leachate flow in leakage collection layers due to defects in ...
leachate flow in leakage collection layers due to defects in ... leachate flow in leakage collection layers due to defects in ...
GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects (a) Leachate phreatic surface in the leachate collection layer Leachate flow in the leachate collection layer Leachate phreatic surface in the leakage collection layer Leachate infiltration Leachate flow in the leakage collection layer Waste Leachate collection layer Primary liner with defect Leakage collection layer Secondary liner (b) Secondary liner Boundary of the wetted zone Wetted zone Defect in the primary liner Leachate flow in the leakage collection layer Figure 2. Leachate flow in the leachate collection layer, through a defect in the primary liner, and in the leakage collection layer in the case where the leakage collection layer is filled with leachate in a certain area around the primary liner defect: (a) cross section; (b) plan view of the secondary liner. wetted zone and the average head of leachate on top of the secondary liner in the wetted zone: this information is necessary to calculate the rate of leakage through the secondary liner. The case where the primary liner has several defects is treated; the defects are either randomly distributed (“random scenario”) or their location is such that the rate of leakage through the secondary liner is maximum (“worst scenario”). Finally, the 218 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4
GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects paper provides an approximate value of the time required for steady-state flow conditions to exist and the value of the time required for leachate to travel from the higher end to the lower end of the leakage collection layer. The defects in the primary liner are assumed to be small in all directions, such as holes in geomembrane liners. Therefore, the results of the study presented herein are mostly applicable to the case where the primary liner is a geomembrane used alone. However, the results of the study may also be useful for the case where the primary liner is a composite liner consisting of a geomembrane placed on a geosynthetic clay liner (i.e. a bentonite layer encapsulated between two layers of geotextile). The leakage collection layer considered in the study can be a layer of granular material (e.g. sand or gravel) or a layer of geosynthetic drainage material (e.g. geonet or geocomposite consisting of a geonet core and two geotextiles). The results of the study are particularly useful for the case of relatively thin leakage collection layers, such as those consisting of geosynthetic drainage materials. 2 ASSUMPTIONS 2.1 General Assumptions The following general assumptions are made: S The leakage collection layer, the primary liner, and the secondary liner have a uniform slope, β. The thickness of the leakage collection layer is t LCL (Figure 3). S The leachate collection layer is assumed to be a porous medium. Therefore, the flow of leachate is governed by equations for flow in porous media, such as Darcy’s equation for the case of laminar flow. Furthermore, the porous medium is assumed to be homogeneous, i.e. it does not contain large open spaces such as pipes and channels. Therefore, the flow of leachate is not treated using equations for flow in pipes and channels. S Flow in the leakage collection layer is laminar (i.e. Darcy’s equation is applicable) and the leakage collection layer material is characterized by its hydraulic conductiv- Q t LCL β Figure 3. General assumptions. GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4 219
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GIROUD et al. D Leachate Flow <strong>in</strong> Leakage Collection Layers Due <strong>to</strong> Geomembrane Defects<br />
(a)<br />
Leachate phreatic<br />
surface <strong>in</strong> the<br />
<strong>leachate</strong> <strong>collection</strong><br />
layer<br />
Leachate <strong>flow</strong><br />
<strong>in</strong> the <strong>leachate</strong><br />
<strong>collection</strong><br />
layer<br />
Leachate phreatic<br />
surface <strong>in</strong> the<br />
<strong>leakage</strong><br />
<strong>collection</strong> layer<br />
Leachate <strong>in</strong>filtration<br />
Leachate <strong>flow</strong><br />
<strong>in</strong> the <strong>leakage</strong><br />
<strong>collection</strong> layer<br />
Waste<br />
Leachate<br />
<strong>collection</strong> layer<br />
Primary l<strong>in</strong>er<br />
with defect<br />
Leakage<br />
<strong>collection</strong> layer<br />
Secondary l<strong>in</strong>er<br />
(b)<br />
Secondary l<strong>in</strong>er<br />
Boundary of the<br />
wetted zone<br />
Wetted zone<br />
Defect <strong>in</strong><br />
the primary<br />
l<strong>in</strong>er<br />
Leachate <strong>flow</strong><br />
<strong>in</strong> the <strong>leakage</strong><br />
<strong>collection</strong> layer<br />
Figure 2. Leachate <strong>flow</strong> <strong>in</strong> the <strong>leachate</strong> <strong>collection</strong> layer, through a defect <strong>in</strong> the primary<br />
l<strong>in</strong>er, and <strong>in</strong> the <strong>leakage</strong> <strong>collection</strong> layer <strong>in</strong> the case where the <strong>leakage</strong> <strong>collection</strong> layer is<br />
filled with <strong>leachate</strong> <strong>in</strong> a certa<strong>in</strong> area around the primary l<strong>in</strong>er defect: (a) cross section;<br />
(b) plan view of the secondary l<strong>in</strong>er.<br />
wetted zone and the average head of <strong>leachate</strong> on <strong>to</strong>p of the secondary l<strong>in</strong>er <strong>in</strong> the wetted<br />
zone: this <strong>in</strong>formation is necessary <strong>to</strong> calculate the rate of <strong>leakage</strong> through the secondary<br />
l<strong>in</strong>er. The case where the primary l<strong>in</strong>er has several <strong>defects</strong> is treated; the <strong>defects</strong><br />
are either randomly distributed (“random scenario”) or their location is such that the<br />
rate of <strong>leakage</strong> through the secondary l<strong>in</strong>er is maximum (“worst scenario”). F<strong>in</strong>ally, the<br />
218 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4
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