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Ponds are common in the United States for water treatment, farming, thermal storage, and more. Creating ponds for the former two applications is notoriously tricky as gas buildup has led to failures of polyethylene geomembrane lining systems due to a phenomenon the industry refers to as whales. Whales are visible bubbles that form when the geomembrane experiences pressure below the liner that exceeds the overburden pressure above the geomembrane. Whales can happen when a geomembrane is intact and does not have any defects that cause leakage.
This article will discuss the source and consequences related to whales and explore how a unique pond liner solution can relieve the gas buildup to prevent whales.
Understanding geomembrane whales
Poor venting or drainage can lead to pressure build-up under geomembranes known as whales. Whales are troublesome because they can stretch the liner past its elongation at breaking point, causing a bursting failure. Dealing with whales first requires understanding how they form.
There are five known causes for whale generation. The first is gas generation due to the decomposition of organic matter in the subgrade under the geomembrane. Rising temperatures under the geomembrane can speed up the decomposition of organic matter and increase hydrocarbon gas generation. Imperfections or faults along weld zones can allow geomembrane leakage under the liner, causing whales that often reach the pond’s surface (1).
Whales can also occur due to site-specific conditions. For example, coastal ponds affected by a rising water table can lead to water pooling. Surface impoundments on compressible silt and clay soils can lead to an increase in gas pressure over time from compression. With each cause, there are known countermeasures (summarized in Table 1). In general, the lowest geomembrane section should incorporate a drainage system.
Table 1. Causes and removal considerations of geomembrane gas and liquid whales (adapted from reference 1).
|Cause||Release Agent||Base Contour||Removal Method|
|Organic degradation||Subgrade gas||Upward||Top-of-slope vents|
|Degrading volatile organic compounds||Subgrade gas||Upward||Top-of-slope vents|
|Rising water table||Subgrade gas, Subgrade liquid||Upward, Downward||Top-of-slope vents, Outlet pipes or sumps|
|Compressible subgrades||Subgrade gas, Subgrade liquid||Upward, Downward||Top-of-slope vents, Outlet pipes or sumps|
|Geomembrane leakage||Held liquid||Downward||Outlet pipes or sumps|
Engineers can combine a heavy needle punched nonwoven geotextile with an upward-oriented base gradient leading to surface vents for gas uplift. The gradient slope should be uniform to prevent the accumulation of gas that could form localized whales. For liquid uplift, engineers often account for about 500 gal/acre-day. The drainage system should use a geonet or geocomposite that slopes downward toward pipe collectors leading to removal sumps or wells.
- 1. M. Koerner and G. R. Koerner, “Underdrain Design for Geomembrane Lined Surface Impoundments to Avoid Whales/Hippos from Occurring.” Geosynthetic Institute. (2015). Accessed online 1 November 2021 https://geosynthetic-institute.org/papers/paper33.pdf.
- 2. Kelsey, “Drainage and Venting of Geosynthetic Containment Systems.” Geosynthetica. (2017). Accessed online 1 November 2021 https://www.geosynthetica.com/drainage-venting-geosynthetic-containment/.