sure covers is provided in EPA SW-867. More detailed
quate protection, provided the upper 6 inches is no
design criteria and procedures are provided in EPA
coarser than sand and free of rock, fractured stone,
600/2-79-165 and EPA SW-873.
debris, cobbles, rubbish, and roots. A drainage layer
(3) The low permeability layer includes a
selected to meet the requirement for bedding material
minimum 2-foot-thick soil liner and a synthetic
can be used above the liner.
membrane at least 20 mil thick.
(f) Where surface slopes are 3:1 or steeper,
selection, and construction procedures for both synthetic
geotextile fabrics are recommended for placement over
and clay liners are provided in paragraph 6-2.
the synthetic liner. Heavy geotextile fabrics >, 12 oz/yd
(a) soils suitable for the lower liner are native
are increasingly being used in combination with flexible
clay materials, or soils blended with clay, bentonite, or
membrane liners in hazardous waste units to protect the
other additives, which can exhibit, when placed on a firm
base, a recompacted permeability of 41 x 10-7 cm/sec.
are used to protect synthetic membranes, it is important
The soil liner should generally fall into the CL/CH Unified
that they, like the synthetic membranes, be tested for
Soil Classification System, with not less than 50 percent
compatibility with hazardous waste. However, many
by weight passing a No. 200 sieve (U.S. Standard), a
such fabrics are made of polypropylene or polyester
liquid limit between 35 and 60, and a plasticity index
materials and may have compatibility characteristics
above the "A" Line in the plasticity chart of the USCS.
similar to those exhibited by liners of the same materials.
Any additive which increases the soil's vulnerability to
(g) Care must be taken to avoid any penetration
cracking by settlement or excessive shrinkage should be
of the liner. Where inlets or outlets are required (e.g., for
an impoundment), inflow/outflow piping should be
(b) Achievable field densities for cover soil liners
designed to go over the top whenever possible. Energy
are generally less than for base liners, because waste fill
dissipaters may be needed at the pipe inlet/outlets.
areas provide a softer, more flexible construction
Where penetrations cannot be avoided, precautions
subgrade. The designer should obtain laboratory tests of
must be taken to ensure an adequate seal between the
the permeability of representative soil liner samples
liner and any unavoidable penetration. In such cases,
remolded to achievable field densities at moisture
flange-type connections should be considered. EPA
contents greater than optimum to establish construction
SW-870 outlines procedures for sealing between the
procedures for low in-place permeability of the soil liner.
liner and any penetration.
(c) The designer should specify moisture
(h) EPA requires that the liner must also be
conditioning requirements, the thickness of soil layers for
protected from damage by sudden changes in slope; to
compaction, the type and weight of equipment, and the
prevent liners from freezing, they must be located
number of equipment passes required to achieve the
entirely below the frost line. Procedures are provided in
required density/permeability and avoid flexural cracking
during placement. The constructed soil liner shall be
(4) The drainage layer must be at least 12
protected from drying until placement of the synthetic
inches thick, exhibit a permeability of >1 x 10-3 cm/sec,
membrane. Spraying with water or application of an
and be able to carry infiltrating waters to the sides of the
emulsion to prevent drying may be necessary.
cover for discharge.
(d) The EPA guidance documents specify a
(a) The designer should carefully evaluate the
synthetic liner at least 20 mil thick; demonstration of the
drainage layer for its ability to carry waters for discharge,
liner's compatibility with the waste or leachate is not
and the need for a synthetic fabric filter or graded
required in this case, because the liner is not expected to
granular layer to prevent plugging due to infiltration of
be in contact with waste or leachate. Nevertheless, liner
soils from the vegetated soil cover layer. Measures
selection should be based upon its resistance to the
should be considered to preclude piping of the drainage
waste present and to degradation, as well as its ability to
layer at discharge areas.
undergo deflection due to settlement without cracking or
(b) Selection of a clean sand (SP), which
exhibits the required permeability and is able to carry the
(e) The synthetic liner must be protected both
volume of infiltrating water, will not only satisfy the
above and below by a layer of material no coarser than
bedding requirements for the synthetic liner, but may
sand. Sands should be classified as either SW or SP by
also eliminate the need for a granular layer to prevent
the USCS, with less than 5 percent passing the No. 100
plugging; nevertheless, a synthetic fabric filter should be
sieve. In addition, sands which act as filters must meet
considered to ensure the long-term effectiveness of the
filter graduation requirements, such as those shown in
chapter 5 of TM 5-820-2. The synthetic liner can be
(c) Although the EPA guidance documents
placed directly on the soil liner with ade-
indicate drainage collection devices are not necessary, a
perforated drainage collection pipe to intercept and