Influence of particle gradation and moisture content on the repose angle of railway subballast

The repose angle serves as an important parameter for evaluating the interaction and flowability of granular material. In this study, a series of repose angle test using the plate lifting method was conducted to measure the repose angle of subballast with 17 different gradations and 7 varying moisture contents. The effects of the maximum particle size D100, the median particle size D50, and the content of fine particles below 1.7 mm on the repose angle were investigated. Test results shown that the repose angle increases with an increase in D100 or D50, and decreases with an increase in the content of particles smaller than 1.7 mm. The Span parameter is commonly used to describe the distribution width of particle gradation and reflects the uniformity across different size ranges. The upper and lower spans of the gradation were defined. The effect of upper span and lower span shows that the combination of lower span (1.6–1.8) and upper span (1.0–1.2) enhances internal friction between ballast particles. Based on the repose angle performance of subballast, the optimized gradation interval within the standard was identified. Furthermore, the effect of water content on the repose angle of subballast was also investigated, and the results revealed that the repose angle could be roughly divided into four typical stages with the increase of the water content: sudden change stage, horizontal stage, rapid increase stage, and slow decrease stage. The optimized gradation curve can provide a reference for the design of subballast grading in practice.