Geosynthetics August/September 2019 - 20

A minitube blanket for landfill gas collection and containment

The size and weight of the waste compactor, as well as the length of the compactor
teeth, should be considered when designing the thickness of the initial waste layer
over the geocomposite.

In-place behavior

A vacuum loss
was higher in
the traditional
LFG collector as
compared with the
minitube blanket
(65% versus 58%),
indicating a better
distribution of
vacuum along
the trench by
the minitube.

For horizontal LFG collection, the flow and
the head loss in the minitube blanket are
governed by the minitubes (the head loss
in the geotextile layers being negligible). In
a first stage, the low-pressure Muller equation can then be used because the pipes of
the geocomposite follow the same physical
laws as a conveyance pipe for gas collection
(Steinhauser and Fourmont 2015).
The gas flow of the minitube is given
from its water flow using Equation 1
(Faure and Auvin 1995):
(1)
With:
Q: flow drained by the minitube
(Qw: water flow, Qg: gas flow)
qp: discharge capacity of the minitube [(qp)w for water, (qp)g for gas]
i: hydraulic gradient
(iw for water, ig for air)
α, n: constants
ρ: density (ρw for water, ρg for gas)
Compared to water, this ratio is about
28 for air, 22 for CO2 and 37 for CH4.
From Faure and Matichard (1993),
the maximum waterhead in the minitube
function of the collected flow per unit
area is given by Equation 2:
(2)
With:
Δh: waterhead (water column)
d: distance between the minitubes
F: flow of liquid collected per unit area

20

Geosynthetics | August September 2019

The flow drained by the minitube
blanket is also given by Equation 3:
(3)
Then, using Equation 1 and Equation
3 in Equation 2, the head loss in the
minitube is given by Equation 4:

(4)
Lymphea software combines these
equations to determine the flow of
gas collected by the geocomposite as
a function of the horizontal LFG collector length and the applied vacuum.
This software has been developed by
the Laboratoire Interdisciplinaire de
Recherche Impliquant la Géologie et
la Mécanique (LIRIGM) from the
University of Grenoble, France, and
validated by large-scale tests. It can be
obtained from the manufacturer.

Comparison to a
traditional trench
The performance of a minitube blanket
and a traditional horizontal LFG collector were tested side by side in the same
800-foot (245-m) refuse trench at Cedar
Hills Regional Landfill, located in Maple
Valley, Wash. (Ghofrani 2016). The traditional LFG collector was comprised of
a 6-inch (150-mm) HDPE pipe with six
0.5-inch (13-mm) perforations, 60° apart
and 6 inches (150 mm) on center. The
geocomposite was comprised of 1-inch
(25-mm) diameter corrugated polypropylene perforated minitubes needlepunched
between two nonwoven geotextile fabrics.
The tested geocomposite was 3-feet (0.9m) wide with four minitubes.
The performance of a minitube blanket and of a traditional LFG collector
Geosynthetics August/September 2019

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