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

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GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects S primary liner defect frequency from 1 to 300 defects per hectare: 10 -4 m -2 < F

GIROUD et al. D Leachate Flow in Leakage Collection Layers Due to Geomembrane Defects 5 LEACHATE HEAD AND TIME REQUIRED FOR STEADY-STATE FLOW CONDITIONS 5.1 Leachate Head on Top of the Secondary Liner Due to One Defect in the Primary Liner 5.1.1 Method Used to Calculate the Head and Thickness of Leachate To calculate the rate of leakage through the secondary liner, it is necessary to know the head of leachate on top of the secondary liner in the wetted zone. The leachate head is related to the leachate thickness through Equation 3. It is important to note that Equation 3 is valid whether t is an actual leachate thickness (case where the leakage collection layer is not full) or a virtual leachate thickness (case where the leakage collection layer is full in a certain area around the primary liner defect). In Section 5.1, the average leachate thickness in the wetted zone, t avg , will be calculated in the case where only one defect in the primary liner is considered. The average head of leachate on top of the secondary liner, h avg , can then be calculated using the following equation derived from Equation 3: h = t cosb (141) avg avg The average leachate thickness in the wetted zone can be calculated as follows: t avg = V A w actual (142) where: V = volume of leakage collection layer that contains leachate; and A w actual =surface area of the actual wetted zone (whereas the surface areas noted A w given in Section 4.3 are the surface areas of the projection of the wetted zone on a horizontal plane; see Section 4.3.3). It should be noted that V is the volume of leakage collection layer that contains leachate, not the volume of leachate. Since leachate only occupies the pores of the leakage collection layer, the volume of leachate is: V leachate = n V (143) where n is the porosity of the leakage collection layer material. From Equations 63 and 142, the average thickness of leachate over the wetted zone is: t avg V = A / cosb w (144) Expressions of A w are given in Section 4.3 for all relevant cases. Therefore, only V needs to be calculated at this point. As pointed out in Section 4.3.3, four cases were considered in Section 4.3 for the derivation of expressions for A w (the surface area of the wetted zone). The same four cases must be considered for the calculation of V. These four cases are illustrated in Figure 13. However, whereas for A w analytical expressions GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NOS. 3-4 255

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leakage

leachate

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equation

wetted

liner

primary

defects

equations

thickness

layers

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