The degradation of coarse aggregates under shear stresses and its influence on the shear strength is studied, considering the energy consumption by particle breakage during shearing. An analytical model is developed relating the deviator stress ratio, dilatancy, friction angle and particle breakage under triaxial loading. Large-scale triaxial testing of latite basalt has been conducted, and the extent of particle breakage during shearing has been quantified. The breakage of particles under monotonic triaxial loading has been considered within the scope of this paper, and the modelling of particle breakage of aggregates under cyclic loading will be presented in a follow-up paper. The results show that the breakage of particles continues to increase beyond the peak deviator stress. The energy consumption by particle breakage is non-linearly related to the particle breakage index. The model also evaluates the effect of particle breakage on the friction angle of ballast. This study sheds further light on the basic angle of friction, which is independent of the breakage of particles during shearing.