Under the high stress of a 300-m dam, the particle breakage patterns of rockfill material may differ from those under low-stress levels. The existing studies on the particle breakage of rockfill material under ultra-high dams are relatively rare. In this study, by performing a series of large-scale triaxial shear tests under different relative densities and confining pressures, the stress-strain relationships and particle breakage characteristics of a sandstone rockfill material were investigated. The development of four particle breakage indexes before and after the triaxial test, the evolution of the gradation curves, and the applicability of three gradation formulas to the data of this study were analyzed. Based on the distribution of one relative breakage index, its relationship with strength and compressibility was established. Finally, three failure modes for the sandstone rockfill material after the triaxial test were given. And the relationships among failure modes and confining pressure, and particle size were discussed.

The degradation of soil bonding, which can be described by the evolution of bond degradation variables, is essential in the constitutive modeling of cemented soils. A degradation variable with a value of 0/1.0 indicates that the applied stress is completely sustained by bonded particles/unbounded grains. The discrete element method (DEM) was used for cemented soils to analyze the bond degradation evolution and to evaluate the degradation variables at the contact scale. Numerical cemented soil samples with different bonding strengths were first prepared using an advanced contact model (CM). Constant stress ratio compression, one-dimensional compression, conventional triaxial tests (CTTs), and true triaxial tests (TTTs) were then implemented for the numerical samples. After that, the numerical results were adopted to investigate the evolution of the bond degradation variables BN and B0. In the triaxial tests, B0 evolves to be near to or larger than BN due to shearing, which indicates that shearing increases the bearing rate of bond contacts. Finally, an approximate stress-path-independent bond degradation variable B sigma was developed. The evolution of B sigma with the equivalent plastic strain can be effectively described by an exponential function and a hyperbolic function.

Particle crushing is a common phenomenon in granular soils, which can affect the physical and mechanical properties of soils. The index which used to quantify the amount of particle crushing is crucial for the relevant research. Based on Kick's comminution energy consumption theory, size potential regarding as a property of soil and representing the energy of particles in a certain state was defined in this study, and a crushing index was proposed based on it. This index was determined according to a density distribution of particle size that voids the problem of sieving error propagation when using a cumulative grain size distribution curve. The validation of the proposed crushing index showed that it could quantitatively describe the amount of particle breakage, regardless of material, gradation, test type and particle size scale. In addition, the proposed crushing index was considered as a soil property index and could be divided into two categories: the ultimate crushing index and the mobilized crushing index, where the mobilized crushing index is obtained from the ultimate crushing index by replacing the ultimate breakage state with the final breakage state. The test results showed that the mobilized crushing index was equivalent to the input energy ratio between current and final breakage states. This relationship contributes to the understanding of the evolution of the grading curve caused by particle breakage.