Improved Performance of Ballasted Tracks under Impact Loading by Recycled Rubber Mats

Ballasted tracks at transition locations such as approaches to bridges and road crossings experience increasing degradation and deformation due to dynamic and high impact forces, a key factor that decreases the stability and longevity of railroads. One solution to minimise ballast degradation at the transition zones is using rubber energy absorbing drainage sheets (READS) manufactured from recycled tyres. When placed beneath the ballast layer, READS distributes the load over wider area and attenuate of the load over a longer duration thus decreasing maximum stress, apart from reducing the energy transferred to the ballast and other substructure components. Subsequently, the track substructure experiences less plastic deformation and degradation. These mats also provide an environmentally friendly and cost-effective alternative. In this study, a series of large-scale drop hammer impact tests was carried out to investigate how effectively the READS could attenuate impact loads and help mitigate ballast deformation and degradation. Soft and stiff subgrade were used to investigate the load-deformation response of ballast (with and without READS), subjected to impact loads from a hammer dropped from various heights (hd =100 - 250 mm). Laboratory test results show that the inclusion of READS helps to reduce the dynamic impact load transferred to the ballast layer resulting in significantly less permanent deformation and degradation of ballast, apart from significant attenuation of load magnitude and vibration to the underlying subgrade layers

Study on the interface behavior of a geosynthetics-reinforced fouled ballast using the discrete element method

Railways offer an efficient and economic transport mode in many parts of the developed countries including Australia, China and USA. Ballast layer is designed as a load bearing layer for rail tracks and to be free draining, but when the ballast voids are wholly or partially impeded due to the intrusion of fine particles, the ballast can be considered to be fouled. Fouling causes a reduction in the drainage capacity of ballast, thereby reducing the track resiliency and triggering high maintenance costs. Geosynthetics are commonly used in railway construction for reinforcement and stabilization purposes. When railway ballast becomes fouled the beneficial effect of the geosynthetics could decrease significantly. This paper presents a study of how the interface behavior of geosynthetics-ballast copes with fouling using discrete element modelling (DEM) of large-scale direct shear tests. A series of large-scale direct shear tests for coal fouled ballast were carried out in the laboratory and were then simulated in the DEM. Shear stress-strain and volumetric dilation responses obtained from the DEM simulations were in reasonable agreement with those measured experimentally. The contact force distributions of fresh and fouled ballast were captured and shown that the fouled ballast exhibited higher number of contact forces compared to the fresh ballast assembly. This is due to coal fines accumulated in voids among large particles then partially carry and transmit contact forces across the assembly. Strains developed horizontally across the geogrid were also analyzed in this study

Investigating the shear behaviour of fouled ballast using discrete element modelling

For several hundred years, the design of railway tracks has practically remained unchanged. Traditionally, rail tracks are placed on a ballast layer due to several reasons, including economy, rapid drainage, and high load bearing capacity. The primary function of ballast is to distributing dynamic track loads to sub-ballast and subgrade layers, while also providing lateral resistance and allowing for rapid drainage. Upon repeated trainloads, the ballast becomes fouled due to ballast degradation and the intrusion of fines which adversely affects the strength and deformation behaviour of ballast. This paper presents the use of three-dimensional discrete element method (DEM) in studying the shear behaviour of the fouled ballast subjected to direct shear loading. Irregularly shaped particles of ballast were modelled by grouping many spherical balls together in appropriate sizes to simulate representative ballast aggregates. Fouled ballast was modelled by injecting a specified number of miniature spherical particles into the void spaces. The DEM simulation highlights that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increase level of fouling. Additionally, the distributions of contact force chain and particle displacement vectors were captured during shearing progress, explaining the formation of shear band and the evolutions of volumetric change of fouled ballast

Performance evaluation of railway subballast stabilised with geocell based on pull-out testing

A large-scale apparatus was designed and built at the University of Wollongong to evaluate the pull-out strength of rail subballast reinforced with geocells. A series of tests were carried out to investigate the pull-out resistance, mobilised tensile strength (ttensile) and passive strength (tpassive) of a subballastgeocell assembly under a given range of overburden pressure (1 kPa \u3c q \u3c 45 kPa). The interface was held in a vertical alignment to better simulate the interaction between subballast and geocell in accordance with routine track practices. The test results show that the geocell reinforcement provides a considerable degree of passive resistance, where the opening area (OA) and lateral pressure (sn) over the geocell strip are found to be influential factors. A three-dimensional finite element simulation was also conducted. The numerical results show that the tensile strength mobilised in the geocell will increase as the geocell stiffness increases, but causes a reduction in tpassive. A parametric study was also developed to investigate the impact of geocell stiffness and friction coefficient on the passive resistance and mobilised tensile strength. These results indicate that the passive resistance and mobilised tensile strength increase with the increase in overburden pressure (q) and friction coefficient (d)

Performance improvement of rail track substructure using artificial inclusions - experimental and numerical studies

Large and frequent loads from heavy freight and passenger trains often lead to the progressive track deterioration. The excessive deformation and degradation of ballast and unacceptable differential settlement of track and/or pumping of underlying soft subgrade soils necessitates frequent and costly track maintenance. However, artificial inclusions such as geogrids and shockmats can mitigate ballast degradation and improve track performance. A quantitative assessment of the influence of breakage, fouling, and the effects of artificial inclusions on the shear behaviour of ballast can be performed either experimentally or numerically. Numerical modelling can simulate these aspects subject to various types of loading and boundary conditions for a range of material properties so in this study, the stress-strain and degradation response of ballast was analysed through discrete element (DEM) and finite element (FEM) methods. In DEM, irregularly shaped ballast aggregates were simulated by clumping together spheres in appropriate sizes and positions. In FEM, a composite multi-layer track system was simulated and an elasto-plastic model with a non-associative flow rule was used to capture ballast degradation. These DEM and FEM simulations showed a good agreement with large-scale laboratory tests. This paper outlines the advantages of the proposed DEM and FEM models in terms of capturing the correct stress-strain and degradation response of ballast with particular emphasis on particle breakage and fouling, as well as applications of geosynthetic grids and shockmats

Application of model reduction for robust control of self-balancing two-wheeled bicycle

In recent years, balance control of two-wheeled bicycle has received more attention of scientists. One difficulty of this problem is the control object is unstable and constantly impacted by noise. To solve this problem, the authors often use robust control algorithms. However, robust controller of self-balancing two-wheeled bicycle are often complex and higher order so affect to quality during real controlling. The article introduces the stochastic balanced truncation algorithm based on Schur analysis and applies this algorithm to reduce order higher order robust controller in control balancing two-wheeled bicycle problem. The simulation results show that the reduced 4th and 5th order controller arcoording to the stochastic balanced truncation algorithm based on Schur analysis can control the two-wheeled bicycle model. The reduced 3rd order controller cannot control the balance of the two-wheeled bicycle model. The reduced 4th and 5th order controller can replace the original controller while the performance of the control system is ensured. Using reduced 5th, 4th order controller will make the program code simpler, reducing the calculation time of the self-balancing two-wheel control system. The simulation results show the correctness of the model reduction algorithm and the robust control algorithm of two-wheeled self-balancing two-wheeled bicycle

Improved performance of geosynthetics enhanced ballast: laboratory and numerical studies

Ballasted rail tracks form one of the most important worldwide transportation modes in terms of traffic tonnage, serving the needs of bulk freight and passenger movement. High impact and cyclic loads can cause a significant deformation leading to poor track geometry. In order to mitigate these problems, the concept of the inclusion of geosynthetics in rail tracks is introduced. This paper presents the current state-of-the-art knowledge of rail track geomechanics, including results obtained from laboratory testing, field investigations and numerical modelling to study the load-deformation behaviour of ballast improved by geosynthetics. The shear stress-strain and deformation behaviour of geosynthetic-reinforced ballast are investigated in the laboratory using a large-scale direct shear test device, a track process simulation apparatus and a drop-weight impact testing equipment. Computational modelling using the discrete-element method is employed to simulate geosynthetic-reinforced ballasted tracks, capturing the discrete nature of ballast aggregates when subjected to various types of loading and boundary conditions. Discreteelement modelling is also used to conduct micromechanical analysis at the interface between ballast and geogrid, providing further insight into the behaviour of ballast subjected to cyclic loadings. These results provide promising approaches to incorporate into existing track design routines catering for future high-speed trains and heavier heavy hauls

Experimental and discrete element modelling of geocell-stabilized subballast subjected to cyclic loading

This paper presents a study of the load-deformation behaviour of geocell-stabilised sub-ballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10-30 Hz. This frequency range is generally representative of Australian Standard Gauge trains operating up to 160 km/h. The discrete element method (DEM) was also used to model geocell-reinforced sub-ballast under plane strain conditions. The geocell was modelled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating geocell. To model irregularly-shaped particles of sub-ballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced sub-ballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that geocell decreased the vertical and lateral deformation of sub-ballast assemblies at any given frequency. Furthermore, the DEM can also provide an insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally