leachate flow in leakage collection layers due to defects in ...

GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects
ity, k. The laminar flow assumption is applicable to sand and approximately applicable
to gravel and geonets. Furthermore, it is assumed that the hydraulic conductivity,
k, of a given leakage collection layer material has a unique value, that is the hydraulic
conductivity of the saturated material.
S The leachate is assumed to have the same density and viscosity as water. This assumption
should be satisfied in the case of all modern landfills due to the generally low
concentration of chemicals in leachate. As a result, the values of the hydraulic conductivity,
k, of the leakage collection layer material measured using water are applicable
to leachate flow.
S The defects in the primary liner are assumed to have a small dimension in all directions
of the plane of the geomembrane so that the resulting leaks can be treated as
point source leaks. Examples of such defects are circular or quasi-circular holes in
geomembranes with a diameter (or an equivalent diameter) of less than approximately
10 to 20 mm (i.e. a surface area less than approximately 1 to 3 cm 2 ). Examples of
defects that are not consistent with the above assumption are defects with a surface
area larger than approximately 3 cm 2 and defects with a great length such as cracks
or a relatively long length of open seam in a geomembrane.
S The rate of leachate migration through a given defect is Q under steady-state flow
conditions, which are assumed to exist in all cases.
S It is assumed that flows through various defects do not interfere. In other words, the
wetted zones related to different defects do not overlap. (Cases where wetted zones
may overlap are discussed in Section 4.4.5.)
S Capillarity in the leakage collection layer is not considered (which limits the validity
of the study by excluding leakage collection layers constructed with fine sands) and
the secondary liner is assumed to be impermeable (i.e. it is a geomembrane with no
defects, possibly on a clay layer, but not a clay layer alone). Therefore, all of the leachate
that passes through defects in the primary liner flows in the leakage collection
layer.
Other assumptions will be made as required at various steps in the analysis.
2.2 AssumptionsSpecific to the Case Where the Leakage Collection Layer is not
Full
As mentioned in Section 1.2.2, two cases are discussed in this paper: the case where
the leakage collection layer is not full and the case where the leakage collection layer
is full. The former will be the lead case, and results for the case where the leakage
collection layer is full will then be derived from results for the case where the leakage
collection layer is not full.
In the case where the leakage collection layer is not full, the flow rate through the
considered defects in the primary liner is assumed to be small enough that the maximum
thickness of leachate in the leakage collection layer is less than the thickness of the leakage
collection layer. In this case, assumptions regarding the hydraulic gradient and the
shape of the phreatic surface can be made, as described below.
As indicated in Section 1.2.1, the leachate that, has passed through a defect in the primary
liner, first flows more or less vertically through the leakage collection layer upper
220 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4