Total strength envelope and friction angle. Help!

[GeotechEIT123]

Hi everyone, I hope you can help me answer these questions that have been bugging me for a while. For a fine grained material:
Is there an application of the total stress friction angle (say from a CUT)? What does this relationship or line (phi and c) represent? Is this line related to the undrained strength of the material? Is there an use of these parameters in a slope stability analysis or any other geotechnical modeling?

I understand that if you subtract you get your effective strength parameters and that's what we use for modelling long term conditions. But wondering about the total stress parameters.

Thank you!!

 

[geotechguy1]

Thats basically what SHANSHEP is, right?

 

[GeotechEIT123]

Thats basically what SHANSHEP is, right?

I don't think this total stress evelope is related to SHANSEP.

 

[geotechguy1]

The fraction at the start of the SHANSEP equation is essentially just tan(total stress phi) they just always present the product of tan(total stress phi) and hide what it actually is, presumably because they teach students total stress phi is always zero and then later have to un-teach it or do something convoluted to apply the same thing.

 

[GeotechEIT123]

Geotechguy1, first, thank you very much for responding to my messages... Appreciate it!

I'm still confused about how/why to use a frictional parameter (phi) to characterize an undrained condition. I searched for an equation or relationship between SHANSEP and the M-C total stress envelope and found nothing.
The undrained strength (Su) is not a fundamental soil property, so I don't know how you can model it with a fundamental soil function (M-C).
I went through Ladd's paper and GeoStudio modelling notes, and my understanding is that SHANSEP gives you the variation of cohesive strength with depth. In this case, the strength (cohesion or Su) increases due to depth or in-situ effective stress, not due to the applied stress. Once you apply an instant load simulating a short term condition, the effective stress doesnt change, it shouldn't change (is we assume a soil 100% saturated) as the applied load is proportional to the excess pore pressure. In undrained conditions the applied load won't change the effective stress of the sample.

And a SHANSEP constant is the highest simplified case, as the Undrained Strength is not a material property and can have infinite functions depending on the stress history.

 

[EireChch]

The only thing CUT test gives you is drained strength parameters. I was in a similar position to you before and looking at trying to use Cu from CU triaxial, however, if you read Duncans book on soil strength and slope stability it more or less says its not appropriate to use Cu from CUT.

And in practice, there is no such thing as phi for total stress so dont attempt to use it in a stability analysis.

 

[GeotechEIT123]

Thank you EireChch. Agree! I contacted an university profesor and received the same response. Total stress mohr circle is useful for understanding the overall stress state and pore pressure changes, it does NOT directly gover the soil strength in the context of failure.
And total stress phi is not related to Shansep.