Pseudostatic Analyses

[moe333]

The recently published book "Soil Strength and Slope Stability" by Duncan and Wright discusses procedures for conducting pseudostatic analyses. I often evaluate simplified seismic displacements of slopes/embankments and use pseudostatic analyses to estimate yield acceleration.

Duncan and Wright state a single stage pseudostatic analysis may be performed using the R strength envelope from a CU test together with effective stresses and steady state pore water pressure.

The use of effective stresses and steady state pore water pressures with a total stress (R) shear strength envelope seems contrary to me and contrary to the dirth of literature on the subject. I have always used total stresses and no pore water pressure when using a total stress (R) shear strength envelope.

Any thoughts on this?

 

[GTeng]

The R envelope is not the same as the total stress envelope that you probably learned in soil mechanics classes. It is actually intended to represent the variation in total stress shear strength that occurs with effective stresses (that's my understanding of it). Technically, it is not exactly accurate if the stress path or stress state in the field isn't appropriately modeled by the test used to produce the envelope, but a total stress envelope is even worse in my opinion.

 

[ntschwanz]

To add to what GTeng said, the R-test is typically an ICU test which is fully consolidated prior to testing (pore water pressure equals 0 so this is an effective stress state). So when plotting the results you plot the shear stress on the failure plane at time of failure (tau-ff) versus the effective normal stress on the failure plane at time of consolidation (sigma'-fc). Like GTeng said, you essentially are assigning an undrained shear strength to a soil that exists at some effective stress in the field. Limit equilibrium computer programs simply calculate the effective normal stress at the base of a slice and it is now easy to determine the undrained shear strength from the R-strength assignment. (sidenote: remember this not a Mohr-Coulomb plot so the intercept and angle are not c and phi.)

 

[moe333]

Thanks for the replies. Part of the confusion is the strength envelope is referred to by many as a "total stress" envelope, and that the effective consolidation stress is also a total stress (since u=0). I also not that some agencies prefer not to use this envelope for some reason.