Geomembrane Design Part 1: How Gas Bubbles Form

Many geomembrane lining systems fail in wastewater treatment ponds and in ponds in the plantation and livestock sectors. Geomembrane gas bubbles that form beneath a lining system are usually caused by a chain of events: the lining system leaks, liquid collects under the geomembrane, methane gas forms, and the trapped gas gradually lifts the geomembrane. This phenomenon is known as “whales” in geomembranes.

One way to prevent “whale” bubbles is to build a pressure relief system that vents the gas and leakage trapped beneath the geomembrane. Petra Nusa Elshada is experienced in underdrain design, proven by dozens of wastewater ponds installed with this system.

Points to consider in geomembrane design and installation:

1. During design, assume the geomembrane will leak slightly. With good installation this should not happen, but the assumption must be made to avoid unwanted outcomes.
2. Field quality control must not tolerate even the smallest leak in the geomembrane. If a leak is left unrepaired, fixing it later will be very costly.
3. Beneath the geomembrane there must be a leak drainage, collection and disposal system to deal with leaks.
4. Leakage through the geomembrane must be drained away to minimize methane formation. The pond floor must slope toward a specific point (sump pit); it must not be flat.
5. Geotextile will not work as a leak drainage and gas venting system.
6. Joints between the geomembrane and concrete structures made with polylock must be free of gaps and voids.
7. The temperature of the liquid in the pond must stay within the maximum temperature rating of the geomembrane.
8. The installation method must account for the coefficients of thermal expansion and contraction at given temperatures.
9. Use woven geotextile as protection between the geomembrane and concrete.
10. Do not weld seams in areas that are still wet.
11. Make sure the full length of the area to be welded with HDPE geomembrane welding tools is first roughened with a grinder.
12. A network of geocomposite strips will not work well, because the peaks of the gas bubbles will not form in the same place.

Material selection, good design, the installation process and quality control are all essential to prevent gas from becoming trapped beneath the geomembrane liner.