Collapse surface approach as a criterion of flow liquefaction occurrence in 3D FEM models

In order to detect the flow liquefaction occurrence, a mathematical criterion is suggested based on the collapse surface or flow line approach. For this purpose, using Dafalias-Manzari constitutive model the multiaxial form of stress ratio at the onset of flow liquefaction is derived and compared with the current stress ratio as well. This stress ratio criterion is evaluated first for both monotonic and cyclic undrained triaxial tests. In such cases, for definable values of flow stress ratio the flow liquefaction is correctly diagnosed; and otherwise when the stress ratio is mathematically indefinable the flow occurrence is basically not expected. In addition, two centrifuge model tests are also numerically modeled using FEM and the application of suggested criterion is assessed in these models. The sudden drop of void ratio values right at the detected flow time, testify that the stress ratio criterion can properly distinguish the onset of flow liquefaction in each elements of a FEM models. By comparing the flow time of several elements with different locations, it can be concluded that the flow occurrence has a progressive pattern. In other words, the surface elements experience flow liquefaction earlier than more confined elements.