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.

Railway tracks are conventionally built on compacted ballast and structural fill layers placed above the natural (subgrade) foundation. However, during train operations, track deteriorations occur progressively due to ballast degradation. The associated track deformation is usually accompanied by a reduction in both load bearing capacity and drainage, apart from imposing frequent track maintenance. Suitable ground improvement techniques involving plastic inclusions (e.g., geogrids) and energy absorbing materials (e.g., rubber products) to enhance the stability and longevity of tracks have become increasingly popular. This paper presents the outcomes from innovative research and development measures into the use of plastic and rubber elements in rail tracks undertaken at the University of Wollongong, Australia, over the past twenty years. The results obtained from laboratory tests, mathematical modelling and numerical modelling reveal that track performance can be improved significantly by using geogrid and energy absorbing rubber products (e.g., rubber crumbs, waste tire-cell and rubber mats). Test results show that the addition of rubber materials can efficiently improve the energy absorption of the structural layer and also reduce ballast breakage. Furthermore, by incorporating the work input parameters, the energy absorbing property of the newly developed synthetic capping layer is captured by correct modelling of dilatancy. In addition, the laboratory behavior of tire cells and geogrids has been validated by numerical modelling (i.e., Finite Element Modelling-FEM, Discrete Element—DEM), and a coupled DEM-FEM modelling approach is also introduced to simulate ballast deformation.

Recycling waste materials, such as steel furnace slag (SFS), coal wash (CW), and rubber crumbs (RC) for transport infrastructure is environmentally friendly and offers significant economic benefits. This paper presents a fundamental study of the geotechnical characteristics of this blended matrix (SFS + CW + RC). A semiempirical constitutive model for SFS + CW + RC mixtures is proposed within the framework of critical state (CS) soil mechanics and based on the bounding surface plasticity theory. A CS surface is formulated with the changing RC contents in the waste mixtures, and an experimental relationship between the total work input (W_total) and CS parameter (M_cs)$$ is established to capture the energy-absorbing capacity of the matrix. The theoretical model is validated using two sets of data, i.e., very recent triaxial test data obtained by the authors and totally independent test results from a past study conducted on sand-RC mixtures.

Ballasted track progressively deteriorates due to ballast degradation and track deformation under dynamic loading, and this process accelerates when train speeds increase and axle loads become heavier as the railways are seeking to serve the enhanced productivity of the mining and agriculture sectors; on this basis, improving track performance is imperative. One effective solution is to incorporate energy-absorbing materials in the rail track, particularly when these materials are recycled from mining waste and recycled rubber. In this paper the performance of the track specimen with a synthetic energy absorbing layer (SEAL) (i.e., a matrix of recycled rubber crumbs with mining waste) is investigated by a series of large-scale (prototype) cubical triaxial tests. The test results indicate that the inclusion of rubber inside the SEAL matrix has a significant influence on the lateral movement, vertical deformation, ballast degradation, and energy distribution of the track specimen. To facilitate a better understanding of the energy-absorbing mechanism with the addition of rubber, an energy-based analysis has been adopted to identify the critical amount of rubber crumbs needed to efficiently distribute the accumulated energy, hence improve track performance. It is shown that adding 10% of rubber into the SEAL matrix will provide superior performance with less ballast breakage, less vibration (as reflected by the elastic energy), and comparable settlement compared to traditional track.