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

GIROUD et al. D Optimal Configuration of a Double Liner System
tal projection of the length of the liner system in the direction of the slope; k 2 = hydraulic
conductivity of the secondary leachate collection layer material; β = slope angle of the
liner system; and λ is a dimensionless parameter defined by:
qi
q1
l = =
2 2
(2)
k tan b k tan b
2
As indicated by Giroud and Houlihan (1995), Equation 1 is valid only if λ is small
(e.g. λ < 0.01). This condition is always satisfied in the study presented in this paper.
2.3 Methods for Calculating the Rate of Leachate Migration Through Liners
2.3.1 Leachate Migration Mechanism
As indicated in Section 1, the only mechanism of leachate migration through a liner
(whether it is a geomembrane used alone or a composite liner) that is considered in this
study is advective flow through defects in the geomembrane. The defects are defects
in the geomembrane used alone as well as defects in the geomembrane component of
the composite liner.
2.3.2 Assumption
For the leachate migration rate calculations, it is assumed that the leachate head on
top of the considered liner is uniform over the entire surface area of the liner. Therefore,
the rate of leachate migration through a geomembrane defect is independent of the geomembrane
defect location in the considered liner.
The uniform leachate head is equal to the given average head for the primary liners
(see Section 2.1), and is equal to the average head calculated using Equation 1 for the
secondary liners (see Section 2.2.2).
2.3.3 Unit Rate of Leachate Migration Through a Liner
Since it is assumed in Section 2.3.2 that the leachate head is uniform over the entire
surface area of both the primary and secondary liners, the same methodology for leachate
migration calculation applies to both liners. The methodology consists of: (i) calculating
the rate of leachate migration, Q, through a geomembrane defect; and (ii) multiplying
the rate of leachate migration through a defect by the geomembrane defect
frequency, F, to obtain the unit rate ofleachate migration, q, through the considered liner:
q = F Q
(3)
where the geomembrane defect frequency, F, is expressed as the number of defects per
unit area (e.g. five defects per hectare, i.e. F =5× 10 -4 m -2 ). As a result, if Q is expressed
in m 3 /s, q is expressed in m/s. Herein, the same geomembrane defect frequency, F, will
be used for both the primary and secondary liners of a given double liner system, and
the following notations will be used for the rate of leachate migration through a defect
and the unit rate of leachate migration through a liner: Q 1 and q 1 for the primary liner,
and Q 2 and q 2 for the secondary liner.
2
378 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4