Particle breakage can occurs within railway ballast under relatively lower confining pressure

resulting in a change in the particle gradation. Gradation significantly influences the physical and mechanical characteristics of ballast. There is an increasingly urgent need for constitutive modeling of ballast for the prediction of gradation evolution under loading conditions. Based on the triaxial test results of ballast

a boundary surface model for ballast under relatively lower confining pressure is developed by employing the fractal breakage index defined by Einav and the theory of boundary surface plasticity. By the comparisons of experimental results

it is shown that the proposed model can well indicate the stress-strain behavior of coarse aggregate under low confining pressure

and provides a reliable prediction of gradation evolution caused by particle breakage during loading.