Landfill Liners and Caps
To minimize the amount of primary leachate that has to be treated and disposed of, geomembranes have also been introduced to cap landfills, both to prevent the ingress of precipitation and to capture and remove landfill gas (LFG).
While HDPE has also been used in caps, the preferred material is often linear low density polyethylene (LLDPE) or another more flexible material such as PVC or flexible polypropylene (fPP) that will better accommodate the strains and resultant stresses that are associated with differential settlements within the waste. Thus, the geomembrane must be able to conform to the profile of the differential settlement without inducing stresses that could, for instance, initiate stress cracking. The less crystalline materials such as LLDPE and fPP are not susceptible to SC in the as-manufactured condition. This requirement also clearly demonstrates that the ductility of a geomembrane is far more important than its strength in relation to long-term performance. Strength is only of importance in being able to handle the rigours of installation.
In order to stop liners and caps from sliding down slopes they are often textured or structured to enhance friction with the underlying soil or non-woven geotextile on GCLs and geocomposites. Texturing is a random ‘rough sea’ profile, while structured profiles are engineered profiles designed to interact optimally with different mating surfaces. To keep cover soils from slipping on top of the geomembrane the upper surface may also be textured or structured.