Critical State and the Loosest Jammed State of Granular Materials

Solid-state (i.e., jammed) granular soils can be prepared into different densities characterised by the mean pressure p and the solid fraction ϕ (i.e., different p-ϕ combinations). The limits for jammed states (i.e., the range of possible p-ϕ) are studied theoretically in the literature or through isotropic compression simulations with the discrete element method (DEM). Shearing also causes unjamming and the critical state is an important reference state for shear deformation. How the jamming limits from isotropic compression tests are related to the critical state is examined in this paper by DEM simulations. Two methods are used to generate isotropic samples. One is the isotropic compression method, which is mainly used for studying jamming in the literature. Possible jammed states from this method lie between two compression lines. The varying-friction methods can generate samples with a larger range of p-ϕ. Isochoric shear tests are conducted on isotropic specimens prepared with both methods. Some specimens reach liquefaction (p′≈ 0) and the others reach the critical state. The obtained critical state p-ϕ line is found to be the same as the loosest jammed state line from the isotropic compression method. Additionally, the critical state stress state is also well described by a Coulomb-type equation in the octahedral profile.