Geosynthetics | October 2011
GMA techline answers from Bob Koerner
RE: Pond liner
We have some air pockets under the geomembrane as the pond fills with water. Rather than try to cut and patch the flexible membrane liner (FML), would it not be easier to simply leave the anchor trench unfilled in those areas to “burp” the air out up top … or does that leave us too vulnerable to wind? Couldn’t we just sandbag those areas?
(Mac | England)
Reply: You have altogether quite a common problem.
The original design should have had an underdrain system (for air and/or water) that led up the sideslope to vents. Vents are either stacks, inverted U-shaped pipes, or flap vents.
For your situation, I suggest you cut in 6in.-diameter holes at 15–25ft spacings along the horizontal runout of the FML. Then weld 12in.-square pieces of FML over each hole, but only seam them along the top and two sides.
This allows the air coming up the sideslope to get out of the unseamed bottom of the flap vent and also prevents rainwater or snow melt from getting in under the FML. It is much better than loosening the anchor trench.
Next you will have to “chase” the pockets to the toe of the slope, which might be quite a chore. Let me know how it works.
P.S. These various vents are all shown in “Designing with Geosynthetics” Chapter 5, Designing with Geomembranes; 5.3, Liquid Containment (Pond) Liners.