Application of geomembranes in landfills
Solution Introduction
In landfill engineering, geomembranes (especially HDPE high-density polyethylene geomembranes) are hailed as the "core of the seepage control system." Their application is mainly concentrated in two major systems: the bottom liner and the capping.
Structural Layers of Geomembranes in Seepage Control Systems
Landfills typically employ composite seepage control liner systems to ensure that leachate does not contaminate groundwater.
Horizontal Seepage Control (Bottom):
Main Liner: Typically uses a 2.0mm thick HDPE geomembrane. This is the core barrier against leachate.
Secondary Liner: In a double-liner system, a second geomembrane serves as a backup, with a leachate monitoring layer sandwiched in between.
Capping (Top):
Capping Liner: Uses a 1.0mm-1.5mm thick HDPE or LLDPE (linear low-density polyethylene) geomembrane to prevent rainwater infiltration and the generation of more leachate, and to block the escape of landfill gas (LFG).
Construction Specifications
I. Core Reference Standards:
ASTM D4437: Geomembrane Field Joint Integrity Assessment Standard.
GRI GM13: The most authoritative industry manufacturing and construction specification for HDPE geomembranes.
II. Construction Process Flowchart
[Flowchart of geomembrane installation: Preparation -> Laying -> Seaming -> Testing -> Anchoring]
Subgrade Preparation:
Smoothness: Verticality deviation should not exceed 2cm/㎡.
Substrate Condition: Sharp stones, tree roots, or standing water are strictly prohibited.
Compaction: Typically requires ≥95% (Standard Proctor).
Placement:
Overlap Width: Typically 100mm–150mm.
Slack: 1%–4% expansion/contraction should be allowed to accommodate thermal expansion and contraction (especially in high-temperature regions like Saudi Arabia).
Direction: On slopes, it should be laid perpendicular to the slope direction, and joints should avoid stress concentration areas at the toe of the slope. Welding Specifications:
Double-track Fusion: Used for long, straight seams. An air cavity must be formed for subsequent pressure testing.
Extrusion: Used for complex shapes (e.g., pipe perforations, patches).
Ambient Temperature Requirements: Typically between 5°C and 40°C. For temperatures above 40°C in the Middle East, shading or nighttime operation is required to prevent excessive stretching of the membrane.
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Working Principle
1. The Impermeability Principle: This is the most fundamental function of geomembranes, based on molecular-level barrier properties.
Extremely Low Permeability: The permeability coefficient of HDPE geomembranes (such as the 2.0mm membrane produced by Geoleed) is typically less than 1.0 × 10⁻¹³ cm/s. This means that if a landfill layer relies solely on compacted clay for seepage prevention, leachate may penetrate 1 meter in just a few years; however, with HDPE geomembranes, it could take thousands of years.
Continuous Sealing: Through thermoforming welding technology (double-track extrusion welding), multiple membranes are joined into a large, seamless whole, forming a container similar to a "super plastic bag" that encloses the entire landfill area.
2. Composite Liner Principle: In practical engineering, geomembranes rarely work alone. They are usually used in conjunction with GCLs (Geosynthetic Clay Liners) or Compacted Clay Liners (CCLs) to achieve a synergistic effect ("1+1>2").
Intimate Contact Effect: If there are tiny pinholes in the membrane (which are difficult to completely avoid during construction), water will attempt to pass through. However, if the membrane is tightly adhered to GCLs or clay, the close contact prevents water from spreading laterally between the membrane and the soil layer, limiting leakage to a very small point and significantly reducing the risk of seepage.
Repair Mechanism: When used with GCLs, the bentonite inside expands upon contact with water, automatically filling the tiny punctures in the geomembrane, thus achieving self-healing.
3. Slope Stability and Interface Friction Principle: Landfill slopes are often very steep (sometimes reaching 1:3 or 1:2), where the working principle of the geomembrane involves interface mechanics.
Mechanical Interlocking: On slopes, if a smooth geomembrane is used, the overlying soil will slide down like a slide. Therefore, textured geomembranes must be used on slopes.
Principle: The protrusions on the textured surface (created through spinning or corrugating) generate strong mechanical interlocking forces with the adjacent geotextile (protective layer) or soil particles, increasing the interface friction angle.
This ensures that the entire lining system will not slide or collapse due to gravity during the landfilling process.
4. Chemical & Gas Barrier Principle for Landfill Gas Generation Control: In capping applications, geomembranes (commonly LLDPE) work by blocking the uncontrolled escape of **landfill gases (mainly methane and carbon dioxide)**. They force the gases to flow into a dedicated collection network for power generation or flaring.
Chemical Stability: Because landfill leachate is highly corrosive (containing complex acids, alkalis, salts, and heavy metals), geomembranes utilize the chemical inertness of their polymers to avoid chemically reacting with these pollutants during their 50-100 year service life.
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