Why does the deviatoric stress tend to infinity when modeling undrained behavior with the Hardening-Soil model?

Why does the deviatoric stress tend to infinity when modeling undrained behavior with the Hardening-Soil model?

Why does the deviatoric stress tend to infinity when modeling undrained behavior with the Hardening-Soil model?


ANSWER

This is because, in the original Hardening-Soil model formulation, the shear and volumetric plastic mechanisms are decoupled. The preconsolidation pressure pc, being the hardening parameter of the cap yield surface, depends solely on the accumulated volumetric plastic strain εvp produced by this mechanism. Similar assumption is adopted for the shear yield surface that expands with increasing accumulated deviatoric plastic strain γPS produced by the shear mechanism. The latter plastic mechanism may also produce volumetric plastic strain (due to dilatancy) but it is not coupled with the hardening law for preconsolidation pressure.

Evolution of the effective stress path in the undrained triaxial test assuming uncoupled shear/volumetric plastic mechanisms (OCR = 1)

The macroscopic behavior of cohesionless soils can be reproduced by the model reasonably well, regardless of drainage conditions (i.e. drained or undrained). The ability to represent undrained behavior of overconsolidated, therefore usually dilative, cohesive soils is limited.  Lack of coupling of these two plastic mechanisms leads to an unlimited undrained shear strength increase regardless of the OCR value.


SOLUTION

The major drawback of the model is such that it is unable to appropriately reproduce the undrained shear strength in the case when dilatancy angle ψ is larger than zero. By applying a conservative assumption ψ = 0°, the computed undrained shear strength may be underestimated with respect to the true one (even a few times for larger values of OCR). On the other hand, by assuming ψ > 0°, the undrained shear strength will tend to infinity with the increasing shear strain amplitude.

The coupling of shear and volumetric mechanisms provides a general solution to the problem, it will be available in ZSoil v2016.