Undrained or drained conditions

[fcaldeira]

I have to assure the stability of a slope (clays). What is the correct option for calculus: undrained or drained conditions?

 

[nvlong]

The undrained condition can be used to evaluate the stability of the slope immediately after loads are activated.
In contrast, the draind condition can be used to evaluate the long-term stability of the slope when the loading time is long enought to estimated that all excess pore water pressure have dissipated.

 

[BigH]

Actually, the effective stress analyses can be used in both cases with the caveat that for the short term conditions where there is positive porewater pressures generated, you need to be able to determine the porewater pressures pretty closely. This is difficult (read that time consuming) to do and as a result, in clays, undrained shear strengths are the way that most analyses are carried out. As a rule of thumb, in normally consolidated material, the most critical time in a cut or fill is immediately after the construction - i.e., the long term strength will be greater. For overconsolidated soil (due to negative porewater pressure generation), the long term (drained) strength values are the most critical. Hope this helps.