The Importance of Thermal Expansion and Contraction in Geomembrane Liner

Heat will cause most matter to expand as atoms collect kinetic energy and become excitable. Sometimes, this kind of expansion can cause complications with human-made structures. In early September 2017, a heat wave forced San Francisco officials to slow down their rapid transit operations to prevent the rails from buckling under the combined effects of thermal expansion and the weight of the trains. Similar measures must be taken with other materials that undergo thermal expansion or contraction.

Geomembranes are ubiquitous outdoor construction materials used as a liner against the encroachment of undesirable elements. But even this synthetic material undergoes significant expansion and contraction throughout the day-night cycle. The distinctive waves occurring throughout the plane of a laid geomembrane liner due to thermal expansion has been the topic of much discussion. We will explore the importance of thermal expansion and contraction in geomembrane liners and how installers can reduce the waving effect.

Thermal Expansion/Contraction: A Geosynthetic Property and Curse

Years ago, industry dogma suggested that thermal expansion-induced waves and wrinkles across a plane of laid geomembrane liner would flatten out upon the application of normal pressure by the final soil cover. Getting rid of those waves is a big deal, because the Environmental Protection Agency (EPA) only recognizes the efficacy of composite liners when they are in “direct and uniform contact” with the underlying clay layer. Thus, any persisting wave or wrinkle that becomes a fold is a problem. Just how big is this problem?

A paper by Soong and Koerner showed that waves as small as 14 mm in height do not flatten out upon the application of normal pressure. Additionally, the study showed that the waves actually collapse to form relatively sharp folds. The residual stresses at the folds can be as high as 22% of yield where they are most accentuated. For this reason, thermal expansion has returned to the spotlight (1).

Some have advocated the use of alternate liners, such as those reinforced with other materials. But that approach is a non-starter because these alternatives often fail to meet the minimum HDPE thickness requirement set by the EPA (Table 1).

Table 1. Current waste containment strategies at landfills in the United States and Germany, from reference 2.

These requirements are important, especially in environmental applications like landfill closures where leachate can contain compounds harmful to local ecology and human health. When it comes to containment, nothing has come close to outperforming HDPE liners across multiple temperatures (Table 2).

Table 2. General chemical resistance guidelines of some commonly used geomembranes (from reference 2).

Five Ways to Counter Thermal Expansion and Achieve “Intimate Contact”

There are several ways to deal with thermal expansion. Some strategies adapt to thermal expansion while others avoid it altogether.

One of the most common adaptive strategies is to push, accumulate, cut, and seam. With this approach, a lift of backfill soil is pushed forward using a bulldozer. The height of wave grows as the wave is pushed. Eventually the wave consisting of these layers grow so large that the backfilling process cannot continue. At this point, the waves are cut along their crests. The excess material is then folded over and seamed using extrusion fillet welding.

Other techniques include using a flat sheet between fixing berms; replacing black geomembrane coatings with a reflective white coating; using a temporary tent over the placement area; and working with nature. The last three options are examples of strategies used to avoid the thermal expansion process.