Immediately following 2014’s 10th International Conference on Geosynthetics (September 2014, Berlin), a high-level, one-day workshop on exposed HDPE geomembranes convened at the headquarters of BAM, Germany’s Federal Institute for Materials Research and Testing. The event, Berlin 2, brought together field luminaries who represented not only the international practice but multiple perspectives from geosynthetics: polymer and additive package development, manufacturing, testing, design, and CQA.
Here, we begin to republish material from Berlin 2 as we near the 6-year anniversary of the gathering and the two year mark since the last notes were added to shape the final report.
BERLIN 2 FINAL REPORT
Berlin 2 served as a successor to a similar workshop held at BAM in the mid-1990s. Some of the same participants from the original workshop were present for Berlin 2. The group’s goal: discuss the creation of a protocol for better anticipating the end of (service) life of an installed, exposed high-density polyethylene geomembrane.
The group’s technical notes serve as a final report is now available openly as a PDF download and the participants welcome field discussion.
PARTICIPANTS
Dr. Ian D. Peggs (I-CORP INTERNATIONAL) and Helmut Zanzinger (SKZ) served as moderators. Werner Müller (BAM) hosted. Dr. JP Giroud oversaw the discussion and helped steer the group towards its actions.
Additional participants and presenters (and their affiliations in 2014) include John Cowland (independent consultant), Mike Sadlier (independent consultant), Fred Gassner (Golder), Sebastian Hausmann (SKZ), Daniel Tan (Solmax), Hyun Jin Koo (Fiti), Chris Kelsey (Geosynthetica), Robert Kienzl (OFI), Mauricio Ossa (GSE), Catrin Tarnowski (GSE), Amr Ewais (Queen’s University, Canada), Ana Noval (CEDEX), Markus Grob (BASF), Montse Garcia (Dow), José Miguel Muñoz (Sotrafa), and Vera Olischläger (NAUE).
Contributing but unable to attend due to late developments were Han-Yong Jeon (INHA University) and John Scheirs (ExcelPlas).
BERLIN 2 FINAL REPORT – SECTIONS 1.0 – 3.0
Editor’s note: More sections of the Berlin 2 technical notes will be published on Geosynthetica. We include here the Introduction (1.0), Objectives of Workshop (2.0), and Differentiation between Buried and Exposed Applications (3.0). Additional sections address the Participants (4.0), Workshop Invitation Outline (5.0), Individual Introductory Abstracts (6.0), Morning Discussion Session (7.0), Summary of Morning Session (8.0), Possible Monitoring Tests (9.0), JP Giroud’s Summary of Items Discussed in Morning Session (10.0), Afternoon Discussion Session (11.0), Overall Research Work to Be Done (12.0), JP Giroud’s Summary of Items Discussed in Afternoon Session (13.0), Specific Action Items (14.0), Workshop Summary (15.0), Acknowledgments (16.0), and Documentation (17.0). – Chris Kelsey
TECHNICAL NOTES FROM BERLIN 2
1.0 Introduction
Nearly all engineered systems have a finite service life. This includes geomembrane (GMB) barriers (liners) as part of the system for the containment of valuable product (potable water, gold solutions, etc.) and contaminated/obnoxious liquids/solids (coal ash, meat processing waste, etc.). All exposed high-density polyethylene (HDPE) GMBs will degrade by oxidation in service and fail by quasi-brittle stress cracking. Clearly, facility owners need to know when a liner will no longer perform its containment function.
On 26 September 2014, the day after the end of 10th International Conference on Geosynthetics (ICG) in Berlin, Germany, a workshop of international experts was held at the Federal Institute for Materials Research and Testing (BAM) to discuss “development of a protocol to determine the remaining service life of an already exposed HDPE geomembrane liner”. This followed two shorter workshops on the same topic at EuroGeo 5 in Valencia, Spain (2012) and at Geosynthetics 2013 in Long Beach, USA.
2.0 Objectives of Workshop
The ultimate workshop objective was to develop a protocol that would describe the actual condition of an exposed HDPE GMB, to predict its remaining service life, i.e. the time before the GMB can be expected to fail, and consequently needs to be repaired or replaced to prevent such failure. The discussion was limited to HDPE because of its globally common use in major sectors such as waste and mining and because its durability characteristics are better understood than those for other liner materials.
A skeleton protocol to catalyze the discussion was proposed as follows, based on the assumption that final failure of HDPE GMB would occur by surface oxidation and stress cracking:
- Measure oxidative induction time (OIT) to assess whether or not oxidation protection is still available.
- Measure carbonyl index (CI) to assess extent of any oxidation that has occurred.
- Determine if there is a critical CI at which stress cracking (SC) is initiated.
- Measure stress cracking resistance (SCR) to determine the time until SC is initiated.
Note well that the intent was to discuss barriers already installed that have been exposed to the environmental synergisms that cannot possibly be reproduced in the laboratory. It was not to discuss accelerated testing on virgin materials in an artificially generated environment to determine their expected service lives.
3.0 Differentiation between Buried and Exposed Applications
3.1 Buried GMBs
- Mechanical stresses in GMBs occur only during installation (under “ideal conditions”) and if we ignore folded wrinkles (according to “North American practice”).
- After installation, stresses in GMBs relax and GMBs are practically “stress-free” (under “ideal conditions”).
- The classical “3-stage aging model” (antioxidant {AO} depletion time + induction time + polymer degradation resulting in a modification of the mechanical behavior of the GMB) described by Hsuan & Koerner (1998) is reasonable because the service life, often presented as time to reduce mechanical properties, defined with an acceptance criteria of the original value is sufficient for an ideally designed and ideally installed, buried GMB.
However, buried GMBs are not the topic of this workshop!
3.2 Exposed GMBs
- The full designed stress (or more) may occur at any time – even after many years in service! Therefore, End of Life (EoL) should be based on 100% retained or design-based strength as 100% of the expected stress may occur at any time.
- “2-stage model” (AO depletion time + induction time) is relevant:
- A reduction of mechanical properties cannot be considered as part of the service life of an exposed GMB.
- First cracks occur in the surface after oxidation of the surface
(OIT is close to zero at the exposed surface of the GMB)
Mechanical properties are close to 100% (e.g. in case of phenol based stabilization).
3.3 Examples of functions of exposed GMBs
- Snow pond (Example 1):
GMB is as a barrier to leakage because leakage water can lead to instability of the earth dam surrounding the snow pond. - Water canal for a hydropower station (Example 2):
GMB is used as a barrier to water loss, as leakage will reduce the efficiency of the hydropower station. - Containment of acid or soda in ponds and reservoirs in mining (Example 3):
GMB is specified as a barrier because groundwater or environmental pollution from unlined leakage would represent inefficient (i.e. less profitable) operation. - Tailing reservoirs in mining (Example 4):
GMB is specified as a barrier because leakage from an unlined facility would be detrimental to groundwater or the environment, or such leakage would lead to financially inefficient operations. Regarding tailings ponds, many of them are now designed to bury the geomembrane under a volume of material that may make monitoring impractical or impossible.
It was originally expected that there would be much discussion on the difficulty of making meaningful comparisons between parameters measured on the full thickness of the GMB and parameters that assess surface layer properties. For instance, to compare CIs (typically measured on a surface layer) and OITs, the OITs should also be measured on surface layers and not on full thickness GMB specimens as is usually done. The OIT of a surface layer may be zero while that measured on the full thickness could be quite significant.
Similarly, should GMB specimens used for SCR tests be notched on the exposed/oxidized side or the opposite unexposed side? Or should all GMB specimens for SCR tests be made from melted material and solidified plaques? Or, should a test on an un-notched specimen be performed?
The most serious discussion was on a definition of EoL, for without a definition of EoL, a remaining service life cannot be determined. This varied from appearance of the first stress crack, through appearance of the first penetrating hole (but holes can be repaired, to extend service life), and even to the need for each application to have its own EoL. Certainly most people felt that a more practical EoL than time to 50% of a retained parameter could be developed.
More sections of the Berlin 2 Technical Notes (Final Report) will be published on Geosynthetica soon. Download the full report (PDF) here.
See also: Original article on Geosynthetica on Berlin 2 (October 2014)
