Shansep MC in plaxis 2d for stability analysis

Question

Hi 
 I am using Shansep MC model in plaxis for stabiltiy analyais. 
 I have a question regarding the "THE 'SWITCH' TO THE SHANSEP CONCEPT" in the manual. I am not very clear how can we user switch the MC to the Shansep MC? 
 Do I need to create two groups (MC and Shansep MC) of properties in the material set window for a given type of material? then I use the MC in the initial phase for the stress calculation, and then manually change the MC model to the Shansep MC model in the phase 1? 
 Or I only choose the User-defined option through the Material model combo box in the Material sets window. In the Parameters tabsheet, I selected 'shansep.dll' as the DLL file from the drop-down menu and the 'MC Shansep' as the Model in DLL, the plaxis will conduct the switch automatically? 
 thank you very much in advance

Martin Hawkes · Accepted Answer

Hi Jian, 
 I think the use of the word "switch" leads to some confusion. One often switch's from one material model to another, for example when soil is replaced with concrete. However in the context of the SHANSEP MC model the word "switch" is used to switch the MC strength from an C-Phi strength to a undrained strength (Su), or to update the Su strength based on changing effective stress history (i.e. strength increase from consolidation). 
 To update (switch) the strength you do not need to create two groups of properties. You can start with the SHANSEP MC in the initial conditions. The initial conditions will calculate the starting &sigma;'1 and &sigma;'1max. For a later phase you would enter a 1 as the special option parameter in the General section of the phase calculation parameters. What this does is instruct the material model to update (or "switch") the soil strength in the model to the SHANSEP strength using the values of &sigma;'1 and &sigma;'1max to calculate the OCR. The model keeps track of &sigma;'1max for all subsequent calculation phases. If you want to model the increase in strength due to increased &sigma;'1 or OCR then you would enter a 1 as the special option parameter to update or "switch" the SHANSEP strength. 
 Without this updating strength method the SHANSEP MC model is just like the MC model. 
 Be sure to check the undrained strength in the model from the SHANSEP update (switch) by plotting Stresses menu in the output module to be sure the strength values are reasonable and as expected. 
 Hope this helps, I like plotting the SHANSEP MC undrained strength model a lot, and use it frequently as a reality check. 
 Martin

Dennis Waterman · Answer

Dear Jian, 
 I presume you specified your material set as Undrained B material, in which case you only have to specify the undrained shear strength. An Undrained A material requires a friction angle and cohesion. 
 Having said so, if you would use an Undrained A material with c' = su and phi=0 it would give exactly the same result as and Undrained B material with given su. Hence, if it fails as in your case, it just doesn't have enough strength. 
 With kind regards, 
 Dennis Waterman

Stefanos Papavasileiou · Answer

Dear Jian, 
 For some reason, I know that Martin Hawkes posted the answer but I cannot see it now. I will repost it here: 
 Hi Jian, 
 I think the use of the word "switch" leads to some confusion. One often switches from one material model to another, for example when soil is replaced with concrete. However in the context of the SHANSEP MC model the word "switch" is used to switch the MC strength from a C-Phi strength to an undrained strength (Su), or to update the Su strength based on changing effective stress history (i.e. strength increase from consolidation). 
 To update (switch) the strength you do not need to create two groups of properties. You can start with the SHANSEP MC in the initial conditions. The initial conditions will calculate the starting &sigma;'1 and &sigma;'1max. For a later phase, you would enter a 1 as the special option parameter in the General section of the phase calculation parameters. What this does is instruct the material model to update (or "switch") the soil strength in the model to the SHANSEP strength using the values of &sigma;'1 and &sigma;'1max to calculate the OCR. The model keeps track of &sigma;'1max for all subsequent calculation phases. If you want to model the increase in strength due to increased &sigma;'1 or OCR then you would enter a 1 as the special option parameter to update or "switch" the SHANSEP strength. 
 Without this updating strength method, the SHANSEP MC model is just like the MC model. 
 Be sure to check the undrained strength in the model from the SHANSEP update (switch) by plotting Stresses menu in the output module to be sure the strength values are reasonable and as expected. 
 Hope this helps, I like plotting the SHANSEP MC undrained strength model a lot, and use it frequently as a reality check. 
 Martin

Shansep MC in plaxis 2d for stability analysis

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