UNDRAINED INSTABILITY RESPONSE OF GRANULAR MATERIAL IN FLEXIBLE BOUNDARY PLANE STRAIN CONDITIONS

Instabilities in granular materials are marked by the development of heterogeneous deformations in "single-element" experiments. However, with the onset of instability, it becomes a boundary value problem. The present study uses a flexible boundary plane-strain (FB-PS) apparatus to characterise the instability behaviour under undrained conditions. Consolidated undrained tests are performed with two different boundary conditions, i.e., flexible and mixed boundaries, to study the influence of boundary rigidity. Flexible boundary signifies employing butyl rubber flexible membranes along all sides. While in mixed boundary conditions, additional rigidity is provided along the principal loading direction (σ<inf>1</inf>) using thin aluminium sheets with flexible butyl rubber membranes. The initiation of instability is delayed with relatively flexible boundary conditions.