Like politics, religion, or how to build a fire, everyone has an opinion. There are actually pros and cons to either leaving the geomembrane exposed (no cover) or burying it (complete cover). Let’s look at the issues.
Not all geomembrane materials can be left exposed because of the effect of either UV radiation or changes in temperature. The latter causes thermal expansion-contraction swings in highly crystalline products such as most polyethylenes, particularly HDPE. For those products, long life exposure is not an option because the life will be greatly reduced, even if a lighter color is substituted.
The advantages of covering a liner include:
- No UV exposure; theoretically longer life
- Minimal thermal changes
- Protection from foot and vehicle traffic
- No uplift concerns
The advantages of an exposed liner include:
- Less damage potential during overburden installation
- Ability to view and monitor the geomembrane condition at all times
- Ability to remove or repair sections
- Ability to use steeper side slopes because overburden sliding concerns are eliminated
- No contaminated overburden which will require disposal
- No differential loading due to shifting overburden
- No maintenance of overburden
- No added issues with the head drop from containment area to O/W separators (geomembrane must be placed lower to account for overburden thickness)
- Lower construction cost
Designers will often provide materials and features in a lining system that will incorporate the advantages of both covered and exposed liners, including:
- Utilizing a geomembrane with a proven, long-term UV life, and negligible thermal-expansion contraction.
- Covering selected areas where foot traffic or other disturbances are expected during the project life. Exposed membranes have been covered by other membrane products (walkmats) in the roofing industry for many years.
- Providing a porous media (geonet, geotextile) under the geomembrane and providing a positive slope to the dikes from the center. This is a particularly good practice in a geomembrane-lined liquid impoundment for passive venting of the subgrade.
Take a look at the project and consider the facility operation. Innovative design allows exposed membranes to offer many features also inherent in a covered application.