Particle breakage characteristics of rockfill materials and correction of Wan-Guo hardening rule

The particle breakage problem of super-high rockfill dams is a hot issue in the current researches. For the rockfill materials of the world's highest concrete faced rockfill dam, Shuibuya Dam, the consolidated drained tri-axial tests with the maximum deformation of 25% are carried out, and 31 high-quality test curves are obtained.  On this basis, the power function relationship between the critical shear stress and the mean stress is yielded, so is the relationship between the particle breakage ratio and the confining pressure. The logarithmic function relationship between the critical state stress ratio and the confining pressure is obtained. The test data reveal that obvious particle breakage of the rockfill materials happens under high or super high pressures. The particle size of many rockfill materials becomes smaller and its grading curve changes, which induces the shear stress and axial strain test curve to bend down. Therefore, the criterion for the critical state of rockfill materials under high confining pressures must be "strain unchanged volumetric" rather than "shear stress unchanged". Finally, the Wan-Guo hardening rule of Rowe’s stress dilatancy equation is modified by use of the breakage factor term.