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 4.2.6.18 Liner materials include soils and, in particular, clay soil,
admixed liners, flexible polymeric membranes, sprayed-on linings, soil
sealants, and chemical absorptive liners. The purpose of the liner is to
prevent the movement of water and its associated contaminants through the base
of the landfill and into the underlying formations. The liner may be fabri-
cated in one of two ways. It may be constructed onsite, such as when soil
materials are placed and compacted, or it may be a manufactured flexible
membrane placed on the site during construction.
4.2.6.19 Many landfills have had clay liners placed at their base.
Clay minerals may be kaolinite, illite, or montmorillonite. A typical clay
will contain one or more of these clay minerals and possibly will be mixed
with other fine-grained soil materials such as silt. Clay minerals have a low
hydraulic conductivity and therefore will significantly retard the movement of
any leachate through them. Permeabilities for most soils containing greater
than 25% clay are in the range of 10-8 cm/s to 10-5 cm/s.
4.2.6.20 Clay liner thicknesses of 5 ft (1.5 m) or greater have been
required at some sites. The success of the clay liner will not only depend
upon its original characteristics, but also upon the method of liner installa-
tion. Best results are achieved by placing several individual layers or
lifts. Each lift is compacted before the next layer is placed. The degree of
compaction achieved will be a function of the compacting equipment, the
thickness of the lift, and the moisture content of the soil. The soil
moisture content is a significant factor. Soil that is either too dry or wet
will be less than optimum for compaction.
4.2.6.21 Admixed liners are formed-in-place liners. These include
asphalt concrete, soil cement, soil asphalt, and bentonite clay liners. The
liners are formed by mixing the lining material with the natural soil at the
base of the landfill. The resultant mixture hardens or modifies the charac-
teristics of the soil material to provide a low-permeability barrier. Each of
these approaches has been employed successfully in the lining of impoundments.
4.2.6.22 Flexible polymeric membranes are manufactured materials that
are 0.020 to 0.120 in. (0.51 to 3.0 mm) thick. The liner material is
manufactured in rolls that are 48 to 96 in. (1.2 to 2.4 m) wide and hundreds
of feet long. A uniform bedding material, such as sand, is placed at the base
of the landfill prior to the installation of the liner. The particle size of
the subgrade material usually should be less than 3/4 in. (19 mm). The base
grade on the liner should be a minimum of 2% if gas release from materials
beneath the liner is anticipated. Gas vents may also be necessary in order to
adequately allow release. The liner is installed by unrolling the sheets of
plastic and then using specialized equipment to form bonded seams between the
individual sheets. The liner is usually extended up the side of the landfill
to the ground surface where it is anchored.
4.2.6.23 Soil sealant and chemically absorptive liners are two other
approaches that have been used on a limited basis for retarding the movement
of the materials through the base of impoundments. These approaches are also
being tested for use in landfills.
4.2.6.24 Groundwater. Groundwater pollution hazards are determined
by examining annual fluctuations in the depth of the water table. A site
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