Initiation of flow liquefaction in granular soil slopes: drained versus undrained conditions

The mechanism of flow liquefaction in granular soils under drained conditions compared with that under undrained conditions is a subject of great interest. This paper discusses the diverse views in relation to the initiation of instability and the conceptual difficulties brought by the discrepancies. It is shown that the constant shear drained (CSD) stress path is not a conventional stress-controlled test, and overlooking this fact may result in misleading conclusions. The difficulties inherent in the CSD tests have resulted in different criteria for the detection of the onset of instability in experiments and thereafter to different views. Through systematic simulations for the CSD, ACU and ICU stress paths under a range of initial states, it is shown that the instability line in the stress space is state dependent, and its gradient can be well related to the state parameter defined using the critical state concepts. For a given soil, the relationship is nearly unique regardless of the CSD, ACU or ICU stress path, and the critical state concepts and the second-order work criterion work well. Theoretically, it is shown that the conditions for instability under the drained and undrained stress paths are the same, regardless of the constitutive models chosen.