Introduction

Railways have emerged as the preferred mode of transportation worldwide, offering reliability, environmental friendliness, efficiency, and cost-effectiveness for both passengers and cargo. Ballasted rail corridors, widely utilized in railway infrastructure, provide numerous advantages such as lower construction costs, easy availability of materials, and simplified construction processes. These tracks consist of layered systems where interface friction and self-interlocking of the ballast layer contribute to track stability. In the context of track stability, the shear resistance at the sleeper–ballast interface plays a crucial role, especially under the increasing demands of faster and heavier trains, which induce significant lateral sliding of the ballast. The lateral stability of railway tracks is influenced by various types of sleeper–ballast interfaces, including concrete sleepers, timber sleepers, and under sleeper pads (USPs) attached to concrete sleepers. To gain insights into this aspect, a laboratory-based investigation using large-scale direct shear apparatus was conducted, focusing on the shear and degradation behavior of ballast at concrete–ballast, timber–ballast, and USP–ballast interfaces. Notably, the study incorporated the use of recycled USPs manufactured from granulated end-of-life rubber tires, aligning with the growing interest in employing waste materials for civil infrastructure construction. To further understand the shear behavior of ballast, 3D numerical models were developed using the discrete element method (DEM). These models simulated large-scale direct shear tests with different sleeper-ballast interface arrangements, enabling a comprehensive exploration of interface shear behavior. The bonded particle model (BPM) within the DEM framework accounted for the effects of particle breakage that occur during shearing. By comprehensively examining the behavior of ballast at different sleeper–ballast interfaces and incorporating recycled materials, this study contributes to optimizing track stability in response to the demands of faster and heavier trains. Additionally, it sheds light on the potential utilization of waste materials in civil infrastructure construction, aligning with the engineering community's growing interest in sustainable practices.

Methodology, Results, and Conclusions

Large-scale direct shear tests were conducted to examine four different interfaces under normal stress of 60 kPa: ballast–ballast, ballast–timber, ballast–concrete, and ballast–USP. Fig. 1 illustrates the test setup, where a 300 mm thick full-height ballast sample was used for the ballast–ballast interface, while the other interfaces included a combination of ballast and timber, concrete, or USP attached concrete sleeper. The loading plate was positioned on top of the ballast after filling the test cylinder, and the normal stress was applied using a lever arm system. The test involved subjecting the bottom cylinder to a constant shear displacement rate of 4 mm/min, reaching a maximum of 60 mm displacement (equivalent to a 15% shear strain).

The shape of the angular ballast aggregate significantly influences interface shear behavior. To address this, the discrete element method (DEM) was utilized as a numerical tool for granular materials. In this study, a novel approach was adopted to generate realistic ballast particle shapes in DEM by employing computer-aided design (CAD) templates derived from 3D scanning of original ballast particles. A library of CAD templates representing various shapes and size ranges of ballast particles was developed, incorporating approximately 12 spheres of different diameters per particle to optimize computational efficiency while preserving particle shape. Bonds between the spheres within each particle were introduced based on the bonded particle model (BPM).

The shear stress-strain variation of ballast at different interfaces was compared between DEM simulation results and experimental results, as shown in Fig. 2c. The highly angular ballast aggregates demonstrated superior self-interlocking ability, resulting in the highest shear resistance. Due to the relatively lower hardness of timber sleepers compared to concrete sleepers, the former exhibited higher shear stress. The ballast aggregates effectively interlocked with the USP surface, offering greater resistance to shearing compared to timber and concrete surfaces. The numerical results from DEM showed good agreement with the laboratory results.

Acknowledgment: Accelerating Higher Education Expansion and Development (AHEAD) Operation of the Ministry of Higher Education funded by the World Bank (Grant No: AHEAD/RA3/DOR/STEM/No.63) is appreciated for the support.

Keywords: Large-scale testing, interface shear, recycle rubber, discrete element method