Recycled materials in railroad substructure: an energy perspective

Given that the current ballasted tracks in Australia may not be able to support faster and significantly heavier freight trains as planned for the future, the imminent need for innovative and sustainable ballasted tracks for transport infrastructure is crucial. 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, as well as waste tyre cells and granulated rubber to improve the stability of track substructure including ballast and subballast layers. This paper reviews four applications of these novel methods, including using recycled rubber products such as CWRC mixtures (i.e., mixtures of coal wash (CW) and rubber crumbs (RC)) and SEAL mixtures (i.e., mixtures of steel furnace slag, CW and RC) to replace subballast/capping materials, tyre cells reinforcements for subballast/capping layer and under ballast mats; and investigates the energy dissipation capacity for each application based on small-scale cyclic triaxial tests and large-scale track model tests. It has been found that the inclusion of these rubber products increases the energy dissipation effect of the track, hence reducing the ballast degradation efficiently and increasing the track stability. Moreover, a rheological model is also proposed to investigate the effect of different rubber inclusions on their efficiency to reduce the transient motion of rail track under dynamic loading. The outcomes elucidated in this paper will lead to a better understanding of the performance of ballast tracks upgraded with resilient rubber products, while promoting environmentally sustainable and more affordable ballasted tracks for greater passenger comfort and increased safety

Multilaminate mathematical framework for analyzing the deformation of coarse granular materials

© 2020 American Society of Civil Engineers. Coarse granular materials such as railway ballast and rockfill are often subjected to three-dimensional (3D) stress conditions including the influence of intermediate principal stress. Modeling the deformation and breakage of these materials under the presence of intermediate principal stress is important for assessing their long-term performance. This paper presents a mathematical model to describe the mechanical behavior of granular materials incorporating the intermediate principal stress and capture particle breakage. The model formulation encompasses interparticle contact planes using a multilaminate mathematical framework based on generalized plasticity and associated critical state concepts. The model that has been calibrated based on recent experimental data on latite basalt, captures the stress-strain and volumetric strain behavior for a range of confining pressures under triaxial compression. This paper also describes the influence of intermediate principal stress on the strength and deformation response of selected granular materials following 3D stress paths. It is evident from the results that the current modeling technique successfully captured the effects of particle breakage, intermediate principal stress, and confining pressure on the shear behavior of various granular assemblies. The results also highlight the influence of intermediate principal stress in reducing the peak deviatoric strength of the material. The model predictions were validated using four independent sets of past experimental data on crushed basalt, limestone, sandstone, and granite aggregates

Constitutive Modelling of the Deformation and Degradation of Railway Ballast Using Multi-laminate Framework

Coarse granular materials such as railway ballast used in the shallow layers of railway tracks are often subjected to moving loads, which cause complex stress conditions involving the rotation of principal stress axes. Further, these materials are involved with particle breakage, which is also affected by the stress changes in the track. Computer modelling incorporating constitutive relationships is an effective technique for assessing the deformation behaviour of coarse granular materials under such conditions. This paper presents a constitutive model for coarse granular materials in multilaminate framework, considering classical plasticity and critical state concepts. A criterion for particle breakage in multi-laminate framework and its effect on the stress-strain behaviour has been incorporated using constitutive relationships in multilaminate framework. The influence of minor principal stress and principal stress rotation on the stress-strain, volumetric strain and particle breakage of these materials have been discussed

Behaviour of ballast under principal stress rotation: Multi-laminate approach for moving loads

© 2020 Elsevier Ltd Railway tracks are subjected to millions of loading cycles over time and at high speeds, these moving trains induce dynamic amplification of vertical stresses and rotation of principal stress axes in the track layers. It is important to predict and analyse the behaviour of ballast under these loads with complex stress paths involving principal stress rotation. In this paper, a constitutive model based on a multi-laminate framework is used to predict the deformation and degradation of ballast under complex stress paths. The yield and plastic potential surfaces are developed based on a non-linear critical state and bounding surface plasticity concepts. The proposed model is validated with independent test data to capture the influence of confining stress, loading frequency, Cyclic Stress Ratio (CSR) and Shear Stress Ratio (ητ) on the permanent strain response. Furthermore, the response of ballast under traffic loading stress paths with different CSR and ητ is analysed. These model predictions show that higher CSRand ητ values lead to exacerbated particle breakage of ballast, large and unstable axial strains and dilatant volumetric strains. Furthermore, a stability surface is proposed based on model predictions, to estimate the allowable CSR and ητ for a stable response

Shakedown response of recycled rubber-granular waste mixtures under cyclic loading

Understanding and quantifying the long-term deformation behaviour of granular materials under repeated loads is imperative for ensuring the longevity of railway tracks. One of the most relevant characteristics of granular materials under repeated cycles of loading and unloading is their ability to achieve a relatively stable state (shakedown) after being subjected to initial compression. The shakedown response of blended rubber-granular waste mixtures under triaxial test conditions has been studied by past studies highlighting the influence of the rubber content, confining stress, and cyclic loading amplitude. However, a clear methodology for estimating shakedown yield limits of these granular mixtures has not been discussed in detail. The current study highlights the influence of peak shear strength of these mixtures under static loading on their shakedown response in cyclic loading conditions. It is observed that the variation of static shear strength with rubber contents and confining stresses is found to affect the shakedown response. A unified method of estimating the shakedown limit is proposed by analysing permanent axial strains with normalised cyclic stress ratio at different loading cycles. The proposed method is validated through two independent sets of drained cyclic triaxial test data on coal wash-rubber crumb mixtures and rail ballast

Constitutive Modelling of the Deformation and Degradation of Railway Ballast Using Multi-laminate Framework

Coarse granular materials such as railway ballast used in the shallow layers of railway tracks are often subjected to moving loads, which cause complex stress conditions involving the rotation of principal stress axes. Further, these materials are involved with particle breakage, which is also affected by the stress changes in the track. Computer modelling incorporating constitutive relationships is an effective technique for assessing the deformation behaviour of coarse granular materials under such conditions. This paper presents a constitutive model for coarse granular materials in multilaminate framework, considering classical plasticity and critical state concepts. A criterion for particle breakage in multi-laminate framework and its effect on the stress-strain behaviour has been incorporated using constitutive relationships in multilaminate framework. The influence of minor principal stress and principal stress rotation on the stress-strain, volumetric strain and particle breakage of these materials have been discussed