Experimental investigation of mixing railway ballast grains with different form using large-scale direct shear box apparatus

Abstract

This paper investigates the effect of grain form on the mechanical behaviour of a railway ballast aggregate using large-scale shear box tests. The speciality of the research is that – contrarily to most of the experimental studies, where the shape content of the assemblies are not manipulated – here, the grains were carefully sorted and the ballast samples were assembled with the desired size and form content. A new sorting method was developed for laboratory-scale samples to separate compact and non-compact (flat, elongated or bladed) grains. This method utilises the combination of standard bar sieves and shape index calliper. The geometries of the grains in the resultant assemblies were thoroughly investigated using modern 3D imaging-based shape characterisation methods. With the adequate parameter thresholds, good correspondence was found between the standard industrial and the modern imaging-based methods, which allows the virtual reproduction of the introduced laboratory sorting technique. Three types of andesite railway ballast assemblies were tested experimentally: a fully compact, a fully non-compact and a mixed one. The mixed assembly had the highest internal friction angle which proves that it can be beneficial to mix grains with different types form. The amount of broken mass and the failure modes of the grains were also recorded, and it was found that the non-compact assembly had the highest rate of breakage. The results of the study also serve as a good basis for the calibration of numerical models.