landfill geomembrane

Landfill Geomembrane

1  What a Landfill Geomembrane Is — and Which Polymer You May Use

A landfill geomembrane is the low-permeability barrier in the composite liner of a sanitary landfill: it keeps leachate out of groundwater and rainfall out of the waste mass. The material is almost always high-density polyethylene — HDPE, or LLDPE where flexibility is needed. Chinese practice names it directly: GB/T 51403-2021 §3.1.2 lists the permitted liner materials and HDPE geomembrane stands first, and the only product standard referenced for it is CJ/T 234 (HDPE) or CJ/T 276 (LLDPE for landfills). EVA is not on that list. EVA softens at lower temperature, swells in solvent-rich liquids and has weaker environmental stress-crack resistance — the three properties leachate attacks hardest — so it is never accepted as the primary barrier of a landfill, whatever its thickness. This note therefore describes the HDPE / LLDPE membrane that landfill work actually specifies. HDPE is chosen because it combines a permeability below 1×10⁻¹³ g·cm/(cm²·s·Pa), yield strength 22 N/mm at 1.5 mm, resistance to the full pH range of leachate, and carbon-black stabilisation that gives a design life of decades under cover.

2  Manufacturing Process

Three-layer co-extrusion. Virgin HDPE resin (density ≥ 0.940 g/cm³, MFI ≤ 1.0 g/10 min at 190 °C / 2.16 kg) is compounded with about 2.0–3.0 % carbon black plus antioxidant, UV absorber and thermal stabiliser — the carbon black is the UV screen, so its content and dispersion are controlled as specification items, not as colour. The melt is extruded through a flat die as a core with two skin layers, passed over a chill roll, thickness-gauged on line, trimmed and wound. Textured product is made by co-extruding a rough skin (sprayed / embossed, 0.3–0.6 mm profile) on one or both faces, or by laminating a nonwoven. Roll width is normally 6.0–8.0 m and length 50–150 m, so a 6.0 m width minimises field seams — GB/T 51403 §3.2.1 asks for not less than 6.0 m for that reason.

3  Advantages

▪ Absolute barrier: hydraulic conductivity effectively that of the polymer itself, independent of compaction, moisture content or settlement — the failure mode of a clay liner;

▪ Deformation tolerance: CJ/T 234 sets ≥ 700 % break elongation and ≥ 300 h environmental stress-crack resistance, so the membrane follows differential settlement of the waste mass without tearing;

▪ Mechanical toughness: yield strength 22 N/mm and puncture 480 N at 1.5 mm resist gravel, stone and construction traffic before the protection geotextile is placed;

▪ Ageing resistance: decades of service under cover; 1600 h UV exposure retains ≥ 50 % of standard OIT;

▪ Chemical resistance: resistant to leachate over the full pH range, to the organic acids and to the salts present in MSW leachate — the property EVA and PVC do not have;

▪ Weldable and testable: wide rolls (6.0–8.0 m) mean fewer seams; seams are heat-fused and every metre can be pressure-tested, giving a verifiable rather than assumed barrier.

4  Specifications & Stock — CJ/T 234, Smooth Sheet

Table 1  CJ/T 234-2006 physical and mechanical requirements, smooth HDPE geomembrane (MD and CMD)

5  Where Each Grade Goes — Thickness by Location

Table 2  Location → grade, thickness and the governing clause

6  Installation Points

▪ Subgrade first: clear roots, stone and debris; ≥ 93 % compaction at the bottom, ≥ 90 % on slopes, surface evenness within 20 mm; no horizontal seam on slopes steeper than 10 % or within 1.5 m of the toe.

▪ Anchorage and sequence: anchor trench at least 800 mm × 800 mm and at least 800 mm back from the slope crest, with rounded rather than right-angled corners; anchorage platforms where the height step exceeds 10 m. Lay slope first then bottom, from low to high, seams parallel to the slope direction, allowing about 1.5 % slack — never stretched.

▪ Seaming: overlap 100 ± 20 mm for double-track hot wedge, 75 ± 20 mm for extrusion welding; trial weld daily and after any change of weather; textured sheet needs its profile dressed back at the seam. Below 5 °C or in rain, stop.

▪ Testing: pressure test every double-track seam at 0.15–0.25 MPa and require 100 % pass; vacuum-box or spark test every extrusion seam; destructive samples at the specified rate. Then place the 600 g/m² protection geotextile immediately — an unprotected membrane is the most common failure.

▪ Repair: patch with the same resin, extending at least 100 mm beyond the defect; no patch on a patch. Repair records, seam numbers and test results form the handover file.