DESIGN METHODS & CONCEPTS
Landfill gases are generated from the biodegradation of solid waste in a landfill. The actual rate of gas generation depends on waste composition, moisture content, age, etc. The purpose of a gas collection layer is to facilitate the collection of the generated gases so that they do not cause uplift of the cap. The typical configuration of a landfill gas collection layer is presented in Figure 7.1. The primary design criterion for geocomposites is to provide enough flow capacity to reduce the landfill gas pressure to an acceptable level in terms of factor of safety for slope stability, as illustrated in the following equation:
Equation 7.1
u = γ × t × cosβ - ((FS × γ × t × sinβ) / tanδ)
max
cover
s
cover
cover
cover
Where:
​
u = allowable gas pressure (kPa)
γ = cover soil density (kg/ m3)
t = soil cover thickness (m)
FS = factor of safety against sliding
δ = interface friction angle (degrees) for geocomposite-geomembrane interface
β = slope angle
max
cover
s
cover
The incoming flow rate for landfill gas will be gauged in terms of flux. The equation used to calculate the landfill gas flux is presented as follows [Thiel, 1998]:
Figure 7.1 Schematic of a landfill gas collection layer.
Equation 7.2
q = r × t × γ
g
waste
g
waste
Where:
​
q = landfill gas supply rate (m/sec)
r = landfill gas generation rate (m /sec/kg of waste)
t = thickness of waste (m);
γ = unit weight of waste (kg/m ).
g
g
waste
waste
3
3
The required transmissivity of the gas drainage layer can be calculated as follows:
θ = ( q × γ / u ) × [D / 8]
greg
max
2
g
g
Equation 7.3
3
g
greg
Where:
​
γ = unit weight of gas (kg/m )
θ = required gas transmissivity for geonet or geocomposite (m /sec per m width).
D = slope distance between drains (m).
3
Ultimate gas transmissivity can be calculated using Equation 7.4
Equation 7.4
θ = θ × FS × RF × RF × RF × RF
ultimate
greg
in
cr
cc
bc
Where:
​
FS = overall factor of safety
RF = reduction factor for intrusion
RF = reduction factor for creep to account for long- term behavior
RF = reduction factor for chemical clogging
RF = reduction factor for biological clogging
cr
bc
in
cc
Notice that the above equation provides the required transmissivity for the flow of gas, not water. Therefore, transmissivity value from actual in-plan airflow should be used for evaluating geocomposite performance.
​
Table 1 provides density and viscosity values for various fluids for use in Equation 7.5. Again we note that a very significant side benefit of providing a gas collection layer under the final cover is that it will also serve to collect side slope seeps. The seeps would be collected at the toe of the geocomposite gas collection layer, as illustrated in Figure 7.2.
Table 1 Density and viscosity of various fluids [Thiel, 1998]
Figure 7.2 Seep collection at toe of gas collection layer under final cover system.
LFG pressure gradient varies linearly with its maximum at the strip drain location, and zero in the center of the geocomposite gas venting blanket. Maximimum pressure gradient is shown below in equation 7.5:
Equation 7.5
i = q / θ × (D / 2)
max
greg
g
EQUATION SHEET
Maximum Gas Pressure
Landfill Gas Supply Rate
Required Air Transmissivity and Gradient
Solve For Maximum Gas Pressure
Input parameters
β =
(degree),
Slope Angle
γ =
cover
Unit weight of cover protective soil
(kN/m )
3
δ =
(degree),
Interface friction angle for geocomposite-geomembrane interface
FS =
s
dimensionless,
Factor of safety against sliding
t =
cover
(meter),
Thickness of cover protective soil
SOLUTION
(kPa)
Allowable maximum gas pressure underneath the cover geomembrane
µ =
max
γ *
cover
t *
cover
cosβ –
(FS *
s
γ *
cover
t *
cover
sinβ)
tanδ
Solve For Landfill Gas Supply Rate
Input parameters
r =
(m /sec/kg-waste),
Landfill gas generation rate
3
g
t =
(meter)
Thickness of waste
waste
γ =
(kg/m )
Unit weight of waste
waste
3
SOLUTION
(m /sec-m )
3
2
q =
g
r •
g
t •
waste
γ
waste
Landfill gass supply rate
3
Solve For Required Air Transmissivity and Gradient
Input Parameter
3
2
(m /sec/m )
D =
(meter)
Slope distance between drains
μ =
(kPa)
Maximum allowable gas pressure
max
RF =
dimensionless,
Intrusion Reduction Factor
in
FS =
dimensionless
Overall Factor of Safety
dimensionless
Creep Reduction Factor
RF =
cr
RF =
cc
dimensionless
Chemical Clogging Reduction Factor
dimensionless
Biological Clogging Reduction Factor
RF =
bc
(N/m )
Unit weight of gas
γ =
gas
3
Gas generation Rate
q =
g
SOLUTION