The performance and deformation of ballast bed are significantly influenced by the particle morphology (size and shape), the rheology (translation and rotation), and the degradation (breakage and abrasion). Regarding the ballast particle morphology, the ballast particle size is generally measured by sieving and described with the Particle Size Distribution (PSD), while the particle shape is normally classified as three characteristics, the form, angularity, and surface texture. Quantifying particle morphology with current manual methods is difficult to obtain accurate results (often subjective). Concerning the ballast particle rheology, almost all the related studies are based on numerical simulations, e.g. the Discrete Element Method (DEM). A limited number of studies were performed to record the translation and rotation with the electronic devices embedded in ballast layer. However, the numerical simulations can only precisely reflect the ballast particle rheology in quasi-static tests (e.g. direct shear test), and the electronic devices can only record the ballast particle rheology in the limited areas, where they were placed. The ballast breakage could be evaluated by the change of the PSD, but the determination of PSD involves significant errors. Additionally, the manual methods could not fully quantify the ballast abrasion. As a result, more accurate evaluation methods need to be developed and utilised for the validation and confirmation of the degradation-related studies.Towards these limitations, the studies on two-dimensional (2D) and three-dimensional (3D) image analysis methods for granular materials are reviewed, discussing their existing and potential utilisation in railway ballast applications. This paper can be of interest to the researchers, who are dealing with the performance and deformation of ballast bed. Additionally, a special attention can be paid to utilising the image analysis for accurate particle morphology quantification, particle rheology investigation and ballast degradation evaluation.@en

Ballast layer defects are the primary cause for rapid track geometry degradation. Detecting these defects in real-time during track inspections is urgently needed to ensure safe train operations. To achieve this, an indicator, the track degradation rate (TDR) was proposed. This rate is calculated using track geometry inspection data to locate and predict railway-line sections with ballast layer defects. The TDR is determined by the monthly standard deviation of the rail longitudinal level, which is one aspect of track geometry. The Ballast Layer Health Classification (BLHC) is designed by assessing the two successive TDRs before and after track geometry maintenance actions. The BLHC is used to categorize the conditions of the ballast layer, including normal periodic deterioration, abrupt deterioration, effective maintenance, rising deterioration, and severe deterioration. Both the TDR and BLHC were validated through field assessments of ballast layer conditions, where the two indicators were found to be effective in revealing defects. The results indicate that the TDR is sensitive to ballast layer defects, while the BLHC can quickly identify the location of these defects. Consequently, the BLHC can provide real-time guidance for ballast layer maintenance.@en

Ground penetrating radar (GPR) has been applied for ballast layer inspection for two decades, mainly for the analysis of ballast layer fouling levels. However, some issues that affect the inspection quality remain unsolved, such as issues involving the GPR equipment quality (antenna) and the correlation between the GPR indicator and fouling index. With the aim of solving these two issues, in this paper, we investigated the difference between the results of two different antennas, the GPR data processing technique, indicators for the fouling level (by GPR signal processing) and the correlation between the indicators and fouling index (obtained by sieving). The results show that the antenna quality determines the inspection quality. The indicators can reflect the ballast layer fouling level, and they correlate the best with the fouling index (obtained by the percentage of particles passing through a 5 mm sieve size). This study is helpful for the future modification of railway ballast maintenance standards.@en

Ballast layer condition should be more regularly and accurately inspected to ensure safe train operation; however, traditional inspection methods cannot sufficiently fulfil this task. This paper presents a method of ground penetrating radar (GPR) application to reflect ballast layer fouling levels under diverse field conditions (annual gross passing load, cleaning and renewal year, fouling composition and transportation type). The results show that the GPR-based inspection method can assess the ballast layer fouling level with a 1–7% difference from the traditional sieving results. Fouling composition (especially metal materials) has a great effect on the GPR signals, thus affecting the inspection accuracy of ballast layer fouling level. Developing diverse GPR-based fouling indicators (by distinguishing different GPR signal features) can improve the GPR inspection applicability to the diverse field conditions.@en

In practice, the assessment and treatment of rail corrugation are quantitatively based on the corrugation depth. Wheel–rail vertical forces (WRVF), as a direct reflection of wheel–rail interaction, can give expression to the corrugation depth and thus serve as a key parameter for assessing the corrugation. In this paper, we propose an evaluation method for rail corrugation based on the WRVF. First, a 3D wheel–rail dynamic finite element (FE) model was developed with typical parameters of CRTS II slab track and CRH3 vehicle for high-speed railways in China. The accuracy of the model was then validated with the measured WRVF data in the field. Second, using the validated model, the time–frequency domain distribution of WRVF (vehicle speed: 300 km/h) was obtained with consideration of the corrugation wavelength in the range of 40–180 mm. The non-linear least squares method and rational equation were used to fit the function between the large value of WRVF and the corrugation depth value under the conditions of different corrugation wavelengths. Next, effects of the Pinned–Pinned resonance frequency and vibration mode on the fitted parameters were analysed, by which an indicator for corrugation treatment (grinding) was proposed. Finally, the indicator was applied in the monitoring of rail corrugation for high-speed railway lines in the field. The results show that the misjudgement rate of rail grinding decisions (using the proposed indicator) is low with the accuracy at 92.5%. The proposed method can provide a basis for the rail corrugation evaluation and grinding decisions-making.@en

Higher speed, more freight and frequent maintenance increase ballast degradation and reduce the ballast lifespan. To reduce ballast degradation, crumb rubber used as buffering aggregates in ballast bed is relatively unexplored and needs more studies, because using it has the advantage of reusing the waster rubber and absorbing the noises. The effects of crumb rubber (CR) size and percentage on ballast degradation reduction is studied in this paper, and the optimal CR size and percentage are proposed. Three CR size ranges are utilised, i.e., 3 ∼ 5 mm, 10 ∼ 15 mm and 20 ∼ 25 mm, and the percentages are 0, 10, 20 and 30% by weight. Three kinds of ballast material with two size ranges are utilised. The deteriorated ballast particles were generated using Los Angeles Abrasion (LAA) tests, and the ballast degradation was evaluated with the 3D image analysis. The results indicate that ballast abrasion can be alleviated by adding the CR, while the CR has few influences on the ballast breakage. When CR size is close to ballast particle size, the effects of degradation reduction are not obvious. The corner and edge loss are the main types of ballast abrasion, although different ballast materials significantly influence the abrasion type and degree. Most importantly, the image analysis method is proved to have the ability to present ballast degradation process and has great potential for further degradation-related studies.@en