30 Sep 01
(2) Existing Cover. Existing cover materials should be evaluated to determine cover thickness,
material types, and overall conditions.
(3) Landfill Gas. Soil gas surveys or soil gas probes installed on the landfill surface can be used
to determine if the landfill is emitting methane, hydrogen sulfide, or other gases. Empirical methods or
estimates from soil gas surveys can also be used to estimate landfill gas emission rates.
(4) Landfill Composition and Excavatability. It is necessary to determine the composition of
the waste materials for the design of any landfill cover system. The information is used to calculate
potential cover settlement, determine landfill gas emission rates, and evaluate if excavation into the waste
materials is feasible because of contaminant levels or potential health and safety concerns. When waste
regrading or other intrusive work is necessary, the excavatability of the waste material should also be
evaluated. Information required depends on site-specific conditions and could include material types,
water table elevation, leachate levels, moisture content, and site stratigraphy.
(5) Leachate. In some situations, leachate may migrate laterally and exit at the surface of the
existing landfill. The surface of the landfill should be inspected and leachate seepage exit areas surveyed
and mapped. Drainage features to collect this seepage may need to be installed below the landfill cover
before it is constructed. The leachate should be characterized, as its composition may affect the
selection of leachate collection materials and subsequent leachate disposal.
(6) Ground water. It is necessary to define water levels, gradients, flow direction, and ground
water chemistry in all water-bearing units in the vicinity of the landfill cover. Ground water investigations
are usually done during the RI phase. Typically, one or more wells are placed up-gradient, and several
wells are placed down-gradient of the site. The wells are used to determine if the site is contaminating
the ground water. Monitoring well data should be reviewed during the pre-design phase to see if
additional wells are needed.
(7) Foundation Soils. The foundation soils should be characterized by determining material
types and extent, water content, density, depth to ground water, etc. In addition, disturbed and
undisturbed soil samples should be collected to determine specific geotechnical engineering properties.
The type of sample and amount of material required depends on the type of test that will be used.
d. Geotechnical Laboratory Tests. Geotechnical laboratory tests are required to assess the
suitability of borrow sources and to establish soil properties for use in stability, settlement, and drainage
analyses. Typical tests are described in the following paragraphs.
(1) Classification Testing. Classification tests consist of sieve and hydrometer analyses,
Atterberg limits, and moisture content testing. These tests are used to select borrow sites for cover
materials and to design filter and drainage layers. Classification tests are conducted along with
consolidation and shear strength tests on the foundation soils beneath the landfill to determine if stability