Hardening Soil Model Limitation

Question

Hi, PLAXIS team, 
 I'm struggling to understand what the upper limit for material stiffness, i.e. E50 in Hardening Soil Model is? I am exploring different material models for a very low to low strength Siltstone (UCS = 1MPa, E50 = 300MPa) and I was thinking to consider modelling this with HS model, as unloading characteristics of this material is of my great interest. I have run a SoilTest triaxial simulation for this low strength material but the results would not match (or not even close to) the available lab test data, i.e. E50 = 200MPa. Needless to say, the Stress-Strain curve is almost linear elastic - perfectly plastic, rather than elastoplastic, which I suspect that it's due to the high shear stiffness. 
 It would be great to hear your thoughts. 
 Many thanks, Farzam

Dennis Waterman · Answer

Dear Farzam, 
 It's not entirely clear what you're trying to doas you don't provide information on how you perform the triaxial test. Did you define the material as overconsolidated, for instance? Note that in the HS model unloading/reloading is elastic, not elastoplastic. Only primary loading is elasto-plastic. Also, E50 is the stiffness for primary deviatoric loading, which, again, if you modelled your material has overconsolidated may not happen. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Farzam, 
 The results you show are clearly elasto-plastic and not even close to LEPP. If you focus on the first 2% strain you'll see that. So I'm not sure what is your problem? 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Farzam, 
 It's a matter of accuracy. You're doing the test in 100 steps up to 10% strain, hence the steps are 0.1%. With the high stiffnesses this means quite a large (deviatoric) stress step, which is not so accurate. You can increase accuracy by taking smaller steps. I increased the steps to 1000 and the maximum strain to 1% and then I find an E50 of 280 MPa, for instance. 
 With kind regards, 
 Dennis Waterman