This is the third in a series of primers on geosynthetic barrier systems, relevant applications, affiliated issues from the field, and more. Links to Articles 1 and 2 may be found at the conclusion of this installment. – Chris Kelsey, Editor, Geosynthetica
There have been a number of cases in which GCL-only liners have been proposed for wastewater treatment plant (WWTP) lagoon liners but have not performed well. This performance issue has been mostly due to inappropriate soils being used for subgrade and ballast, not because of an inadequacy of the geosynthetic clay liner (GCL) itself. While a leak drainage layer under the GCL is correctly required, this must be done in such a way that the GCL is not damaged. Consider the following for a GCL-only liner:
Criteria for GCLs in WWTP Design
1. Consider an unreinforced GCL on the floor
2. Consider a reinforced GCL on slopes
3. For high hydraulic heads use "composite" GCLs with polyethylene (PE) films attached.
4. Place with film side up as in conventional geomembrane/GCL composite liner. This way the hydraulic head, not just the ballast layer, will generate confining pressure on the GCL.
5. With film on bottom only the ballast layer provides confining pressure and the thin film is forced onto the subgrade which could puncture it. With film up, the GCL cushions the film.
6. Ballast soil should be at least 18 in. thick. In Europe, as much as 1 m is used.
7. Often a 0.75 in. minus aggregate layer is proposed for leakage drainage and collection. This has good drainage performance but is a potentially damaging substrate for a GCL
8. Maximum particle size should be 0.75 in. and there should be 80% fines less than #60 sieve.
9. The ballast layer should be similar
10. Avoid "point" loading on the GCL that can laterally displace the bentonite and puncture the GCL
11. A "uniform confining pressure" is the target
12. Ensure adequate joint overlaps for added powder or to cover bentonite-exposing cut.
13. Overlap at least 6 in. at roll edges and 24 in. at ends.
14. Do not allow the GCL to go through drying/rehydration cycles — this is the fastest way to increase the GCL’s hydraulic conductivity — cracks develop on drying that are not sealed on rehydration.
15. If using a batten strip for underwater sealing remember "uniform’ confinement. Don’t crank on the bolts and laterally displace the bentonite. The strip should not be "pillowed" between bolts.
16. Use fillets in 90 degree corners for uniform support. As with geomembranes, do not bridge corners.
Remember the key words: “uniform confinement.”
- “A Primer and Guide for HDPE Geomembrane Liners in Wastewater Treatment Plant (WWTP) Lagoons — Avoiding Whales”
- "Design Primer: When and If You Can Use GeoElectric Technology for Leak Location Surveys"