Technical Paper by JP Giroud, KL Soderman and K. Badu-Tweneboah

GIROUD et al. D Optimal Configuration of a Double Liner System
1 INTRODUCTION
In many landfills and other waste containment systems where a double liner system
is used, the primary liner is a geomembrane and the secondary liner is a composite liner
that consists of a geomembrane on a layer of compacted clay (Figure 1a). The inverse
configuration where the geomembrane-compacted clay composite liner is the primary
liner and the geomembrane used alone is the secondary liner (Figure 1b) is rarely considered
because: (i) compaction of the clay could damage the geomembrane used as the
secondary liner (and the geonet, if one is used in the secondary leachate collection layer);
and (ii) regulations often prescribe the first configuration.
Geosynthetic clay liners (GCLs) are increasingly used instead of compacted clay in
composite liners. GCLs are usually installed with light equipment that should not damage
the underlying geosynthetics. Therefore, when a GCL is used, both configurations
can be considered: (i) the geomembrane alone as the primary liner and the geomembrane-GCL
composite liner as the secondary liner (Figure 1c); and (ii) the inverse configuration
(Figure 1d). A comparison between these two configurations is presented in
this paper.
It is realized that several criteria and approaches can be used to compare liner systems.
However, only leachate migration due to advective flow of leachate through geomembrane
defects is considered herein. The considered defects are defects in the geomembrane
used alone as well as defects in the geomembrane component of the
composite liner.
The purpose of the comparison presented herein is to provide information that may
be used as part of the process for selecting the optimal configuration for a double liner
system.
2 METHODOLOGY
2.1 Approach
The two double liner systems considered in the study are shown in Figure 2. For each
of the two liner systems, the rate of leachate migration through the secondary liner is
calculated according to the following four steps:
S A given average head of leachate on top of the primary liner is considered.
S The rate of leachate migration through the primary liner due to advective flow
through defects in the primary liner geomembrane is calculated.
S The average head of leachate on top of the secondary liner due to leachate that has
migrated through the primary liner is calculated.
S The rate of leachate migration through the secondary liner due to advective flow
through defects in the secondary liner geomembrane is calculated.
The rate of leachate migration through the secondary liner thus calculated is the rate
of leachate migration into the ground.
It is preferable to discuss leachate head calculation prior to discussing leachate migration
calculations due to numerous references to leachate head in leachate migration cal-
374 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4