Using the geomembrane in a slope model gets complicated and is based on the problem you want to solve.
For slope stability and interface friction angle you set up an idealized case with the height of slope you are looking at.
For global stability you may want to put in 2 piezometric lines, one for the membrane lined materials applying its own phreatic condition and the second phreatic line for the foundation materials.
If you are relying on the membrane for strength, you would need to model the material with a built in reinforcement tool in the software, looking at the strength at failure and determining when that will be for the membrane specified.
GeoEnvGuy (Geotechnical) thank you for answering … am using GeoStudio and there is no material model for geomembrane and we can't use Mohr coulomb as well so as the membrane is 3mm thick it doesn't contribute for the stability so shall I left it out during slope/w analysis?????
If you want to know if the loosely packed materials on top of the liner will slide off you can make a mohr coulomb material of limited thickness to see if it will slide off. Like previously stated this would be an idealized case. The manual calculation for this is presented in the designing with geosynthetics textbook, slope/w optimizes the top and bottom entrance and exit to the slide plane.
For global stability you would not include this material but have 2 phreatic levels related to the materials which would be contained by the liner. So you would need to show which material has which phreatic conditions applied.
