AREA A/B ENGINEERING REPORT - Waste Management

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begins to be observed. Oxidation reactions then proceed slowly throughout the service life of
HDPE geomembranes and, eventually, the geomembrane will likely become brittle to the extent
that it is considered to have reached the end of its service life (Rowe & Sangam, 2002). In their
research report for the USEPA, Koerner & Hsuan (2002) select this point as the 50 percent
reduction in a specific design property, such as tensile stress at break, although they note that
even with this reduction in design property the geomembrane can still function, albeit at a
decreased performance level. With this conservative endpoint defined, the service life of HDPE
geomembranes was estimated to be on the order of a thousand years: approximately 200 years
for antioxidant depletion, over 20 years for induction of geomembrane oxidation, and 750 years
for 50 percent degradation of strength properties (Bonaparte, et al., 2002a).
Although a geomembrane may lose strength over time, Rowe & Sangam (2002) highlighted that
the real service life of a geomembrane depends on the hydraulic and diffusive properties of the
geomembrane. Thus, a geomembrane may lose strength while still performing satisfactorily as a
barrier. Accordingly, the true hydraulic and diffusive service life of a geomembrane may
significantly exceed the service life determined based on the degradation of the physical and
mechanical properties, especially if the tensile stresses are minimal. Furthermore, burial or
submersion of a geomembrane can lessen the rate of antioxidant depletion and geomembrane
oxidation by decreasing the availability of oxygen. In the case of a geomembrane liner for a
MSW landfill, biodegradation of waste will probably consume most of the available oxygen
above the liner well before the end of PCC period (perhaps even as soon as shortly after the
start of PCC).
The low-permeability soil component of a Subtitle D composite liner typically consists of a
compacted clay liner (CCL), a geosynthetic clay liner (GCL), or a GCL overlying a CCL.
Significant experience with the use of engineered low-permeability soil components in landfill
liner system designs has been gained over the past three decades. To function adequately over
its required useful life, a CCL or GCL must maintain a hydraulic conductivity no greater than its
design value during this timeframe. As discussed by Rowe (1998), provided that a GCL has been
properly designed, installed, and protected from desiccation, and provided that appropriate
attention has been given to the chemical compatibility of the low-permeability soil layer with the
anticipated leachate, the GCL should meet its hydraulic conductivity criterion for hundreds to
thousands of years when used in a composite liner in a MSW landfill. For a CCL under these
same design and construction constraints, the service life is even longer, on the order of thousands
of years. This is the reason that CCLs are used in liner systems for containment of critical wastes
(e.g., radioactive waste) and are relied upon to protect HHE over exceptionally long periods of
time (i.e., tens of thousands of years).
MD10186.doc 127 29 March 2009