Behaviour of ballast under principal stress rotation: Multi-laminate approach for moving loads

Railway tracks are subjected to millions of loading cycles over time and at high speeds, these moving trains induce dynamic amplification of vertical stresses and rotation of principal stress axes in the track layers. It is important to predict and analyse the behaviour of ballast under these loads with complex stress paths involving principal stress rotation. In this paper, a constitutive model based on a multi-laminate framework is used to predict the deformation and degradation of ballast under complex stress paths. The yield and plastic potential surfaces are developed based on a non-linear critical state and bounding surface plasticity concepts. The proposed model is validated with independent test data to capture the influence of confining stress, loading frequency, Cyclic Stress Ratio (CSR) and Shear Stress Ratio (eta(tau)) on the permanent strain response. Furthermore, the response of ballast under traffic loading stress paths with different CSR and eta(tau) is analysed. These model predictions show that higher CSR and eta(tau) values lead to exacerbated particle breakage of ballast, large and unstable axial strains and dilatant volumetric strains. Furthermore, a stability surface is proposed based on model predictions, to estimate the allowable CSR and eta(tau) for a stable response.