In this paper, the discrete element method is used to explore why differing amounts of breakage, quantified using Hardin’s relative breakage parameter (Br), are associated with the critical state line (CSL) and the normal compression line (NCL) at similar stress levels. Virtual samples initially containing more than 20,000 spherical particles were isotropically compressed to a range of confining pressures up to 56 MPa and subjected to triaxial compression, both considering and disregarding particle crushing. A particle crushing model was developed for these simulations which is both computationally tractable and gives macro-scale results qualitatively in agreement with laboratory tests. The CSLs are both linear in q–p' space. A curved peak envelope, corresponding to a curved Mohr–Coulomb envelope, is obtained for the crushing simulations which is absent when crushing is disabled. Consideration of particle crushing reduces the peak stresses and the volumetric response is much more contractive with crushing at high p'. These simulations capture the behaviour in Br–p' space expected from published laboratory tests. The difference in behaviour along the NCL and CSL is explained by the larger fluctuations of contact force during triaxial shearing than during isotropic compression which was quantified using a newly-defined measure, contact number ratio. Particle crushing continues after the critical state is attained, contributing to counteract the dilation induced by particle rearrangement.