On manifestations of delayed failure and the effect of dilatancy in transient poro-elasto-plastic analyses of slopes and excavations

Depending on shear strength and soil permeability, an excavation may initially be stable but fail after some time, thus raising the practically important issue of stand-up time. This paper uses coupled hydraulic–mechanical analyses to investigate how delayed failure manifests itself. A relatively simple geotechnical problem is considered and provides valuable insights into the mechanism of delayed failure. The results presented are thus also relevant to other geotechnical problems, such as the stand-up time of tunnel headings. Emphasis is placed on the effects of plastic dilatancy as it affects ground response not only quantitatively but also qualitatively. Under the frequently made simplifying assumption of a constant positive dilation angle, coupled analyses inevitably lead to a constant deformation rate at failure, whereas failure is (commonly) associated with accelerating displacements. Models that allow for shearing under constant volume lead to accelerating displacements but inherently exhibit numerical stability problems close to the failure state. In order to determine stand-up time, it is essential to evaluate the numerical results in their entirety, including the time development of the displacement, stress and pore pressure fields.