Can anyone provide some information/websites about stability of landfill slope?
stability criteria, mode or mechanism of failure, enviromental impact of landfill...
For any slope stability evaluation you need the geometry of the slope and engineering characteristics of the fill material as well as the groundwater condition. Given this data, you could evaluate the slope stabilty under static and dynamic conditions. Characteristics of landfill materials are documented in a number of literature including "The Geotechnology of Waste Management" (I don't have the specifics of the publication handy, but you could search it on the web). Once the appropriate materials model is established, and with representative material parameters, any slope stability program could be used for two dimensional analysis of the problem. The engineer should analyse a series of cross sections with different groundwater conditions to determine the least factor of safety.
Mode of failure may be anything from circular to irregular.
Environmental impact of a liandfill is a completely different topic.