Overview
The paper investigates the performance and alterations of a Geosynthetic Clay Liner (GCL) after 12 years of exposure in a composite liner system at a landfill in central California. The study focuses on understanding the effects of prolonged exposure on the GCL’s hydraulic and geochemical properties.
The paper "Hydraulic and Geochemical Characteristics of a Geosynthetic Clay Liner Exhumed from an Exposed Composite Liner" was authored by Thomas R. Williams, Craig H. Benson, Kuo Tian, Nazli Yeşiller, and James L. Hanson.
Key Findings
· Types of GCL Tested
o The study involved samples from a single type of GCL supplied by one manufacturer. The GCL consisted of granular sodium bentonite encapsulated between two geotextiles bonded by needlepunching.
· Hydraulic Conductivity Variability
o Hydraulic conductivity of the exhumed GCL samples varied significantly, ranging from 9.7 × 10^-12 m/s to 2.6 × 10^-6 m/s.
o Samples from the top of the slope exhibited low hydraulic conductivity (~10^-11 m/s), while those from the toe showed higher values (~10^-8 m/s or higher), indicating greater permeability and degradation due to prolonged exposure.
· Swell Index (SI) and Cation Exchange
o The SI of the exhumed bentonite varied between 3 mL/2 g and 24 mL/2 g, compared to 32 mL/2 g for virgin GCL.
o A significant drop in SI was observed downslope, attributed to the replacement of sodium (Na) by calcium (Ca) and magnesium (Mg) ions from the subgrade.
o GCL samples with SI < 15 mL/2 g showed high hydraulic conductivity (>10^-7 m/s), while those with SI > 15 mL/2 g maintained lower hydraulic conductivity (<6.0 × 10^-11 m/s).
· Bentonite Erosion and Thinning
o Extensive bentonite erosion was noted, particularly at the toe of the slope, leading to thinning of the GCL. This contributed to increased hydraulic conductivity despite some samples retaining high SI values.
· Thermal Gradients and Moisture Migration
o Diurnal thermal gradients caused moisture migration within the liner system, leading to condensation at the geomembrane-GCL interface. This migration contributed to bentonite erosion and moisture accumulation downslope.
· Implications for Design
o The findings emphasize the need to protect GCLs from prolonged exposure to atmospheric conditions to prevent degradation.
o Proper covering and maintenance of composite liners are crucial to maintaining GCL integrity.
o Understanding the interaction between GCLs and subgrade materials is vital to prevent cation exchange and maintain low hydraulic conductivity.
o The study suggests a combined parameter of SI and mass per unit area (SI × MPA) to ensure low hydraulic conductivity.
Design Implications for GCLs
· Protection from Exposure:
o To prevent degradation, composite liners should be covered promptly to avoid prolonged exposure to thermal cycling and moisture migration.
· Material Selection and Testing:
o High-quality bentonite with a high swell index and sufficient mass per area is essential.
o Site-specific testing should be conducted to assess potential cation exchange impacts from the subgrade.
· Thermal Management:
o Implement strategies to manage thermal gradients to reduce moisture migration and associated bentonite erosion.
· Ongoing Monitoring and Maintenance:
o Regular monitoring and maintenance of liner systems are essential, especially in areas with significant water level fluctuations or exposed conditions.
It is worth highlighting that granular bentonites (like the one tested) are more susceptible to erosion and their performance in erosive conditions (exposed composite liner systems) becomes more reliant on the sodium (Na) content. It also means GCLs that are granular are more susceptible to cation exchange when in the presence of calcium. Powder bentonites are better in comparison for maintaining impermeability and factory properties in erodible and higher calcium environments. There are also factors such as the quantity and quality of the polymer used (if any) to enhance the bentonite's stability under these conditions. Some believe adding polymer masks the low content of sodium in some bentonites, but in many cases, polymers improve long-term bentonite performance, particularly when the right polymer is added in the correct ratio.
Conclusion
The study provides valuable insights into the performance and degradation mechanisms of GCLs exposed to atmospheric conditions over an extended period. These findings highlight the importance of protecting GCLs, selecting high-quality materials, and understanding site-specific interactions to ensure long-term performance in landfill and other containment applications.
By addressing these factors, engineers and designers can improve the reliability and durability of geosynthetic clay liners, promoting safer and more effective environmental management practices.
and durability of geosynthetic clay liners, promoting safer and more effective environmental management practices.
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