Triaxial Tests to Assess the Effects of Densification, Lateral Spreading and Grain Breakage on Settlement Behaviour of Full-Size Railway Ballast

Predicting the rate of deterioration of ballasted track due to cumulative plastic settlement is a major challenge for railway infrastructure owners. For track on a stiff subgrade, more than 50% of the total deformation originates from the ballast through grain rearrangement (compaction), lateral spreading and possibly grain breakage. Despite much research, knowledge of the mechanisms by and extent to which these three processes contribute to permanent settlement is limited, and permanent settlement is still usually estimated empirically. This paper describes the rationale and initial results of triaxial tests on full-size granite railway ballast, carried out to investigate the relative importance of the processes responsible for ballast settlement. Specimens of full-size railway ballast 600 mm high and 300 mm in diameter were subjected to cycles of deviator stress between 10 kPa and 50, 100 or 200 kPa. Tests were carried out over up to 500,000 load cycles at a frequency of 3 Hz and an effective confining stress (cell pressure) of 30 kPa. For the ballast tested, grain rearrangement and lateral spreading were the dominant effects on the evolution of ballast permanent settlement. Grain breakage was insignificant except at the highest test load.