Amanda Klimchuk, Richard MacDonald, Wesley Ferris, Higher Ground Consulting
August 15, 2016
Executive Summary
PTAC retained HGC to investigate the impact that liner material selection, construction techniques, operational techniques, and environmental conditions may have on the performance of engineered liner systems. The investigation focuses on the specific application of synthetically lined earthen ponds in western Canadian climates. The objective of this study is to close existing knowledge gaps not currently addressed in the industry, bring consistency to produced water pit construction, and mitigate overall industry risk. The work included a literature review, case study reviews, and interviews with industry representatives, regulators, researchers, and geosynthetic vendors.
Common liner failure mechanisms include physical liner damage (particularly during operations), welding deficiencies, and stress cracking. Stress cracking represents a significant concern for many operators that were interviewed. In the case study reviews, there were two instances where stress cracking behavior was observed, despite the fact that a third-party laboratory determined the stress crack resistance of failed samples met the requirements of industry standards (GRI-GM13).
The root cause of stress cracking is generally related to compounding factors, rather than one single independent factor. Potential contributing factors include the following:
Each of these factors can contribute to the mechanism of failure. Cold-weather conditions are of particular concern as they exacerbate the brittle nature of most plastics, like HDPE. Welding in cold temperatures can be of particular risk as the temperature variances can elicit thermal shock, resulting in additional stresses which may lead to stress cracking. Fortified liners may help achieve desired performance characteristics based on the expected chemical constituents in the stored water. These may be used in tandem with composite liners to provide an effective and reliable form of containment. LLDPE is also a good alternative to consider in installations where stress cracking is of potential concern, as LLDPE’s appear to be less susceptible to stress cracking.
Research has shown that during installation, issues with welded seams are responsible for nearly 80% of leaks, with 61% of these leaks related to extrusion welds (Nosko et al. 1996, 2000). General liner damage caused during operations were associated with the majority of post-construction liner failures based on industry interviews. Based on available research, very few instances of leaks and defects have been associated with liner installation projects with a rigorous QA program (Giroud & Bonaparte 1989). Similarly, experience shared by industry members indicate significantly less holes, defects, and failures following implementation of third-party QA. Post-installation QA/QC tests assist in identifying liner defects and serve as an additional risk mitigation as they may be conducted at any point in the lifecycle of the pond.
In Alberta, Directives 55 and 58 provide guidelines for the design, construction, and operational requirements associated with storing process-affected waters in Alberta. The AER’s application process follows a risk-based, case-by-case approach whereas the BC OGC has issued a document titled Management of Saline Fluids for Hydraulic Fracturing Guideline, which follows a more prescriptive approach. Saskatchewan regulates produced water under Information Guideline 97-01, which notes a preference towards steel for primary containment over synthetics, as “punctures and tearing problems have been experienced in the field with synthetics as primary liners” (IG 97-01).
Improperly investigated or constructed pond earthworks can lead to settlement issues that place the geosynthetic liner under stress and may ultimately lead to failure. Generic specifications, such as those provided by the GRI, are often appropriate for routine, low-risk projects but may not be adequate for the storage of aggressive solutions including brine. Appropriate geotechnical investigation, site selection, engineering design with double-liner systems and groundwater management, the inclusion of construction QA/QC programs, and contractor prequalification can dramatically reduce the likelihood and consequences of liner failure. Limiting access to lined areas, managing liquid levels, frequent leak detection monitoring, and ongoing inspections are good practices to incorporate to further reduce the frequency and severity of liner failure.
Moving forward, further research is recommended in the following areas:
# 15-WIPC-09