TM 5-814-5
with perforated pipes that carry the leachate to a
strands are crossed to form a grid. It can serve as a
collection point. Figure 3-7 shows examples of
protective layer or a drainage media. When placed
collection systems.
over the liner, it protects the liner from gravel or
(c) Underlying the leachate collection system
crushed stone in the drainage layer and can replace
must be a composite barrier consisting of a flexible
all or part of the drainage layer. Geotextile fabric
membrane liner (FML), over a minimum depth of 2
must be placed between the soil cover and the
feet of compacted soil having a hydraulic
drainage layer and also between the soil or drainage
layer and the geonet.
(2) Pipe Location. Pipe location and placement
most cases, a layer of clay with a minimum 3-foot
depth should be specified. While the USEPA has
are critical to leachate collection system perfor-
established a minimum FML thickness of 60-mil for
mance. Failure of the collection system may result
high density polyethylene (HDPE), and 30-mil for
from crushing or displacement of pipe caused by
other FML materials, a greater thickness is
equipment loading and/or differential settling. A
recommended. The type of material and its
pipe is best protected when it is placed in a trench,
thickness must be selected based on the conditions
with careful consideration given to proper bedding
anticipated. HDPE is more chemically resistant and
and to solid waste loading conditions over the pipe.
stronger than PVC and other types of material due
The trench provides added protection for the pipe,
to its rigidity. However, PVC and more flexible
especially during placement of the first lift to waste
materials conform better to settling and large
when the pipe is most susceptible to crushing.
(3) System Redundancy. Design redundancy is
deformations in the underlying material than does
the rigid HDPE. Liner manufacturers should be
important to minimize the effect of any single
consulted to determine acceptable materials.
failure. The system should be able to remove
Generally, more than one material will be
leachate from any point in the facility by more than
acceptable for each design. Figure 3-8 shows a
one pathway, such as through the gravel bedding in
typical liner system.
the trench as well as the piping system. One of the
(d) Liner Placement. Where settlement is a
primary ways to provide redundancy is to design
concern or the underlying soil contains coarse
collection laterals so that drainage requirements can
material, a geonet layer can be used as a cushion
be met by the gravel layer alone if flow through the
under the liner. Geonet is a net-like product where
pipe is restricted. Collection laterals are pipes and
3-6
Previous Page
