Hardening Soil Model Limitation

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