Environmental Engineering Reference
In-Depth Information
16.8. Composite liners
Composite liners are now commonly used as standard liner systems. They
consist of a geomembrane (usually high-density polyethylene, HDPE) overlying a
mineral barrier (usually a CCL or, in some cases, a GCL). In the case of a composite
liner, the geomembrane provides the primary resistance to advective contaminant
flow and diffusion of some contaminants. The clay component of the composite
liner (CCL or GCL) serves to reduce leakage through any holes or defects in the
geomembrane and also provides some attenuation of contaminants (e.g. volatile
organic compounds, such as dichloromethane, toluene, benzene, etc.) that can
diffuse through an intact geomembrane (see [SAN 01] for more details).
The geomembrane component of a composite liner is essentially impervious to
liquid flow when devoid of holes or defects. Water can still move through the
geomembrane by diffusion, but the water transmission rates are very low. Effective
hydraulic conductivities corresponding to water diffusion are on the order of 10
-14
m/s to 10
-15
m/s for most geomembranes [GIR 89a, GIR 89b]. However, defects in
the geomembrane can occur even with carefully controlled manufacture and damage
can be found even in sites where a strict construction quality program has been put
in place [BOU 02]. For example, Rollin
et al
. [ROL 99] indicated that despite the
implementation of good quality assurance control programs on 10 sites surveyed in
France and Canada, leaks were detected during the installation phase. The recent
papers on leak detection surveys are indicating that most of the damage occurs after
installation.
A recent survey conducted by Nosko and Touze-Foltz [NOS 00] summarized the
results of electrical damage detection systems installed at more than 300 sites and
covering more than 3,250,000 m
2
. This survey showed that the majority of damage
(71%) was caused by stones (Figure 16.24), followed by heavy equipment (16%).
Interestingly, most of the failures (78%) were found to be located in the flat areas of
the liner (bottom liner, see Figure 16.25); only 9% of perforations were found at the
corners and edges of the landfills. Although defects in the geomembrane (tears, cuts,
etc.) generally result from construction activities, sometimes they can also occur due
to poor manufacturing. McQuade and Needham [MCQ 99] presented the results of
an extensive survey of 111 leak location surveys conducted on landfills and other
geomembrane-lined facilities in the UK. It was found that 48% of these sites (53
sites) had no defects. It was noted that relatively few defects were detected in seams,
indicating improved welding methods, testing and quality assurance control of field
seams over the past decade. The bulk of the defects were caused during placement
of cover materials. Table 16.2 gives some examples of the types of holes or defects
that a geomembrane can suffer and their possible causes.
