Liner Systems

Liner systems are containment elements constructed under the waste to control infiltration of contaminated liquids into the subsoil or groundwater. The contaminated liquid, or leachate, may be part of the waste itself or may originate from water that has infiltrated into the waste.

Liner systems consist of multiple layers which fulfill specific functions. The description presented below refers specifically to llandfill liner systems. However, the main characteristics of liner systems are similar for other applications. Landfill liner systems may consist, from top to bottom, of the following functional layers:

1.Protective layer
This is a layer of soil, or other appropriate material, that separates the refuse from the rest of the liner to prevent damage from large objects.

2.Leachate collection layer
This is a high-permeability layer, whose function is to collect leachate from the refuse and to convey it to sumps from where it is removed. Frequently the functions of the protective layer and the leachate collection layer are integrated in one single layer of coarse granular soil.

3.Primary liner
This is a low-permeability layer (or layers of two different low-permeability materials in direct contact with each other). Its function is to control the movement of leachate into the subsoil.

4.Secondary leachate collection layer or leakage detection layer
This is a high-permeability (or high transmissivity, if geosynthetic) layer designed to detect and collect any leachate seeping through the primary liner. This layer is used only in conjunction with a secondary liner.

5.Secondary liner
This is a second (or backup) low-permeability layer (or layers of two different low-permeability materials in direct contact with each other). Not all liner systems include a secondary liner.

6.Drainage layer
In cases where the liner system is close or below the water table, a high-permeability (or high transmissivity, if geosynthetic) blanket drainage layer is generally placed under the liner system to control migration of moisture from the foundation to the liner system.

7.Subbase
This layer is generally of intermediate permeability. Its function is to separate the liner system from the natural subgrade or structural fill.

These layers are normally separated by geotextiles to prevent migration of particles between layers, or to provide cushioning or protection of geomembranes.

As indicated above, liner systems may have a primary liner only or may include primary and secondary liners. In the first case it is called a single-liner system, and if it has a primary and a secondary liner it is called a double-liner system. Also, each of the liners (primary or secondary) may consist of one layer only (low-permeability soil, geomembrane, or GCL) or adjacent layers of two of these materials, in which case it is called a composite liner. There are multiple combinations of these names, some of which are given below as examples (obviously there are many more combinations):

• Single synthetic liner: primary liner only, consisting of a geomembrane.
• Single soil liner: primary liner only, consisting of a low-permeability soil layer.