Critical state of rockfill materials considering particle gradation and breakage

The theories of critical-state soil mechanics are satisfactory in describing the stress-strain characteristics of fine-particle soils and have become the foundation for establishing various constitutive models for clay and sandy soils. As for rockfill materials, under the influences of some factors such as stress, density and gradation, their deformation characteristics are very complex, and their particles are easily broken under high stress. Whether there exists the only critical state or not is worthy of being discussed. Through a series of large-scale triaxial shear tests on rockfill specimens with different gradations and densities under different confining pressures, the critical state of the rockfill materials and the influencing factors are investigated. The test results show that, under different gradations, densities and initial consolidation stresses, when the shear strain is large, the specimens all tend to be in the critical state, and the values of the critical state relate to the initial density, gradation and particle breakage. And in the - plane, there exits the only critical stress ratio M for rockfill materials. Then, in the - plane, the critical state lines are basically parallel, and their intercept can be directly obtained according to the initial density and gradation. A mathematical expression for the critical state of rockfill materials considering particle gradation and breakage is proposed by comparing and analyzing the critical states of various rockfill specimens.