Design considerations regarding the interface shear strength of geomembrane liners in mining applications (Lupo, 2009)

1. Importance of Interface Shear Strength

• Geomembrane liners are widely used in heap leach pads, tailings storage facilities, ponds, and covers.

• The interface shear strength between the geomembrane and the surrounding soils/geotextiles controls liner system stability, especially on steep slopes.

• Both interfaces — geomembrane with underliner (bedding soil) and overliner (cover/drainage material) — must be assessed.

  
  

2. Testing Is Critical (Not Generic Values)

• Interface friction varies widely (from 3° to 53°).Designers cannot rely on literature values — project-specific testing is essential.

• Standard test: ASTM D5321 (Direct Shear Test).

• Peak and residual strengths are recorded.

  • Residual values are more appropriate for design, since localised slip or settlement during operation often reduces friction to residual levels.

  
  

3. Residual vs Peak Shear Strength

• Peak shear strength is achieved at small displacements but is not stable long-term.

• Residual shear strength accounts for the realistic, post-slip condition of the interface.

• Experience in mining (e.g., heap leach pad failures) confirms the importance of using residual strength values in stability analyses.

4. Design Implications for Slopes

• Many mining facilities are built on steep terrain (slopes >26°, or 50%).These slopes are often steeper than the residual friction angle (20–25°) of common interfaces.

• Designers must either:

  • Increase interface shear strength (by material selection or modification), or

  • Flatten slopes to reduce shear stresses.

  
  

5. Case Study: Clay Bedding vs Blended Soil

• Tests on smooth and textured LLDPE vs native clay gave very low residual friction (5.8–6.6°) — far below safe design thresholds.

• Solution: Blend clayey liner bedding (high plasticity, PI=40) with silty sand (40% sand + 60% clay).Result: residual friction increased to 27.6° while maintaining low permeability (< 10⁻⁶ cm/s).

• Lesson: Bedding soils must balance friction and hydraulic performance.Designers can improve interface shear strength by reducing plasticity or introducing granular components.

6. Key Design Considerations for Engineers

1. Material Pairings: Test both smooth vs textured liners and all relevant bedding/cover options.

2. Drainage Conditions: Wet or saturated drainage layers can reduce interface strength — test under realistic moisture.

3. Compaction & Moisture: Laboratory specimens should mimic field compaction and water content.

4. Slope Angles: Compare slope inclination directly with residual friction angles.

   • Slopes steeper than residual friction require engineered measures (e.g., soil blending, geogrid reinforcement, benches).

5. Data Use: Develop a linear or power-law shear strength function (shear vs normal stress) from residual test data for stability modelling.

   
   

7. Practical Guidance for Designers

• Always specify project-specific direct shear tests on both interfaces (liner bedding and overliner).

• Select bedding soils not just for low permeability, but also for sufficient friction.A clay with high plasticity may require modification or blending.

• Use residual shear parameters (not peak) in slope stability analyses — especially for heap leach pads and tailings dams where loading is non-uniform.

• Textured geomembranes often provide better interface friction than smooth liners, but still require testing.

• Target residual friction angles that exceed slope angles + FS (e.g., >1.3 factor of safety).

8. Actionable Recommendations

• For steep slopes (>1:3), design should include:

  • Blended bedding soils (clay + sand mix) to raise friction to 25–30°.

  • Textured geomembranes or geocomposite layers for friction improvement.

  • Drainage layers that avoid excessive fines (reduce lubrication effects).

  • Bench/berm design to break up continuous slope height.

• Verify all interface shear values post-construction if materials differ from design assumptions.

  
  

Key Takeaway for Designers

Liner stability is interface-driven. Use residual interface shear strength values, test all materials under site-specific conditions, and design bedding/overliner soils to optimise both friction and hydraulic performance. Never rely on generic friction data — low friction angles (<10°) are possible with clayey bedding and can cause failure unless addressed. If introducing coarse soils we can reduce import soil costs (clay) and protect the liner with a cushion geotextile that enhances clogging capability along with massively improved friction on slopes. That's 4 benefits from one concept. Cheaper and more durable liner system!