The most critical components of a landfill cap are the barrier layer and the drainage layer. The barrier (sealing) layer can be low-permeability soil (clay) and/or made from geosynthetic clay liners (GCLs).
A flexible geomembrane liner may also be required and if so, is placed on top of the barrier layer. In addition for stoney sub-soil materials a protection geotextile “blanket” may be needed, over and/or below the flexible geomembrane liner.
The soils used as barrier materials are usually clays that are compacted to a hydraulic conductivity no greater than 1 x 10-9 m/sec for UK landfills generally, but less stringent permeabilities may be justifiable and may be used where acceptable to the Environmental Regulator (UK – EA).
Compacted sealing layers are generally installed in 200mm minimum lifts to achieve a thickness of 600 mm or more.
Many people talk of using a composite capping system. A composite cap/barrier uses both soil (clay usually) and a geomembrane, and making best of the advantages of the properties of each. A geomembrane when installed is essentially if intact so impenetrable by water to be thought of as impermeable, but if it geomembrane barrier develops a leak, the soil component provides a second line of defence and prevents significant leakage into the underlying waste.
The purpose of landfill capping is to shield humans and the environment from the harmful effects of the landfill contents and limit the migration of the contents by reducing inflow of water from the surface, and greatly reducing gas escape. A cap will always restrict surface water infiltration into the contaminated landfill contents to reduce the possibility of contaminants leaching from the site after landfill closure.
A Landfill capping Landfill Capping is the most common form of remediation because it is generally less expensive than other technologies and effectively manages the human and environmental risks connected with a remediation place. That being said, the process of capping is still very expensive
Hydrogeological studies must be carried out to guarantee the drainage of any water that, by running between the membrane and soil mass, can reduce to zero the soil/membranes’ coefficient of friction. If such a situation was present the soil mass overlying the impermeable membrane would become unstable at many landfils subject to slippage in a veneer fashion.
The Geosynthetic material known as “Pozidrain” may be able to provide these functions with higher performance and lower cost than conventional crushed stone filters. Also, Pozidrain is also used by landfill operators who want to gain the revenue from every last once of waste into their landfill before landfill closure. The thickness of this material makes this possible as it is much thinner than a stone layer. This allows more waste to be put in the landfill before the planning consented top of site levels are attained.
Pozidrain has been specially designed to be compatible with both HDPE and clay liners and to give the optimum performance over the whole life of the landfill closure capping. Pozidrain will enhance the performance of the GCL or HDPE liners by providing an additional barrier that prevents the majority of the water or gas reaching the liner.