Undrained shear strength using Undrained A check

Dear Support Team,

In PLAXIS Reference Manual, section 9.3.3 (i. e. PLAXIS 2D CE manual) we can find the hint:

Particularly when the soil strength has been defined by means of effective strength parameters (Undrained (A) it is useful to plot the mobilised shear strength τmob in a vertical cross section and to check this against a known shear strength profile.

So the results for Undrained A should be checked by comparing the mobilised strength, τmob.

On the other hand, in Tips and Tricks (https://communities.bentley.com/products/geotechnical1/w/wiki/46010/how-to-check-undrained-shear-strength-using-undrained-a) we can see the picture with circles that refer to the limit line, which means that it is the maximum strength, τmax, not τmob:

Could you please inform us which strength should be used for checking: maximum or minimum?

Best regards,
Rushan Gizzatullin

  • If you want to evaluate if in an Undrained A analysis the (known) undrained shear strength is not violated, you should check the current stress state. From this stress state, you can use the principal stresses to determine Mohrs circle and its radius: (σ'_1 - σ'_3)/2  

    This should not be larger than the maximum allowed undrained shear strength (c_u value). 

    See e.g. above for a Undrained A materail :

    • the stress state for the green Mohr-Circle as at the limit of the effective strength, but the stress state is still below the undrained shear stress;
    • the orange circle indicates that the maximum allowable stress is reached within the effective strength parameters definition. If you compare it with the undrained shear strength c_u, the orange stress state violates it;
    • the blue circle is well below the effective strength limit (the red MC-line), but as you can see, the top of the Mohr-Circle is above the undrained shear strength profile. 

    As you can see, to evaluate if the undrained shear strength is not violated, please use the current mobilized shear stress and compare it with the undrained shear strength value.

    Note that for Undrained A (effective stress and effective stiffness) the undrained shear strength in an outcome based on the stress path and stress state; it is not input. 

    Answer Verified By: Rushan Gizzatullin 

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