Investigating the Critical State Using Laboratory Ring Shear Tests

The critical state is the state at which a mass of soil is sheared at a constant volume, constant effective stress, and constant shear stress. This state is reached at an undrained or constant volume condition and at large strains at which all potential net particle reorientation and crushing are complete. Triaxial compression tests are most commonly used to assess critical state conditions; however, as a result of the limited displacement of the device, triaxial testing may not reach a true critical state. In contrast, the ring shear test allows very large shear displacements where all net particle reorientation and crushing are complete. At the large displacements achieved in the ring shear, particle damage and grain crushing can lead to contractive behavior in initially dense materials. These effects result in critical state conditions that can differ significantly between the triaxial and ring shear devices.