Hello,
I have been wondering a little on the modelling of undrained behavoir in clays using the Hardening Soil Undrained A model, especially with regards to the prevalence of anisotrophy of clays in undrained loading. In order to model this behavoir, I have been using the NGI-ADP model, but this does not allow for any consolidation aspects to be considered.
When modelling the completely drained behavior, using the friction angle and cohesion from a traxial test should as far as I am aware yield correct results, however I am worried that these parameters would overestimate the strength in undrained loading, since the anisotrophy cannot be modelled. Do you have some thoughts about this? Would this primarily give overestimation of Msf in safety analyses, or also underestimate the resulting forces in regular loading phases? Would it perhaps be more correct to adjust the parameters to resemble the direct shear strength when using HS Undrained A?
Dear Jonas,
With anisotropy I presume you mean the concept that the triaxial compression and triaxial extension path give a different strength due to the fact that the Mohr-Coulomb criterion is not a equally sides hexagon? The shape of the M-C criterion (in deviatoric plane) depends on the friction angle, when the friction angle equals zero the shape is a perfect equally sides hexagon and so the strength in compression and extension are the same. This is the reason why an Undrained B or Undrained C drainage type will give the same strength in compression and extension: the friction angle is zero. The NGI-ADP model was created to overcome that problem.However, for an Undrained C calculation the friction angle is the effective (drained) friction angle and thus the compression and extension strengths are not the same, hence you do have the anisotropy as it should be. You could for instance do a quick simulation with SoilTest to see what undrained shear strength in compression and extension you would get for a certain paramter set and calibrate your parameters accordingly. For instance, the ratio Eur/Eoed have a strong influence on the reached undrained shear strength.
With kind regards,
Dennis Waterman