The presentation by Drew Crossley and Attila Marta from REE discusses observations and lessons learned from over 100 geosynthetic-lined structures, emphasizing the importance of design, construction, and quality assurance in ensuring the long-term performance of geomembranes (GMBs). Here’s a detailed summary highlighting the key points and figures:
Intent of the Presentation
The authors, with extensive experience in various aspects of geosynthetic-lined structures, aim to share issues observed in lining systems that have been operational for 5 to 20 years and discuss how these issues can be addressed during the design and construction phases.
Examples of Geosynthetic Lined Structures
Single Composite HDPE Lined Coal Seam Gas (CSG) Dams
Double Composite HDPE Lined CSG Dams
Bituminous Geomembrane (BGM) Lined Ponds
BGM Erosion Protection
HDPE Bladders
Challenges and Issues Observed
UV Degradation and Wind Loads: Exposed geomembranes are subject to UV degradation and wind stresses.
Fluctuating Fluid Levels: Trapped stresses occur due to fluctuating fluid levels, potentially leading to system failure.
Temperature Variability: Daily temperature changes of 20-25°C and annual changes of 50°C impact the integrity of geomembranes.
Chemical Exposure: Stored solutions vary widely, including raw water, tailings decant solutions, CSG water, concentrated brine, caustic solutions (pH14), and acid mine drainage.
Stress Cracking in the Heat Affected Zone (HAZ)
Stress cracking is a significant issue, particularly in the heat-affected zones of welds. This can be initiated by cyclic movement, constant bending, and tension.
Cyclic Loading: Day/night temperature fluctuations cause movement in geomembrane wrinkles, leading to stress cracking.
Bending: Stress cracks often occur at transitions, such as from crest to batters with a slope of 1V:4H or 1V:3H.
Excess Stress and What Can Be Done
Weld Quality: Ensuring consistent, high-quality welds is crucial, yet challenging given the current reliance on discrete, destructive tests.
Use of White Geomembranes: White GMBs reduce material temperature, thermal expansion, and the risk of stress cracking in exposed applications.
Covering Geomembranes: While this can reduce stress, it may not always be economically feasible.
Interim Anchoring: Anchoring across or down a batter can limit geomembrane movement due to thermal expansion, reducing trapped slack from water level fluctuations.
Storage and Handling Damage
Proper storage and handling are critical, as damage during these processes can lead to long-term risks. Examples include damage from dragging rolls, creating hard-to-detect defects in the liner system.
Design Evolution
Rub-Sheets: The design of rub-sheets has evolved to prevent issues such as trapped water, with newer methods like reverse stitch welds reducing problems.
Under Liner Environment
Issues such as evaporation, condensation, swelling, and erosion under the liner can lead to washout and other damage. High points and low points in the liner can exacerbate these issues, causing significant damage over time.
Summary
Geomembranes and their designs have significantly evolved over the past 15 to 20 years. However, continued evolution in design, construction practices, and quality assurance processes is necessary to address field issues.
Weld Quality: Remains a critical factor in geomembrane performance. Ensuring a consistent, high-quality weld is essential, and designers need to specify these qualities effectively.
Material-Specific Specifications: As geomembranes evolve, weld specifications must also evolve to be material-specific, ensuring long-term durability and performance.
This presentation underscores the complexities involved in the design, installation, and maintenance of geomembranes, highlighting the need for ongoing improvement in practices to ensure their effectiveness over time.
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