TM 5-814-7
(a) Selecting design slopes which will minimize
carry water from the drainage layer to surface drainage
the damaging effect of settlement, i.e., use 4 percent
facilities may be a better alternative than granular
construction slopes for upper surfaces over fill areas
drainage discharge areas.
where settlements can be expected to be uniform, due to
(5) The soil layer for vegetation should be a high
placement procedures and a uniform depth of fill, and
quality topsoil at least 2 feet thick, and capable of
use 10 to 33.3 percent slopes (10:1 to 3:1 horizontal to
sustaining vegetation.
vertical slopes) over perimeter and interim fill areas,
(a) The vegetation must be a persistent but
where the depth of fill increases significantly due to the
shallow-rooted species which will minimize erosion, while
perimeter excavation, and can result in settlements
not penetrating below the vegetative and drainage layers
which decrease the construction slope by 10 percent or
(EPA SW-867 and EPA 600/2-79-165). The vegetated
more (see figure 6-10).
soil layer must also have an erosion rate of < 2.0 tons
(b) Using uniform fill placement and solidification
per acre per year using the US Department of Agriculture
procedures which minimize differential settlement and
Universal Soil Loss Equation (USLE). This equation and
enable prediction measurements for the order of
data for its use are described in EPA SW-867 and EPA
settlement that can be expected after closure.
600/2-79-165.
(c) Staging final closure to delay placement of
(b) As noted, steeper perimeter slopes must be
final cover where substantial settlement is expected
provided with surface drainage systems capable of
(may require an extension in the 180-day limit for
conducting run off across the slope without damaging the
closure, and placement of an expendable interim cover).
vegetated soil cover. Stability against slippage under
(4) Design slopes should be selected to allow for
saturated or seismic conditions must also be
any settlement. Final slopes should be at least 3 percent
demonstrated.
to prevent ponding due to irregular surface areas, but
c. Design considerations. Because hazardous
less than 5 percent to prevent excessive erosion.
waste fills can undergo settlement, and any damaging
Perimeter slopes may be steeper, but must be provided
effect of settlement on final cover must be repaired
with surface drainage systems capable of conducting run
during the post-closure period, the designer should
off across the slope without forming erosion rills and
assess the potential for uniform settlement of the waste
gullies. Steeper slopes must be evaluated for stability
fill, recommend operating practices which minimize
against slippage under saturated or seismic conditions,
differential settlement, and select construction slopes
and for acceptable resistance to erosion.
which minimize the damaging effect of settlement.
(1) Settlement of waste fills generally occur due to
6-8. Special design elements
(a) Mechanical consolidation: a decrease in void
a. Regulatory requirements. Regulations within
space related to applied load(s) of the fill and soil cover
sections of 40 CFR 264 establish design, construction
and their depth.
and maintenance requirements for structural integrity of
(b) Biological decomposition: a decrease in
impoundment dikes, overtopping controls, and wind
volume by loss of solids.
dispersal controls.
Requirements related to air
(c) Displacements: differential
settlements
emissions have not been established, but are expected
which result from liquefaction of saturated layers, creep
to be developed in the future by EPA. The specific
of the waste fill, and/or collapse of drums placed prior to
regulations are summarized in table 6-7.
the ban of such practice.
b. Design considerations for dikes. Since dikes
(2) In new facilities, where design procedures
are the principal containment components of surface
minimize foundation settlement, and placement
impoundments and are partially or completely above
procedures minimize differential settlement of the fill,
ground, it is essential that they be designed, constructed
consolidation of the waste fill will be the primary source
and maintained with sufficient structural integrity to
of settlement. The potential for settlement should be
prevent failure. Dike slopes must be stable at all times,
analyzed for the following conditions: compression of the
especially during rapid drawdown of waste liquids; they
foundation and compression of the waste due to
must also be protected against erosion due to wave
dewatering, liquefaction, primary and secondary
action, wind, rain or animal intrusion. Dikes must be
consolidation, biological oxidation of organics, * and
designed so that excessive stresses are not put on the
chemical conversion of solids to liquids. EPA SW-873
foundation.
provides current state-of the-art design information to
(1) To accomplish these goals, the designers
determine settlement, and additional studies are being
must evaluate the materials of construction, liner type(s),
performed for EPA.
weather factors, loads imposed by wastes, drainage
(3) The following provisions should be
systems, and the hydrologic and geotechnical
considered to minimize damage by anticipated
characteristics of the site. Analyzing the stability of the
settlement:
pro-
* Calculate assuming one pound of organic matter will be
destroyed for each two pounds of oxygen consumed
in a BOD5 test.
6-25
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