Introduction: Increasing demand for high-speed rail and heavier freight transport is posing a huge challenge for railway transport authorities in many countries around the world. The rapid growth in population, urbanization, as well as increasing congestion in highway transport and growing demand for energy (e.g., fuel), are forcing the railway authorities to improve the rail track transport system. The implementation of faster passenger trains and heavy haul freight networks is inevitable. This causes large undue pressure on the track structure, excessive track layer settlement, and material degradation which ultimately leads to frequent derailments, accidents and costly maintenance. Therefore, the adoption of improved techniques in rail track structures is imperative for the sustainable development of modern track infrastructure. In order to improve track performance, increase the longevity of tracks, and reduce the cost of maintenance, artificial inclusions such as resilient rubber elements can be beneficial. Rubber has an excellent energy absorbing capacity and lessens track vibrations significantly. In this respect, using resilient rubber elements underneath the concrete sleepers (i.e., Under Sleeper Pads - USPs) and on top of a stiff subgrade (i.e., Under Ballast Mats - UBMs) are popular ways of restoring the resiliency of stiffer track substructure. 

Method: The disparity between the actual particle size used in a track foundation and scaled-down particles used in a specimen for conventional shear and triaxial testing leads to misleading stress-strain responses in the laboratory due to the size-dependent compressibility, dilation, and crushing aspects of soil aggregates. Thus, large-scale testing apparatus with a specimen of actual particle size provides more insightful and tangible practical outcomes to geomechanical problems especially for ballast particles used in rail track substructure. This study focused on a comprehensive assessment of the geotechnical characteristics of railway ballast stabilized with rubber elements (USP and UBM of 10 mm thickness) and the associated response of tracks under cyclic and shear loads using large-scale laboratory triaxial and direct shear testing and numerical simulations.

Results and Conclusions: The large-scale laboratory testing and numerical simulation results showed that the energy-absorbing characteristics of rubber elements reduce the amount of deformation and degradation of ballast in the rail tracks. The rubber elements distribute the loading from moving trains more uniformly by increasing the effective contact area, which also contributes to the reduction in pressure and track degradation. A substantial reduction in track damage and extended track longevity would embrace blatant ‘green’  implications (i.e. less quarrying raw materials and environmental degradation) while accruing substantial annual savings to the rail asset owners. UBMs used in this study were manufactured from the recycled shredded tyre. In terms of national benefits, manufacturing these rubber mats from recycled rubber tyres would reduce their volume at spoil tips and contribute to more effective land use, especially in expensive urban suburbs.

Acknowledgement: The support provided by the Accelerating Higher Education Expansion and Development (AHEAD) Operation of the Ministry of Higher Education funded by the World Bank (Grant No: AHEAD/RA3/DOR/STEM/No.63) is gratefully appreciated.

Keywords: ballast, rubber, degradation