Abstract: Much experimental evidence suggests that for granular materials, the ultimate grain size distribution is not an arbitrary cut-off value of particle size (of 0.074 mm) proposed by Hardin in 1985 under extremely large confining pressure. A modified definition of Hardin’s breakage index is presented for crushable granular materials to characterize two processes of temporary or perpetual termination of breakage by further developing the concept of breakage. A series of consolidated drained large-scale triaxial tests are conducted for modeled rockfills, and nonlinear relationships are developed to describe appropriately the variation of particle grading, breakage index and the stress-strain-volume change response of modeled rockfills. The analysis of these results indicates that: (1) The particle size distributions of rockfills have some remarkable changes, which is demonstrated from the evidence that the characteristic particle sizes decrease and the grading indices increase with the increase of confining pressures ; (2) In the peak deviator stress or critical state, the breakage index increases, and the corresponding stress ratio or the internal frictional angle decreases with the increase of confining pressures, and the internal friction has an inherent relationship with the breakage index ; (3) During the same shearing process, the ratios of breakage index, plastic volumetric strain and deviator stress to plastic deviator strain are not in-phase, which leads to a complex effect of particle breakage on stress and deformation behaviors of modeled rockfills.