Design methods & concepts
The primary purpose of the geocomposite in a leachate collection system (LCS) is to provide in-plane transmission of leachate that reaches the geocomposite after permeating through the overlaying waste and protective soil cover. The function of the LCS is to convey leachate while controlling the head on the underlying geomembrane. The maximum head must be smaller than the thickness of the drainage system and must be smaller than the required maximum value, typically 0.3 m (1 ft). Since the geocomposite is commonly designed to convey flow in an unconfined condition (i.e., the liquids are completely maintained within the composite), the buildup of the head is very small.
In a typical landfill cell configuration, the LCS consists of two sections with two different slopes: a steeper upstream side slope; and a flat downstream cell slope. Figure 5.1 shows a typical cross-section of a landfill leachate collection system with two slopes:
Figure 5.1 – typical landfill leachate collection and removal system configuration
The required transmissivity for geocomposite in the upstream and downstream section can be calculated as follows:
θ =
up_req
q L
h
up
sinβ
up
EQUATION 5.1
sinβ
q (L +L )
θ =
down
h
up
down
down_req
EQUATION 5.2
WHERE
θ = Design required transmissivity at upstream section
θ = Design required transmissivity at downstream section
L = Length of the liquid collection layer in upstream section
L = Length of the liquid collection layer in downstream section
β = Slope for the upstream section
β = Slope for the downstream section
up
down
up
down_req
up_req
down
The ultimate transmissivity for the two section can be calculated using equation (5.3) and (5.4)
θ = θ × RF × RF × RF × RF × FS
up_ult up_req in cr cc bc
EQUATION 5.3
θ = θ × RF × RF × RF × RF × FS
down_ult down_req in cr cc bc
EQUATION 5.4
WHERE
θ =
Specified 100-hour transmissivity of a candidate geocomposite layer at upstream section
θ =
Specified 100-hour transmissivity of a candidate geocomposite layer at downstream section
RF = Reduction factor of intrusion
RF = Reduction factor of creep
RF = Reduction factor of chemical clogging
RF = Reduction factor of biological clogging
FS = Overall factor of safety
up_ult
down_ult
in
cr
cc
bc
GRI-GC8 does not make recommendations for the impact of temperature on the transmissivity. It is the responsibility of the user to perform additional testing to calculate a suitable reduction factor for temperature where long term exposure to high temperatures is expected.
LANDFILL LEACHATE
EQUATION SHEET
Input Parameter Values
q =
h
(m/s)
Impingement rate
Upstream
Downstream
β =
up
o
Slope for the upstream section
L =
up
m
Length of the liquid collection layer in upstream section
RF =
in
Intrusion Reduction Factor
RF =
cr
Creep Reduction Factor
RF =
cc
Chemical Clogging Reduction Factor
RF =
bc
Biological Clogging Reduction Factor
FS=
Overall Factor of Safety
0.38 RAD
β =
down
o
Slope for the downstream section
L =
down
m
Length of the liquid collection layer in downstream section
Permeability of the adjacent soil
RF =
in
Intrusion Reduction Factor
RF =
cr
Creep Reduction Factor
RF =
cc
Chemical Clogging Reduction Factor
RF =
bc
Biological Clogging Reduction Factor
FS=
Overall Factor of Safety
0.021 RAD
SOLUTION (Upstream)
θ =
up_req
Design required transmissivity at upstream section
m²/s
θ =
up_ult
m²/s
Specified 100hr transmissivity of a candidate geocomposite layer at upstream section
SOLUTION (Downstream)