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Landfill Gas Pressure Hydraulic Transmissivity  -Design Calculator

Problem Statement

Landfill gas (LFG) pressure underneath a lined cover system can significantly reduce the effective normal stress on the liner, which can affect cover soil stability.The Landfill Gas Stability Calculator can be used to verify the factor of safety of a landfill cover subject to landfill gas pressure underneath a geomembrane liner.

Large-scale landfill cover slope failures have been recorded over the past decade to be directly attributed by an inadequate LFG venting layer.  According to recent studies, based on intrinsic permeability theory into gas transmission rates, the rate of LFG transmissivity is ten times lower than the hydraulic transmissivity in any porous media.  In the past, however, this relationship was believed to be inverse, i.e. the air transmissivity was believed to be 100 times greater than the hydraulic transmissivity.  The resulting miscalculations significantly under design the required transmissivity of the LFG venting layer, which in turn may cause landfill cover slope failures.

Hydraulic Transmissivity vs. LFG Transmissivity

The gas transmissivity can be converted to a hydraulic transmissivity for the same drainage medium. The intrinsic permeability variables for common liquids and gases are listed in Table 1.

Eq. (1)

where:

θH20

 Hydraulic transmissivity (m2/s)

θGAS

 gas transmissivity (m2/s)

μgas

 Dynamic viscosity of gas (N-s/m2)
μH20 Dynamic viscosity of water (N-s/m2)
γGAS Unit weight of gas (kN/m3)

γH20

 Unit weight of water (kN/m3)
 

Table 1. - Intrinsic Permeability Variables for Common Fluids and Gases (70oF)
  Density, ρ Unit Weight, γ Dynamic Viscosity, μ  Kinematic Viscosity, υ
  slug/ft3 kg/m3 pcf N/m3 Centipoise lb-s/ft2 N-s/m2 ft2/s m2/s
Water 1.94 1000 62.4 9800 1.01 2.12E-5 1.01E-3 1.09E-5 1.01E-6
Air 2.34E-3 1.2 .0753 11.8 .018 3.78E-7 1.79E-5 1.63E-4 1.48E-5
CO2 3.55E-3 1.83 .114 17.9 .015 3.15E-7 1.50E-5 8.88E-5 8.21E-6
Methane 1.29E-3 .666 .0416 6.54 .011 2.31E-7 1.10E-5 1.79E-4 1.65E-5
LFG(*) 2.53E-3 1.31 .0815 12.8 .0132 2.77E-7 1.32E-5 1.09E-4 1.01E-5

*55% CO2  ,45% CH4



Input Values
Gas Transmissivity vs. Hydraulic Transmissivity
θlfg m2/s
μLFG N-s/m2
μH2O N-s/m2
γLFG kN/m3
γH2O kN/m3


References

Richardson, G.N. and Zhao, A., (2000), "Gas Transmission in Geocomposite Systems", Geotechnical Fabrics Report, March, pp. 20-23, 2000.

Thiel, R.S. (1998), "Design Methodology for a Gas Pressure Relief Layer Below a Geomembrane Landfill Cover to Improve Slope Stability", Geosynthetic International, Vol. 5, No. 6 pp. 589-617.


 Copyright 2001 Advanced Geotech Systems.  All rights reserved.