Shansep MC in plaxis 2d for stability analysis

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 σ'1 and σ'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  σ'1 and σ'1max to calculate the OCR. The model keeps track of σ'1max for all subsequent calculation phases. If you want to model the increase in strength due to increased  σ'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

Hi Martin,

Thank you for your kind explanation. I have figured out the problem by entering 1 for the special option. 

We know that shansep mc model include the parameters for MC model and for Shansep model.

When I use Shansep MC for stability analysis of tailings dams under the post-liquefication loading conditions, I use the liquefied undrained shear strength ratio for tailings for the Shansep model parameters, but what would be the friction angle for the tailings in the MC model parameters. As in the undrained loading condition, I assumed it would be zero but this does not work in the initial phase as the model fails. I try to use the effective friction angle at the critical state for tailings, it can work this time. But this seems not reasonable. 

This seems not a problem in geo/slope as it not requires the input of the cohesion and internal friction angle, only the undrained shear strength ratio is required. 

Thank you,

Jian 

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

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 σ'1 and σ'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  σ'1 and σ'1max to calculate the OCR. The model keeps track of σ'1max for all subsequent calculation phases. If you want to model the increase in strength due to increased  σ'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

Dear Stefanos,

Thank you for your reply. I have figured out it by using 1 in the special option. I have another question regarding the use of Shansep for tailings. 

I am using Shansep MC mode for stability analysis. As I choose “user defined” in the material model, the drainage type can only be undrained A, drained, non-porous, there is no option undrained B. So I used undrained A and it was required to input the c and phi for the Mohr Coulomb parameters and also alpha, power, G/Su, etc. for Shanspe model parameters, as shown in the below figure.

If I understand correctly, the MC parameters will be used in the initial phase to calculate the initial stress, and then it will switch to Shansep MC to update the Su based on the calculated initial stress. At the same time, the model will behave as undrained B, with the friction and dilation angles being switched to zero, and C=Su.

I was wondering what is the value of C and phi I should choose for tailings in the MC model parameters. I chose c=0 and phi=0, the model failed as there was no strength. I also tried to use C=Su,min and phi=0, the model also failed. The model can work when I use c=0, and phi=effective friction angel at the critical state, but this seems not reasonable. Because for tailings, I suppose it should have no cohesion and low value of friction angle if MC model parameters should be set, my case is different from the case shown in the manual for shansep, which is usually for clay.

Thank you for your help in advance.