An elastic-viscous-plastic model for overconsolidated clays

A comparative study of different model families for the constitutive simulation of viscous clays

“…The isotachs of the OVP and NVP models are bounded by their elastic response at very high velocities, with slope equal to κ . This upper bound actually depends on the initial conditions of e and p and has been previously referred to as “instant time line” by other authors . Simulations with the OVP model show that a lower bound exists because of the consideration of a minimum isotach.…”

“…Particularly, the NSFS model and the VHP model incorporate only the maximum isotach in their formulations. This curve has been previously referred to as “instant normal compression line” by Yao et al or “maximum void ratio isotach” by Fuentes et al OVP models incorporate a minimum isotach, referred to as “static yield surface” in previous studies . The NVP model is the only one incorporating neither the maximum nor the minimum isotach.…”

“…The maximum isotach has been introduced into some models, because it is inferred that above the normal consolidated state OCR=1, there must be a compression line for which the creep time is “0.” The line then describes the normally and instantaneously compressed behavior of clays, thus the name “instant normal compression line.” For geotechnical analysis, engineers may be used to have a bound of the void ratio, which implies a bound of the stress ratio, as otherwise macro pores and their impact on the constitutive modeling should be treated. Because these effects are not subject of the models described here, we find it appropriate to introduce a maximum bound of isotachs.…”

“…This upper bound actually depends on the initial conditions of e and p and has been previously referred to as "instant time line" by other authors. 37,66 Simulations with the OVP model show that a lower bound exists because of the consideration of a minimum isotach. Below this curve, the existence of isotachs for this model is not possible.…”

“…This is, however, not true: The literature offers different theoretical approaches to develop the model formulation, presenting important discrepancies between them, of which users should be aware of. One may distinguish, for example, viscous models based on nonstationary flow surface (NSFS), viscoplastic models based on overstress functions (OVP), others based on potential laws similar to Norton's law (NVP), and yet recently, the so‐called visco‐hypoplasticity (VHP), among other formulations . Model developers know that it is possible to formulate viscous models with identical material parameters but having different formulations.…”