Effect of particle angularity and size distribution on the deformation and degradation of ballast under cyclic loading

Ballasted railroad tracks are still used all around the world because of their resiliency to repeated wheel loads, low cost construction and ease of maintenance, but current ballast gradations worldwide vary in particle size and uniformity for reasons that are not always clear. This research aims to investigate the angularity (shape) of ballast aggregates and the influence of particle size distribution on its deformation and degradation. To achieve this aim, a three-dimensional laser scanner and the triaxial apparatus designed and built at the University of Wollongong were used respectively to carry out a three dimensional characterisation of ballast aggregates and cyclic large-scale triaxial tests. The deformation and degradation of ballast under high and low track speeds was investigated by using two different loading frequencies, and then a fractional order constitutive model was proposed to predict the long term deformation of ballast by using the fractional rate for strain accumulation. These experiments indicated that shape of ballast particles varied according to their size, such that as the particles increased in size, ballast became more rounded and less columnar. Moreover, a two-dimensional characterisation would underestimate particle sphericity, so a new index called ‘ellipsoidness’ was suggested to facilitate a three dimensional analysis of particle shape. This ellipsoidness decreased as the particles increased in size, which indicated relatively more irregularity in coarser ballast. The parent ballast aggregates became increasingly regular after cyclic tests while smaller aggregates created from ballast degradation seemed to have a lower ellipsoidness.