Experimental Investigation of Coal-Fouled Steel Slag Ballast–Granite Ballast Mixtures under Static and Cyclic Loading

The performance of coal-fouled steel slag ballast (SSB), granite ballast (GB), and 60%SSB+40%GB (SSB60) mixture were assessed using direct shear and cyclic testing apparatus. Additionally, chemical composition, mineralogical phases, and morphology of SSB were examined through X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscope (SEM), respectively. The XRF results signify that the iron content in SSB was significantly higher than that in GB. SEM images clarify that the surface texture of SSB is rougher than that of conventional GB. Furthermore, direct shear test (DST) results revealed that with the increase in the void contamination index (VCI), both the friction angle (φ) and dilation angle (ψ) of SSB, GB, and SSB60 were reduced. Moreover, the cyclic test results confirmed that, irrespective of the VCI, SSB60 samples underwent a lower extent of vertical settlement (Sv) and lateral displacement (ld) and, hence, lower volumetric (εv) and shear strain (εs) than did conventional GB samples. Furthermore, breakage (Bg), resilient modulus (Mr), damping ratio (D), and track stiffness (k) of the SSB, GB, and SSB60 were reduced with an increase in the VCI. The results further suggest that the performance of SSB60 was drastically reduced for VCI>40%; hence, the VCI of 40% may be considered as threshold fouling percentage.