The effect of unsteady excess pore water pressure on soil stability

Based on hundreds case studies for deep excavation in Taipei soft clay strata, it has been found that the changes of force equilibrium present significant effect on soil internal stress system. It changes from steady state to unsteady state promptly and then turns back to steady state slowly. During the change, the apparent total stress is a constant but inside the saturated soil structure is in unsteady condition. It is a time-dependent phenomenon and determines different models of soil behaviors. Only when all stresses balanced and all particles displacements stopped, the soil structure achieves steady and stable condition. This paper introduces a conceptual soil model based on logical inference. It can be considered as the first course for the unsteady soil stress. Further studies are underway to verify the accuracy of such concept.