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 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 (ru,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 (αis and βis) and Lode’s angle (θis) 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.