Particle breakage and shakedown behavior of granular material under cyclic loading conditions

The geomechanical performance of railway ballast is critical to the long-term stability of heavy-haul track systems. Limestone aggregates are often disregarded for such applications due to their susceptibility to particle breakage and permanent deformation. This study presents an experimental investigation into the cyclic response of limestone ballast under loading conditions representative of heavy-haul freight operations. Long-term permanent deformation tests were conducted under varying cyclic stress ratios and confining pressures. Particle breakage was quantified using the Particle Breakage Index ( Bg ) and the Ballast Breakage Index ( BBI ), while single-particle crushing tests were performed to evaluate particle strength. The results identify a critical cyclic stress ratio ( n _crit = 0.80) governing both permanent deformation and particle breakage. For cyclic stress ratios below this threshold, the material exhibited stable behavior (shakedown regime) and limited particle breakage. Conversely, for n _crit ≥ 0.80, progressive deformation (plastic creep regime) and a marked increase in particle degradation were observed. Increased confinement significantly reduced degradation and promoted shakedown behavior, even under higher stress levels. These findings demonstrate that the cyclic stress ratio controls not only deformation behavior but also particle integrity, while enhanced confinement mitigates ballast degradation. The results support the conditional feasibility of limestone ballast for heavy-haul railway applications.