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

GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects
1 INTRODUCTION
1.1 Leakage Collection Layer
Many landfills, especially those containing hazardous waste, are lined with a double
liner system. This paper addresses the design of the drainage layer, called “leakage
collection layer”, located between the two liners, i.e. the primary liner located above
and the secondary liner located below the leakage collection layer. The purpose of the
leakage collection layer is to collect the leachate that migrates (“leaks”) through the
primary liner and to convey it toward collector pipes. The collector pipes then convey
the leachate to a sump where it is removed from the landfill. The leakage collection layer
also allows detection of liquid migrating (“leaking”) through the primary liner, and
as a result it is also called “leakage detection and collection layer”. It should not be
called “leak detection layer” since it does not detect individual leaks.
1.2 Leachate Flow
1.2.1 Description of the Flow
To reach the leakage collection layer, leachate first flows through a defect in the primary
liner (Figure 1a). In this paper the only mechanism of leachate migration through
the primary liner that is considered is advective flow through defects in the liner. Phenomena
such as permeation or diffusion of leachate or its constituents through a liner
are not considered in this paper.
Leachate flow is governed by the hydraulic gradient. After it has passed through a
defect in the primary liner, the leachate flows more or less vertically through the leakage
collection layer upper part, which is unsaturated (Figure 1a). When the leachate reaches
the saturated part of the leakage collection layer, it flows downgradient, which for a
small fraction of the leachate consists of first flowing upslope then turning gradually
to flow downslope with the rest of the leachate (Figure 1b). As a result, the leachate
flows only in a portion of the leakage collection layer called the wetted zone. The
boundary of the wetted zone has approximately the shape of a parabola (Figure 1b),
which will be demonstrated in Section 2.2.
1.2.2 Definition of the Two Cases, “Not Full” and “Full”
The case described above and illustrated in Figure 1 is the usual case where the leachate
phreatic surface in the leakage collection layer is not in contact with the primary
liner. In this paper, this case is referred to as “the case where the leakage collection layer
is not full”. The case where the leachate phreatic surface in the leakage collection layer
is in contact with the primary liner is shown in Figure 2. In this paper, this case is referred
to as the case where the leakage collection layer is full in a certain area around
the primary liner defect, or more simply “the case where the leakage collection layer
is full”. In this paper, both cases will be analyzed: the case where the leakage collection
layer is not full and the case where the leakage collection layer is full.
216 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4