In this article a conceptual model for long term stability of slopes and embankments on soft soils is presented, which is based on the Critical State Soil Mechanics framework. Using experience gained from predominantly clayey soils, the effects of viscosity, strain rate and structure on the material behavior are discussed while particular attention is paid to the long term strength. The resulting design approach suggests the use of the concept of instability lines as limiting boundary for compression or shearing yield and subsequent strength increase or decrease respectively.
The applicability of this conceptual approach for the safeguarding of the long term stability of slopes and embankments on a wide range of soils is discussed and recommendations for further research and developments are presented.