30 Sep 01
(1) General. Potential stability problems for a landfill cover could result from the foundation soil,
or the waste, or at cover material interfaces. The stability of a landfill is controlled in broad terms by the
Properties of the foundation soil.
Strength characteristics and weight of refuse.
Inclination of slopes.
Leachate levels and movements within the landfill.
Frictional resistance of cover material interfaces.
Ability of the cover to freely drain infiltration.
Quantification of the geotechnical properties of waste materials is very difficult; therefore, geotechnical
investigations of these materials are rarely undertaken. The following stability issues should be
addressed during design.
(2) Design Criteria.
(a) Cover Component Interfaces. The stability of geosynthetic interfaces normally controls
the design of side slopes of a cover rather than the stability of the waste fill or foundation. Cover
component stability is assessed by analyzing the frictional resistance between each adjacent layer.
EPA/625/4-91/025 describes procedures for analyzing the stability of cover systems and determining
strength requirements for reinforcement materials. Typical interface friction angles between adjacent
geosynthetics or between the geosynthetics and adjacent soils range from 8 to 25. Because of this
large variation in friction angles, it is important to have a requirement in the specifications that the
contractor conduct interface friction tests on the actual project materials and submit the results to the
designer for approval. Interface friction values should be determined on samples of the materials to be
used for construction. Placement, loading, and wetting conditions used during testing should also be
representative of field conditions. ASTM D 5321 describes the procedure used to perform interface
friction tests. Reinforcement layers can be incorporated into the design to prevent tensile forces from
developing in the geosynthetic components of the cover. Typically, geotextiles or geogrids are used for
this purpose. When reinforcement layers are utilized, the material must have sufficient tensile strength
and must be properly anchored. Benches on the landfill cover can also be used to provide additional
stability for the cover system.
(b) Factor of Safety for Cover Interfaces. Typically, a minimum factor of safety of 1.5