a successful (effective) geoelectric integrity survey on a geomembrane liner requires four basic boundary conditions:
- A conductive medium immediately above the geomembrane – sand, gravel, soil, a wet geocomposite, leachate, etc.
- A conductive medium through the leaks being sought – leachate, sand, etc.
- A conductive medium immediately below the geomembrane – sand, GCL, soil, etc.
- No electrical connection between the conductive media above and below the liner other than through the holes being sought
This will facilitate a continuous electrical circuit between the current injector electrode placed in the soil above the liner, through the soil, through a primary leachate drainage composite, through the leaks, then through the GCL/sand under the (single) liner to the current return electrode in the subgrade soil, or through the GCL/sand below the primary liner to the current return electrode in the secondary sump.
Thus, there are two important factors of which to be aware:
- In a single liner the survey must be performed with geomembrane exposed around the periphery of the cell – the soil above the liner must not be in electrical contact with the subgrade soil.
- In a double liner with only a geotextile/geonet/geotextile composite, commonly termed “geocomposite” (it is only one of many geocomposites) Condition 3 above is NOT met. The “geocomposite” is NOT conductive.
The only way to survey the primary liner of a constructed double liner with a “geocomposite” LDS is to backfill the LDS with water which becomes the conductive medium under the geomembrane. This is time-consuming, costly, and can be damaging to the liner if it is uplifted. Additional ballast may be required above the liner. Of course, the survey is only effective where the LDS water is in contact with the geomembrane. Depending on the gradient of the floor it may not be possible to survey the high end. And it is not possible to survey the slopes. So, if an integrity survey is required as the final stage of CQA, or if one might be required to find a closure-threatening leak, plan ahead for that underside conductive medium. The options are:
- GCL, ensuring it is not desiccated when covered
- Conductive geomembrane, taking care at welds – fusion welding must “break” the conductive layer on the underside, and conductivity must be assured from panel to panel
- Conductive geotextile
- Conductive geotextile as top layer of a “geocomposite."
While the benefits of geoelectric surveys cannot be disputed (NYSDEC is about to require them on secondary liners also) all lining systems are not immediately survey-able.
Simply remember the four boundary conditions and apply them to each installation.
Consider, carefully, that the alternative is an ineffective survey that finds nothing even though leaks are present.
Ian D. Peggs, Ph.D., P.E., P.Eng., is president of I-CORP INTERNATIONAL Inc. He can be reached at icorp@geosynthetic.com. NOTE: geosynthetica welcomes Tech Notes from all readers. Contact the editors for more information: chris@geosynthetica.net.
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