Smarter Transport Infrastructure Embracing Granular Waste And Recycled Rubber – A Circular Economy Perspective

Transport infrastructure in Australia is predominantly composed of compacted granular materials used as base, sub-base, ballast, sub-ballast and capping layers for roads and railways. Replacing traditional natural rockfill with granular wastes and other forms of recycled materials such as rubber is becoming increasingly crucial in a circular economy seeking reduced capital and maintenance costs for sustainable infrastructure. Recent extension of ports and reclamation of low- lying land has also considered compacted granular waste as an alternative structural fill. In Australia, ballasted rail tracks offer the most common mode of transportation including both commuter and freight networks. However, ballast and other granulates progressively degrade under dynamic and impact loads. The degree of degradation will be accelerated due to the growing demand for elevated speeds of passenger trains and heavy axle freight trains. It is, therefore, necessary to develop novel and cost-effective technologies to enhance the longevity and performance of transport infrastructure through amended design and construction. Over the past two decades, numerous studies have been conducted under the leadership of 1st Author to investigate the ability of recycled rubber mats/pads, as well as waste tyre cells and granulated rubber to improve the stability of substructure materials for both railways and roads. This keynote 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 stability. The findings reflect the following: (i) the inclusion of recycled- rubber based synthetic energy absorbing layers significantly attenuates the magnitude of the dynamic load with depth and particle breakage, (ii) an alternative solution by using coal wash-rubber crumb 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) for railways significantly reduces permanent vertical and lateral deformation of the track as well as reducing ballast degradation, (iv) waste tyre cells infilled with granular aggregates effectively increase the stiffness and bearing capacity of the capping layer and assist in mitigating excessive lateral displacement, and (v) field tests indicate geogrids and shock-mats are efficient methods to reduce the substructure displacement and particle breakage. These research outcomes provide promising approaches to transform traditional transport infrastructure design practices to cater for future high axle rolling stock and for enhanced longevity and reduced maintenance of all modes of transport corridors.