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

GIROUD et al. D Optimal Configuration of a Double Liner System
is a composite liner (Column B, Figure 2b). Each of the four pairs of cells (1A - 1B, 2A
- 2B, 3A - 3B, and 4A - 4B) corresponds to one of the four cases defined in Section 3.1.
The first of these four pairs of cells is related to the case presented in detail in Section
3.2. Each of these eight cells comprises four lines: one line for the result of each of the
four steps defined in Section 2.1.
Comparing Columns A and B in Table 1, it appears that calculated values of the rate
of leachate migration into the ground (i.e. the rate of leachate migration through the
secondary liner, q 2 ) are smaller when the primary liner is a composite liner and the secondary
liner is a geomembrane liner than with the inverse configuration.
4 DISCUSSION AND CONCLUSIONS
4.1 Discussion of the Results
4.1.1 Liner System Configuration
The purpose of this paper was to compare two configurations of double liner systems:
(i) a double liner system where the primary liner is a geomembrane and the secondary
liner is a composite liner; and (ii) a double liner system where the primary liner is a composite
liner and the secondary liner is a geomembrane. In the double liner systems used
for the comparison, the composite liner consists of a geomembrane on a GCL (Figure
2). The comparison was performed by calculating the rate of leachate migration due to
geomembrane defects. The results of the calculations, presented in Table 1, show that
the rate of leachate migration into the ground is significantly less with the second configuration
(Figure 2b) than with the first (Figure 2a). Therefore, if a design engineer has
to choose between the two double liner systems described above, it may be recommended,
on the basis of the comparison presented in this paper, to use the composite
liner as the primary liner and the geomembrane liner as the secondary liner. However,
this recommendation is mostly applicable to the case where the composite liner consists
of a geomembrane on a GCL. If the composite primary liner consists of a geomembrane
on a layer of compacted clay, compaction of the clay component of a primary liner may
damage the underlying geosynthetics, i.e. the geonet secondary leachate collection system
and the geomembrane secondary liner, which would likely cause more leachate
migration through the secondary liner.
It should be remembered that, as mentioned in Section 1, the comparison presented
in this paper is based on advective flow calculations. A different ranking of the liner
systems might have been obtained if other migration mechanisms (such as diffusion)
had been considered.
4.1.2 Magnitude of Leachate Migration Rate
The calculated rates of leachate migration into the ground (q 2 in Table 1) are extremely
small. The largest value of q 2 calculated, 1.42 × 10 -17 m/s, is equivalent to 5 × 10 -3
liters per hectare per year, which should cause virtually no pollution. However, it should
be remembered that mechanisms of contaminant migration other than advective flow
have not been considered in this study. Nevertheless, it is clear that double liner sys-
386 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4