Smart Solutions in a Circular Economy for Advancing Railroad Design and Construction Using Recycled Materials

Ballasted rail tracks are the most common type of transportation infrastructure. However, ballast progressively degrades under dynamic and impact loads. The degree of degradation will be accelerated due to the increasing demand for elevated speeds of passenger trains and heavier axle loads for freight trains. It is, therefore, necessary to develop novel and cost-effective technologies to enhance the longevity and performance of tracks through amended design and construction. Over the past two decades, a number of studies have been conducted by the researchers of Transport Research Centre (TRC) at the University of Technology Sydney (UTS) to investigate the ability of recycled rubber mats/pads, as well as waste tyre cells and granulated rubber to improve the stability of track substructure materials including ballast and sub-ballast layers. This paper presents an overview of these novel methods and materials based on comprehensive laboratory tests using iconic testing facilities. Test results from comprehensive laboratory tests and field studies have indicated that the use of energy-absorbing rubber inclusions can substantially improve overall track stability. The findings reflect the following: (i) the inclusion of recycled-rubber based synthetic energy absorbing layers (SEAL: SFS-CW-RC mixture) significantly attenuates the magnitude of the dynamic load with depth and ballast breakage, (ii) an alternative solution by using CW-RC mixtures as capping layer is also introduced in this study, and the compressibility of the rubber is captured by cyclic compression triaxial tests, (iii) the installation of under ballast mats (UBM) significantly reduces permanent vertical and lateral deformation of ballast as well as ballast degradation, (iv) waste tyre cells infilled with granular aggregates effectively increase the stiffness and bearing capacity of the capping layer and help mitigate track displacement, and (v) field tests indicate geogrids and shockmats are efficient methods to reduce the track displacement and ballast degradation. These research outcomes provide promising approaches to transform traditional track design practices to cater for future high-axle rolling stock carrying heavier loads.