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Track Design, Dynamics and Modelling
Published in Simon Iwnicki, Maksym Spiryagin, Colin Cole, Tim McSweeney, Handbook of Railway Vehicle Dynamics, 2019
The subgrade, also called the substructure, is the foundation of the track bed, which carries the static and dynamic loads from the track and train and distributes these loads deep into the underlying ground. The surface layer of the subgrade requires a certain strength to bear train dynamic loads over the long term and should have enough stiffness to limit its elastic deformation to a certain range. In addition, the subgrade should have sufficient depth to ensure that the dynamic stress transmitted to the bottom layer of the subgrade is smaller than the load-bearing capacity of the underlying natural ground. The subgrade is a very important component of the track structure, and not recognising this has been the cause of track failure and poor track quality; for example, uneven subgrade settlement can cause interface damage of slab tracks [3,4] and can disable the proper functioning of sleepers of ballasted tracks [5,6]. The subgrade composed of geomaterial is a potentially weak and unstable part of railway lines. Therefore, subgrade deformation control is critical for the success of subgrade design.
Ground penetrating radar applications and implementations in civil construction
Published in Journal of Structural Integrity and Maintenance, 2023
Macy Spears, Saman Hedjazi, Hossein Taheri
An implementation of GPR in the maintenance of infrastructure is seen in the assessment of railway tracks where the condition of the tracks is evaluated to determine if any parts need to be replaced. The track-bed consists of two main components that can be referred to as the superstructure and substructure; the superstructure consists of the rails, fastening system and sleepers, whereas the substructure is used to describe the granular layers below the superstructure: ballast, sub-ballast and subgrade (Artagan & Borecky, 2020; Ciampoli et al., 2019). Using the penetrating radar method, the ballast thicknesses and quality can be observed, important for ensuring the layers of materials are able to bear the stress from trains. The condition of the ballast can be evaluated from the dielectric properties or reflected energy, which depend on moisture content, surface slope and aggregate dielectric characteristics (Al-Quadi et al., 2010). The ballast layers can be divided into three layers: clean, mostly clean and fouled or sub-ballast. Clean ballast consists of having high air void volume, mostly clean ballast has some finer grains in air void volume, and fouled ballast has air voids mostly filled with finer grains.
Development of a simplified design approach for shallow ballasted track forms with geocells reinforced sub-ballast
Published in International Journal of Rail Transportation, 2022
Based on the track bed samples, a number of design options are proposed in Table 6 for each line in accordance with Network Rail’s track bed standard [11] for conventional designs and using a similar approach used in Site 1 to provide alternative geocells designs. From the table, it can be noted that the required total thickness based on the conventional standard design can reach up to 0.85 m, which can be costly and difficult to construct. Therefore, a geocell design was proposed as an alternative to reduce the total thickness required to 0.5 m. It should be noted that sand fill was used as sub-ballast to also prevent subgrade erosion and improve track bed drainage.
Reliability, availability and maintainability analyses for railway infrastructure management
Published in Structure and Infrastructure Engineering, 2019
Shuhratjon Hidirov, Hakan Guler
Track bed layers have an important role in track performance with respect to track support stiffness, maintenance of track geometry and drainage. The general term for track bed layer refers to both ballast and sub-ballast layers. Ballast fouling causes track deterioration. Ballast fouling materials come from different sources which are ballast breakdown, infiltration from ballast surface, sleeper wear, infiltration from underlying granular layers and subsoil infiltration (Selig & Waters, 1994). Ballast fouling reduces the bearing capacity and the drainage function of the ballast bed. In addition, highly fouled ballast layer causes some problems related to absorbing shocks and noise levels.