Instability Conditions of a Silty Sand under Monotonic Loads in Simple Shear Tests: Fid-Bau Portal

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

Mele, Lucia / Kocaman, Kadir / Özocak, Aşkın / Flora, Alessandro

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 $η^{*}$ (modified apparent viscosity) was introduced. Depending on the strain rate, $η^{*}$ allows to identify stability conditions more clearly than second-order work; (2) the value of the pore pressure ratio at the onset of instability ( $r_{u, is}$ ), ranging between 0.30 and 0.50, is linearly linked to the state parameter $ψ$ ; and (3) at the onset of instability, the dependence of both the major and minor principal stress directions ( $α^{is}$ and $β^{is}$ ) and Lode’s angle ( $θ^{is}$ ) on $ψ$ 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.