Geomembrane performance: Lessons learned—Part 1
By Abigail Beck
While intact, geomembrane lining systems provide excellent liquid containment. With good quality control, it is possible to install a geomembrane with few or no holes. However, geomembranes are exposed to a variety of adverse conditions that can reduce, or even destroy, the integrity of the geomembrane during its operational life.
The long-term integrity of exposed geomembrane has typically been evaluated by measuring the engineering properties of the geomembrane. At any stage of the life of a geomembrane, a service life prediction can be made based on its material properties at that time. However, case studies, including the three presented in this article, show that these theoretical service life predictions may not be mirrored in field conditions.
This article details the current methods for assessing remaining service life and provides an explanation of why existing ponds may not be performing as predicted.
With the exception of mining clients who may lose valuable solution due to pregnant solution pond leakage, it is rare for pond owners to volunteer to assess the performance of their pond, perhaps since the damage found will only mean more cost to repair once the leakage is verified.
This article discusses three case histories where the integrity of ageing, exposed geomembranes were evaluated. Two of the installations had liner integrity surveys performed by conscientious site owners; the third site is required by the state to perform annual visual inspections. The mechanical properties of the geomembrane material was not tested on any of the ponds; the material was only visually inspected for evidence of ageing such as stress cracking.
The typical ageing process for HDPE geomembranes involves three stages (Hsuan and Koerner, 1998).
During Stage I, the antioxidants present in the geomembrane formula are consumed at a rate dependent on the exposure conditions. Antioxidants typically make up 0.5% to 1.0% of the geomembrane (Hsuan and Koerner, 1995). The presence of antioxidants in a geomembrane can be quantified using the standard Oxidative-Induction Time (OIT) test (ASTM D3895) or the high-pressure OIT test (ASTM D5885).
In modern geomembranes, the initial OIT value can be around 130 minutes, which represents the time it takes for the geomembrane sample to reach the exothermic peak when held isothermally at 200 C (392 F) at the given pressure for the test method in an oxygen environment.
Stage II begins when the antioxidants are depleted to the point where the geomembrane is no longer resistant to oxidative degradation. The OIT value at the beginning of Stage II lies somewhere around 0.5 minute or 1.0 minute.
There is uncertainty due to the difficulty in measuring such low values (Rowe et. al., 2009). During Stage II, the geomembrane retains its mechanical properties although the antioxidants are depleted.
The beginning of the decline in physical performance of the geomembrane marks the beginning of Stage III. Stage III ends when the mechanical properties of the geomembrane are decreased to 50% of the original values.
The length of Stage III can depend on which physical property is measured (Rowe et. al., 2009). Albeit arbitrarily, the service life of the geomembrane is considered to be the sum of the time it takes the geomembrane to complete the three stages. Using the aforementioned criteria to predict the service life of geomembranes, exposed geomembranes in pond applications in four different climatic zones have been verified to remain functional after more than 30 years (Tarnowski and Baldauf, 2006).
A more complete method for evaluating the remaining service life of exposed geomembranes was presented by Peggs (Peggs, 2008). It was noted in this article that the surface layer of the geomembrane will become depleted of antioxidants before the core of the geomembrane because of its greater exposure to UV.
As the surface layer loses its tensile strength and is exposed to expansion and contraction due to temperature fluctuations, cracks can begin in that weak surface layer. Due to the nature of HDPE as well as the increase in density with time, cracks in the surface layer will quickly propagate through the core of the geomembrane, resulting in a breach through the geomembrane.
This can occur before an OIT test registers the complete depletion of antioxidants, since the core of the geomembrane, which forms a significant portion of the geomembrane sample tested, still retains antioxidants (Peggs, 2008).
A forensic study of a pond in Calgary, A.B., Canada, vividly illustrates the unfolding of this process (Rowe et. al., 2003).
The composite-lined pond was decommissioned after a 14-year service life. When the pond was drained, the presence of leachate under the geomembrane creating a “waterbed” effect showed the failure of the geomembrane portion of the composite liner.
Geomembrane samples were taken from the various exposure conditions (on the floor under layer of sludge, under level of leachate, in the anchor trench, above the level of leachate, etc.). In addition, borehole samples of the 3m-thick compacted clay liner were taken and sampled at various depths to assess the extent of leachate migration.
Contaminant transport modeling was used in conjunction with the measured chloride leachate concentrations throughout the clay samples to assess when the geomembrane ceased functioning. It was concluded that the geomembrane stopped functioning between 0-4 years after installation. It is believed that maintenance activities in the pond may have created some of the observed defects in the geomembrane.
Testing and scrutiny of the samples of geomembrane with varying degrees of exposure revealed the progression of geomembrane failure as the geomembrane material aged. As expected, the samples representing the fully exposed condition showed more deterioration in mechanical properties than the other samples.
Interestingly, the OIT values for the covered conditions were in the range of five minutes, while the OIT values for the exposed conditions were less than two minutes. Cracks were observed in the geomembrane on the slopes where it was fully exposed and the antioxidants were basically depleted but not under the level of the leachate where low levels of antioxidants were still present.
Therefore, the defects creating the leakage through the geomembrane were not likely due to the aging of the geomembrane material. This is supported by the early failure date indicated by the extent of leachate migration through the compacted clay liner.
Site 1 contained eight stormwater ponds of varying sizes, with whales present during the wet season in several of the ponds, indicating a failure of the geomembrane. A visual assessment of the geomembrane material showed no obvious signs of ageing.
A total of 2,074 holes were located during the liner integrity survey and the holes were recorded with the type of damage encountered:
- 53% of the holes found were due to equipment damage, including gouges in the geomembrane that matched the exact geometry of the squeegees used to clean the geomembrane every year, as shown in Figure 1.
- questionable activities were witnessed by liner integrity survey operators, such as driving equipment over the geomembrane, as shown in Figure 2.
Site 2 is visually inspected on an annual basis, typically by the same engineer. In 2010, cracking appeared in many of the extrusion welds, which had not been present the previous year, as shown in Figure 3. This is likely due to the particular extrudate resin or extrusion welder settings used for the repair patches installed in response to the findings of a previous inspection.
In general, the overall geomembrane quality appeared to retain its mechanical properties. The few areas of stress cracking in the geomembrane sheet were located in the heat affected zones adjacent to extrusion welds above the level of the leachate, as shown in Figure 4.
Little has been documented regarding the long-term performance of geomembranes other than HDPE (Koerner et. al., 2005).
The approximately 15-year-old reinforced polypropylene liner at Site 3 may have entered Stage III of its service life as indicated by the color of the geomembrane and the holes encountered with exposed reinforcement fibers where the resin portion had completely degraded, as shown in Figure 5.
Aside from the evidence of ageing, many of the holes encountered were caused by angular road base that had rolled into the pond from the surrounding access road. Poor pond design did not provide a restraining device for loose road gravel immediately adjacent to the anchor trench. The gravel on the pond bottom then punctured the geomembrane when maintenance personnel walked in the pond and inadvertently stepped on the rocks.
Maintenance activities in ponds can cause more damage than benefit as annual cleaning activities are carried out. It has been previously emphasized that pond design should include maintenance procedures and a plan to ensure the longevity of a geomembrane containment system well after the date of construction (Rowe et al., 2003, and Tarnowski and Baldauf, 2006).
This article further demonstrates the necessity for such measures. At the minimum, pond cleaning procedures should be provided to site owners as part of the design and installation of their pond. As part of these procedures, prohibited activities in geomembrane-lined areas should be detailed. In addition, perhaps an improved pond design should include a more puncture-resistant geomembrane material such as PVC. Most importantly, the value of performing a liner integrity survey as part of construction and regular pond performance evaluation cannot be underestimated.
It is an unfortunate reality that containment systems required by law to be installed with an impermeable lining system appear to be failing years after their installation, while service life predictions on paper remain in excess of 30 years. Although a useful tool in evaluating the remaining service life of the geomembrane material, material property tests such as the OIT test taken from a few swatches of geomembrane cannot evaluate the performance of an entire lining system.
A complete pond performance evaluation must contain an additional tool such as a geoelectric leak location survey, the state-of-the art, field-proven method of locating holes in existing geomembranes.
Abigail Beck, P.E., M.S., a project engineer for Ausenco Vector, has eight years of solid-waste engineering and more than 50 million sf of geoelectric liner integrity survey experience; firstname.lastname@example.org.
Geomembrane performance–Part 2: In the August/September issue of Geosynthetics, “Patch extrusion welding as a geomembrane failure mechanism.”
ASTM D3895. “Standard test method for oxidative-induction time of polyolefins by differential scanning calorimetry”, Annual Book of ASTM Standards.
ASTM D5885. “Standard test method for oxidative-induction time of polyolefin geosynthetics by high-pressure differential scanning calorimetry”, Annual Book of ASTM Standards.
Hsuan, Y. G. and Koerner, R. M., (1995), “Long-term durability of HDPE geomembrane: Part 1–Depletion of antioxidant”, GRI Report 16, 35.
Hsuan, Y. G. and Koerner, R. M., (1998), “Antioxidant depletion lifetime in high density polyethylene geomembranes”, Journal of Geotechnical Geoenvironmental Engineering, Vol. 124, No. 6, pp. 532-541.
Koerner, Robert M., Hsuan, Y. Grace and Koerner, George R., (2005), “Geomembrane Lifetime Prediction; Unexposed and Exposed Conditions”, GRI White Paper #6.
Peggs, Ian, (2008), “How long will my liner last?”, Geosynthetics, Vol. 25, No. 5, p. 56.
Sangam, Henri P. and Rowe, Kerry R., (2002), “Effects of exposure conditions on the depletion of antioxidants from high-density polyethylene (HDPE) geomembranes”, Canadian Geotechnical Journal, Vol. 39, pp. 1221-1230.
Rowe, Kerry R., Sangam, Henri P., and Lake, Craig B., (2003), “Evaluation of an HDPE geomembrane after 14 years as a leachate lagoon liner”, Canadian Geotechnical Journal, Vol. 40, pp. 536-550.
Rowe, R.K., Rimal, S. and Sangam, H., (2009), “Ageing of HDPE geomembrane exposed to air, water and leachate at different temperatures”, Geotextiles and Geomembranes, Vol. 27, pp. 137-151.
Tarnowski, C. and Baldauf, S., (2006) “Ageing resistance of HDPE-geomembranes – Evaluation of long-term behavior under consideration of project experiences”, Geosynthetics, J. Kuwano & J. Kosaki (eds), Millpress, Rotterdam, N.L.D., pp. 359-362.