The true triaxial compression test is the three-dimensional test, where the intermediate principal stress is not less than the minor principal stress and not more than the major principal stress. The effect of the intermediate principal stress ratio (i.e., the b-value) on particle breakage of rockfill material (RFM) was investigated by conducting a series of true triaxial compression tests under the constant-p′ and constant-b loading condition. It was observed that the relative breakage extent of RFM increased with an increase in confining pressure but decreased with an increase in b-value. The effect of the b-value on the particle breakage of RFM was well predicted by an empirical equation.

The size and shape of particles influence how effectively coarse angular aggregates of ballast interact. The aim of this study was to improve the characterisation of ballast particles using a three-dimensional (3D) imaging method. Various size and shape indices, such as elongation ratio, sphericity and roundness, were determined from the scanned 3D images. A modified index called ‘ellipsoidness’ was proposed to capture adequately the shape of the 3D particles. Variation of these indices with particle size was studied. Comparison of the 3D true sphericity and the corresponding two-dimensional sphericity indicated that the latter would underestimate sphericity. A modified approach for transforming particle size distribution to constriction size distribution is proposed by capturing the size and shape effects of particles.

The deformation and degradation of ballast is influenced by the size of the aggregates. In this study, a series of cyclic drained triaxial tests was conducted on ballast with different sizes using the large-scale cylindrical triaxial apparatus designed and built at the University of Wollongong, and two different frequencies of cyclic loading were used to simulate low-speed and high-speed trains. From the laboratory results, coarse particles experience less vertical and lateral strains, whereas the volumetric strains decrease and then increase as the coefficient of uniformity increases, regardless of the loading frequency. Resistance to deformation and degradation is found to be improved by increasing ballast density. Different trends between the extent of breakage and particle size are observed for different breakage indices, and accordingly the extent of breakage is characterized into two distinct zones, depending on the coefficient of uniformity (Cu), where the significantly reduced breakage cor...

Traditional plasticity models of granular soils are usually established on different loading and plastic potential surfaces. This paper presents a nonassociated bounding-surface model by incorporating a fractional-order plastic-flow rule that is obtained by performing fractional-order derivatives on the bounding surface. It is found that the flow direction no longer necessarily coincides with the loading direction, even if the same potential and loading surfaces are used. The flexible ability of the model is then validated by simulating a series of experimental results of different granular soils, including ballast, rockfill, and sand, under both drained and undrained loading conditions. It is observed that the proposed model can well capture the key features (e.g., stress dilation, strain hardening/softening, and liquefaction) of various granular soils.