Embankment Stability Analyses using Lesser of Drained and Undrained Strengths on a Slice-by-Slice Basis-PPT Presentation

PPT Presentation

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Conventional limit-equilibrium analyses of embankment stability typically assess end-of-construction conditions using undrained strengths and long-term conditions using drained strengths in separate analyses. This approach may miss the most critical condition for large embankments or projects with long construction periods, where parts of a potential slip surface may have drained while others remain undrained.<br /><br />The presentation describes a “partially drained” analysis that compares drained and undrained strengths for each slice along a slip surface and assigns the lower strength to that slice. The procedure uses drained strengths and effective stresses in the first stage, undrained strengths and total stresses in the second, and the lesser strength on a slice-by-slice basis in the final analysis. Here, “partially drained” refers specifically to a slip surface containing both drained and undrained segments.<br /><br />Three real project cases were analyzed: staged construction of a pier-reinforced embankment and two earth-fill dams. In each case, the partially drained analysis produced a lower factor of safety than either the separate drained or undrained analysis. The reductions relative to the lower of the conventional results were 11.3%, 2.4%, and 8.5%, respectively. Results from two limit-equilibrium software programs closely agreed, with a maximum factor-of-safety difference of approximately 0.002.<br /><br />The authors conclude that separate drained and undrained analyses may not identify the lowest factor of safety for end-of-construction conditions. They recommend considering the partially drained method in such assessments. The work is ongoing, with a journal paper planned after further research.

Keywords

embankment stability

limit-equilibrium analysis

partially drained

undrained strength

drained strength

factor of safety

slip surface

staged construction

earth-fill dams

geotechnical analysis