Defining the critical state line from triaxial compression and ring shear tests

The critical state (or critical void ratio) line is the locus of void ratio-effective stress conditions achieved after shearing a soil to large displacement and after all net void ratio changes and effective stress changes are complete. The triaxial compression test is commonly used to define the critical state line of sandy soils. In this study, we present drained and undrained triaxial compression and ring shear tests that are used to define the critical state line of a silty sand sampled from the Mississippi River near Cape Girardeau, Missouri, USA. All specimens were prepared by pluviating the dry sand through air. The results show that both drained and undrained (or constant volume) triaxial and ring shear tests reach critical state at similar shear displacements prior to the onset of particle damage in the ring shear tests. A unique critical state line can be defined for these states. At larger shear displacements only possible in the ring shear tests, considerable particle crushing happens and dominates the sand behavior and only after very large shear displacements (> 750 cm) particle crushing ceases to continue and a complete particle rearrangement is reached. At this state, the stresses and volume of the sand become constant which corresponds to the critical state of the crushed sand. A unique line can also be drawn for this state. These unique critical state lines indicate that they are not influenced by the drainage conditions and shearing modes.