30 Sep 01
classified by the Unified Soil Classification System (ASTM D 2487).
(b) Design Criteria. Material type and gradation requirements should be specified to prevent
the use of highly plastic soils and to ensure compatibility with the underlying filter layer. In addition, a
maximum particle size should be specified to protect against puncture or other damage to underlying
geosynthetics. The maximum particle size specified will typically range from 9.5 mm (0.375 in.) to 25
mm (1 in.), depending on the type of geosynthetics present immediately beneath the select fill. The use
of angular material should also be avoided because of increased potential for damaging geosynthetics.
The ultimate choice of a select fill material will depend on the availability of economical borrow sources.
The EPA recommends that the total thickness of the cover soils should be a minimum of 600 mm (24
in.) or equal to the maximum frost depth, whichever is greater. Constructibility issues are critical when
placing select fill. Select fill should be placed starting at the toe and advancing up the slope. Placing fill
in a top-down fashion can induce surface slope failures and tension in the underlying geosynthetics. The
first layer of select fill should be placed in a loose lift 380 to 460 mm (15 to 18 in.) in thickness to
protect underlyng geosynthetic materials. Select fill should be placed on geosynthetics by dropping from
a height no greater than 900 mm (36 in.). To further protect geosynthetics from damage, low ground
pressure (57 psi) equipment should be utilized on the first lift of select fill. After the initial lift, soils can
be compacted to a specific density; however, the compaction from placement equipment is usually
sufficient and will enhance the soil's ability to support vegetation. Where temporary haul roads are
located over the geosynthetic components of the cover system, it is recommended that a minimum
thickness of 760 to 920 mm (30 to 36 in.) should be maintained.
d. Filter Layer--Geotextile.
(1) General. A filter layer is normally required between the select fill and the underlying drainage
layer. The filter layer ensures consistent drainage properties by preventing migration of fine-grained soil
particles into the void spaces of the drainage layer below. For a landfill cover, the filter is normally a
geotextile fabric; however, a series of graded granular materials could also be used. The majority of
geotextiles used in cover systems are made from polypropylene or polyester polymers. The design of a
geotextile filter depends on a number of factors, as discussed in the following paragraphs.
(2) Design Criteria. Guide Specification CEGS-2373 should be used in contract documents
when specifying geotextiles for separation/filtration. AASHTO M 288-96 and Koerner (1998) provide
detailed design information about the geotextile properties listed in the following paragraphs.
(a) Soil Retention. A geotextile filter must be designed to prevent the migration of soil
particles from the select fill into the drainage layer. To accomplish this, the openings in the geotextile
fabric must be small enough to retain the soil on the top side of the fabric. In the design of the geotextile
filter, the coarser sized soil particles must be retained. Although some fines will initially pass through the
filter, the coarse particles will eventually block further losses of finer particles if the soil is well graded.
There are a number of methods available to evaluate the retention capability of a geotextile filter. All of
these methods select an appropriate geotextile by comparing the particle size distribution of the soil to a