Page 369 - Materials Chemistry, Second Edition

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340 Waste Management Practices: Municipal, Hazardous, and Industrial
(6 in.). A mixture of dense-rooted grasses and legumes is recommended for establishment on this
layer. Erosion-related soil loss should not exceed 1.8 metric tons (2 tons) per acre per year to min-
imize long-term maintenance. To attain such a level of erosion control typically requires the con-
struction of slopes less than 1:4 and drainage swales placed at 6 m (20 ft) vertical increments.
Erosion from the effects of water is kept under control by vegetation and also by hardening the
cover surface using stones or riprap (U.S. EPA, 1994).
Erosion control maintenance includes routine vegetation management (such as mowing, fertil-
ization, liming, and replanting), repair of any areas undergoing subsidence, and run-on or runoff
control. Sedimentation basins and drainage swales must be inspected after every major rainstorm
and be repaired as needed.

10.5.4 GAS COLLECTION SYSTEM
A minimum of one passive gas vent per acre of cover should be installed to allow for the release of
gas pressure beneath the cover. The gas venting system can use vertical gravel walls, blanket col-
lectors (beneath the barrier layer), or gravel trench drains (also beneath the barrier layer) to collect
landfill gases. The collected gases are routed through the cover using vent pipes as shown previ-
ously in Figure 10.18.

10.5.5 THE LANDFILL CAP
Slope stability and soil erosion are important concerns in the design and installation of landfill
caps. The landfill cover slope must be sufficiently stable to sustain infiltration and runoff from a
24-h, 25-year storm. Side slopes are typically 1:3 to 1:4, and the friction between adjacent layers
must resist seepage forces. On slide slopes, composite liner caps (membranes placed directly
above a low-permeability soil layer) are not advisable (Vesilind et al., 2002). For slopes steeper
than 1:5, a drainage layer should be provided. If sliding occurs, liner systems will be damaged, soil
may enter surface waterways, and the cover will need to be repaired or rebuilt.
Two different cover systems are depicted in Figure 10.29. See also the photos in Figure 10.30.

10.5.6 SUBSIDENCE EFFECTS
Landfill subsidence can be large-scale (‘global’ due to uniform settlement of MSW) or localized
(e.g., collapse of a large void directly below a portion of the cover). In general, global subsidence
does not result in excessive tensile strains on the cover and improves the stability of the cover by
reducing sliding. Localized subsidence, however, can produce depressions on the cover that can cre-
ate excessive strain in cover layers and can cause ponding of water. The impact of tensile strains is
minimized using a flexible geomembrane composed of PVC, low-density polyethylene, or
polypropylene. Ponding of water must be avoided because it can kill or distress cover vegetation,
and the weight of the water can cause expansion of a pond on the cover (U.S. EPA, 1994).

10.5.7 WEATHER EFFECTS
The cover must withstand extreme weather conditions and function with minimal maintenance. The
extreme weather conditions for which a final cover should be designed include heavy rains, extreme
drought, and ground freezing. Cover management for heavy rains includes growing dense-rooted
vegetation, maintaining a modest slope, and constructing adequate conveyances for excess runoff.
Extreme drought is another consideration during the design of the erosion control layer. Certain
plants are more drought-tolerant and should be included in the original seed mixture. Periodic irri-
gation with water and leachate may be required. Freezing of the cover is a concern because of the
impact of freezing on clay permeability. Repeated cycles of freezing and thawing increase the per-
meability of compacted clays by causing large cracks to form. Damage to the clay layer is difficult