Instability Conditions of a Silty Sand under Monotonic Loads in Simple Shear Tests

In order to improve the understanding on static liquefaction phenomenon, this paper presents an insight into the instability mechanism of a silty sand originating from the Sakarya region (Turkey). Monotonic undrained simple shear tests were performed in either strain-controlled or stress-controlled modes. The interpretation of the experimental results allowed us to draw three interesting conclusions on instability conditions: (1) Hill’s stability postulate can be conveniently used to identify the onset of instability for strain-controlled tests, whereas in stress-controlled conditions, an alternative new parameter ${\eta }^{*}$ (modified apparent viscosity) was introduced. Depending on the strain rate, ${\eta }^{*}$ allows to identify stability conditions more clearly than second-order work; (2) the value of the pore pressure ratio at the onset of instability ($r_{u,\mathrm{is}}$), ranging between 0.30 and 0.50, is linearly linked to the state parameter $\psi$; and (3) at the onset of instability, the dependence of both the major and minor principal stress directions (${\alpha }^{\mathrm{is}}$ and ${\beta }^{\mathrm{is}}$) and Lode’s angle (${\theta }^{\mathrm{is}}$) on $\psi$ was also observed. These findings are expected to be of some help for a more consistent approach to the constitutive modeling of the static instability mechanism.