Steel furnace slag (SFS) and coal wash (CW) are two common by-products from the coal-mining and steel industries in Australia. Rubber crumbs (RC) is a material derived from waste tires contributing to environmental problems in most developed countries. Reusing and recycling these waste materials is not only economically beneficial and environmentally sustainable, but it also helps to address geotechnical problems such as track degradation. In this study, SFS, CW, and RC are blended to explore the feasibility of obtaining an energy-absorbing capping layer with properties similar or superior to conventional subballast. Comprehensive laboratory investigations have been carried out to study the geotechnical properties of $\mathrm{SFS}+\mathrm{CW}+\mathrm{RC}$ mixtures, from which seven parameters (including gradation, permeability, peak friction angle, breakage index, swell pressure, strain energy density, and axial strain under cyclic loading) were used to evaluate the properties of these mixtures used as subballast. It was found that a mixture with $\mathrm{SFS}:\mathrm{CW}=7:3$ and 10% RC (63% SFS, 27% CW, and 10% RC) is the best mixture for subballast.

The practical application of waste materials such as steel furnace slag (SFS) and coal wash (CW) is becoming more prevalent in many geotechnical projects. While adding rubber crumbs (RCs) from recycled tires into mixtures of SFS and CW not only solves the problem of large stockpiles of waste tires, it also can provide an energy-absorbing medium that will reduce vibration and prevent track degradation. Thus, the engineering insight into the effect that rubber crumbs have on the dynamic behavior of $\mathrm{SFS}+\mathrm{CW}+\mathrm{RC}$ mixtures is in urgent demand. In this study the influence that RC contents and confining pressures have on the deformation, resilient modulus, damping ratio, and shear modulus was investigated by cyclic triaxial tests. Test results reveal that with the inclusion of RC, the axial strain, volumetric strain, damping ratio, and energy-absorbing capacity of the $\mathrm{SFS}+\mathrm{CW}+\mathrm{RC}$ mixture increase, while its resilient modulus and shear modulus decrease. Based on these properties, an amount of 10% RC is recommended as an optimal blended mix to be used as railway subballast. A three-dimensional (3D) empirical model of the relationship between the maximum axial strain, volumetric strain, and resilient modulus with RC contents and the effective confining pressure was developed, and the energy-absorbing capacity of these waste mixtures has also been analyzed for practical purposes based on their comprehensive parameters.

Small-strain behavior is a key indicator for assessing the performance of compacted fills. Compaction conditions, i.e., initial moisture content and applied energy, govern compaction effectiveness and thus, the structure and matric suction of compacted soil. This paper presents an experimental study of the small-strain behavior of compacted silty sand prepared with different compaction conditions. Specimens with varying initial moisture contents and compaction energies were tested with bender elements to determine the small-strain shear modulus (G₀), while the post-compaction matric suction was measured using the filter paper method and tensiometer. The experimental data suggest a pronounced relationship between G₀and the degree of saturation (S_r) of the as-compacted soil specimens. X-ray computed tomography (CT) scans were performed to examine structural changes of selected specimens upon compaction. The laboratory results are also examined in light of common end-product specifications, which show that it is beneficial to compact the soil slightly dry of optimum moisture content from the modulus point of view.