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 (τtensile) and passive strength (τpassive) of a subballast-geocell assembly under a given range of overburden pressure (1 kPa ≤ q ≤ 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 (σn) 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 τpassive. 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 (δ)

Performance assessment of geocell-reinforced subballast: Modeling and design implications

This paper presents a study of the load-deformation behavior of geocell-stabilised subballast subjected to cyclic loads using a large-scale track process simulation apparatus and numerical modelling. The tests and numerical simulations were conducted to mimic the actual track conditions. Subjected to a given frequency and cyclic loads the predicted load-deformation behavior of the subballast with and without geocell inclusions match reasonably with those measured in the laboratory, and show that geocell could effectively decrease the lateral and axial deformations of the reinforced subballast. The results also provide an insight to design of rail tracks capturing the roles of geocell in decreasing lateral deformation of subballast. Additionally, the numerical modelling carried out in this study can be applied in the preliminary design of track substructure where a wide range of subballast aggregates and geocell mattresses with varying strengths and stiffness can be considered

Modelling of geocell-reinforced subballast subjected to cyclic loading

This paper presents the experimental and numerical studies of geocell-reinforced subballast subjected to cyclic loading. A series of laboratory experiments were conducted using a large-scale prismoidal triaxial apparatus that was subjected to relatively low confining pressures of σ'3 = 10-30 kPa and a frequency of f = 10 Hz. Numerical simulations were performed using the commercial finite element package ABAQUS in three dimensions to realistically model cellular confinement, and to study the effectiveness of geocell reinforcement on subballast. A cyclic loading with a periodic and positive full-sine waveform was adopted to model the geocell-reinforced subballast, which is similar to the load carried out in the laboratory. The results of numerical modelling agreed well with the experimental data, and showed that geocell could effectively decrease the lateral and axial deformations of the reinforced subballast. The numerical model was also validated by the field data, and the results were found to be in good agreement, indicating that the proposed model was able to capture the load-deformation behaviour of geocell-reinforced subballast under cyclic loading. A parametric study was also carried out to evaluate the effect of the subballast strength and geocell stiffness on the mobilized tensile strength in the geocell mattress. It was found that the maximum mobilized tensile stress occurs on the subballast with the lowest degree of stiffness. Also the results revealed that lateral displacement decreased further by increasing geocell stiffness, and geocell with a relatively low stiffness performs very well compared to the geocell with a higher stiffness

Load-deformation Responses of Ballasted Rail Tracks: Laboratory and Discrete-Continuum Modelling

The recent and rapid urbanization and frequent congestion of roads have led to more attention being focused on ballasted tracks for freight and commuter transport. The mechanisms of ballast degradation and deformation, the need for effective track confinement, understanding of interface behaviour, determining the dynamic bearing capacity of ballasted tracks require further insight to improve the existing design guidelines for future high speed commuter and heavier freight trains. The load-deformation behaviour of ballast under cyclic loads is measured in the laboratory using a novel large-scale Track Process Simulation Apparatus (TPSA). A novel coupling model based on discrete element method (DEM) and finite element method (FEM) is developed to predict the load-deformation responses of the ballast assembly considering the interaction of discrete ballast grains and continuum subgrade. In this coupled model, the discrete ballast grains are modelled by DEM and the subgrade domain is modelled as a continuum by FEM. The results indicate that significant settlements are observed during the initial load cycles, followed by gradually increased deformation, arriving at a steady value towards the end of tests. Contact force distributions, stress contours and corresponding broken bonds are captured

Improved performance of railroad ballast using geogrids

Geogrids are commonly used to stabilise and reinforce ballast, and over the various laboratory tests have been carried out to determine how geogrids affect the interface between geogrid and ballast aggregates. This paper presents a critical review and interpretation of the results of large-scale direct shear tests and cyclic tests on key parameters such as the interlocking effects of aperture size and the location of geogrids. Field investigations from sites at Bulli and Singleton as well as findings from Discrete Element Modelling, including the influence zone of geogrid and the linear relationship between geometric anisotropy and stress ratio are examined and discussed. It also includes a presentation and discussion of analytical modelling for quantifying the geogrid reinforcing effect (pull-out tests)