Rail tracks undergo degradation owing to particle breakage and fouling of ballast by various fines including
coal and subgrade soil. As the ballast becomes fouled, its strength and drainage capacity are compromised,
sometimes resulting in differential settlement and reduced track stability. This paper
demonstrates a continuum mechanics based framework to evaluate the detrimental effect of fines on
the strength, deformation and degradation of coal-fouled ballast under monotonic loading. An elastoplastic
constitutive model that considers the effect of fines content and energy consumption associated with
particle breakage during shearing is presented. This multiphase constitutive model is developed within a
critical state framework based on a kinematic-type yield locus and a modified stress-dilatancy approach.
A general formulation for the rate of ballast breakage and coal particle breakage during triaxial shearing is
presented and incorporated into the plastic flow rule to accurately predict the stress–strain response of
coal-fouled ballast at various confining pressures. The behaviour of ballast at various levels of fouling is
analysed and validated by experimental data.