Experimental and Numerical Study on Ballast Flakiness and Elongation Index by Direct Shear Test

Abstract

The cube is considered the best shape for high-quality ballast aggregates. However, the presence of flaky and elongated aggregates (misshapen particles) caused during ballast production is unavoidable. In this regard, the consequence of using this shape of ballast is not well investigated. In this study, first a series of large-scale direct shear tests were carried out under various vertical stresses of 50, 100, and 200 kPa to evaluate the breakage and level of fines in ballast samples categorized by the flakiness and elongation index. Second, the discrete element method (DEM) was employed to simulate tests concerning the breakage of particles. The results showed that there was a 14.7% and 5.2% decrease in strength and friction angle of aggregates with 20% increase in the percentage of flaky aggregates. An aggregate limit of 25% by weight for misshapen particles is recommended to meet the minimum shear stress level of the flaky ballast sample in the Chinese standard. Despite higher shear strength at low vertical stresses compared with cubic aggregates, flaky and elongated aggregates result in higher deterioration and shorter service life in the long term.