On a daily basis, railway convoys carry vast numbers of commuters, freights and bulk goods by traveling along ballasted tracks. Through the years, role of railways in connecting residential and production poles has become crucial, fostering huge improvements in the technical performance of the convoys that, in turn, require adequate support from the track-beds. In view of this, a more effective and timely maintenance is needed for railway ballast in order to ensure suitable safety and functionality conditions at the network level.
According to the literature, a typical railway track-bed can be described as an overlap of two structural elements, namely, the superstructure and the substructure [1
]. The superstructure is composed of the rails, the fastening system and the sleepers, whereas the substructure includes the granular layers that are laid upon the subgrade, i.e., the ballast and the sub-ballast. Specifically, ballast is referred to as a homogeneously graded hard-rock-derived material, usually composed of aggregates with a diameter size ranging between 3 and 6 cm [2
The bearing capacity of the substructure stands as a major concern for designers and maintainers, as differential settlements are usually reported to occur within this system [1
]. This is mainly due to i) a change in the material grading caused by the breakage of the sharp corners of the aggregates under the effects of cycling loadings [5
] and ii) the contribution of fine-graded material from the ballast-sleeper and the wheel-rail friction filling the inter-granular air voids [4
]. This material can be directly poured by the passing freights [7
] or it can migrate upwards or sideways from the subgrade along with capillary water [8
]. In fact, both the occurrences affect a decrease of the shear angle between the aggregates and, therefore, they are mostly responsible for triggering plastic and irreversible deformations of the track-bed [1
]. According to the above, a comprehensive knowledge of the condition of the railway substructure is crucial to prioritise maintenance activities and, in parallel, to maintain the highest safety standards.
Necessarily, the monitoring procedures that allow for such a knowledge have to comply with cost-benefits considerations. As a result, the quality of the monitoring in terms of dedicated funds is typically related to the rate of utilization of the inspected railway stretch [3
]. In such a framework, increasingly efficient inspection techniques are required in order to lower the cost-to-benefit ratio, and allow for a wider assessment of the track-bed conditions over the entire network.
Within this context, it is worth mentioning that the traditional monitoring procedures of visual inspections are still widely diffused. On-site operators are required to evaluate the conditions of the railway track by reporting the presence and the extent of decay for each visible structural component. In regard to ballast, it is clear that this approach allows the assessment of signs of fragmentation and fouling only at the surface level, neglecting deep pollution and fragmentation of the aggregates at the foundation level.
Nowadays, automatic laser-based systems are employed as an integration to the traditional approach. These systems, mounted onto dedicated diagnostic convoys, allow us to monitor the geometric parameters of steel rails (i.e., gauge, rail alignment) and, therefore, to observe anomalous deformations along the track. Even though this approach is evidently more robust than performing only visual inspections, it allows a partial and late assessment of the condition of the substructure. Both fouling and fragmentation are in fact detected only at a very advanced stage, i.e., when the deformation of the rails has started.
In view of this, advanced diagnostic trains equipped with additional systems for the direct assessment of the condition of the ballast have been introduced in the past few years [3
]. Among the other equipment, Ground-Penetrating Radar (GPR) is one of the most employed for this type of service.
GPR is a widespread geophysical technique, which allows us to inspect relevant features of the subsurface using information from the propagation of electromagnetic (EM) fields [8
]. In terms of working principles, a source inner to the GPR system emits an EM impulse that is partially back-reflected and partially transmitted beyond at any given dielectric contrast encountered throughout the medium. The collection of these diffractive occurrences through a receiving station allows us to image the subsurface features, in both two and three dimensions [8
According to the literature, first attempts of using GPR as a potential method for assessing the conditions of ballasted railway track-beds date back to the early years of this century [10
]. In these experimental activities, surveys were conducted using GPR systems with central frequencies below 500 MHz. More recently, high-frequency (1000–2000 MHz) air-launched systems have been mostly adopted [12
], due to the advantages observed in terms of results viability and survey productivity.
It is important to observe that the research focus on the subject area has also changed over the years [17
]. The assessment of the geometry of the track-bed and its overall stability have been first a matter of research [10
], whereas the focus has progressively moved towards the time-domain [14
] and the frequency-domain [12
] analyses of the effects of fouling and fragmentation of ballast on the signal. In addition to this, new avenues in assessing quality of ballast aggregates have been recently explored by means of simulation-based approaches [25
] and numerical models [26
]. To the best of the authors’ knowledge, although crucial pieces of information were derived from these studies, only few laboratory and test-site activities are actually reported in the literature. This is most likely due to the high costs and the labor extensiveness of the operations [2
This work reports the results of an extensive experimental campaign conducted on a test-site located on a real-life railway in Rome, Italy. The main aim of the research is to investigate the viability of GPR in assessing the health conditions of railway track-beds and to evaluate its suitability for implementation into maintenance management programs. In this regard, the effects of fouling and fragmentation of the ballast on the EM signal are analysed and interpreted with a special reference to potential future applications in maintenance and monitoring activities. Modelling is finally proposed to estimate fouling content.
The specific objectives of this study can be summarised as follows: i) understanding the influence of fragmentation and fouling of ballasted track-beds on the EM signal as collected by GPR; ii) defining the role of soil moisture within the detection of ballast decay; iii) providing indications on the most suitable survey configuration (central frequency of the antenna and its polarisation) according to the specific target of the survey.
4. Conclusions and Practical Implications
This work reports the experimental activities carried out on a test-site area within a railway depot in Rome, Italy. A 30 m-long railway section was divided into 10 sub-sections reproducing different various physical and structural conditions of the track-bed.
Combinations of different scenarios of fragmentation and fouling of the ballast were reproduced. The set-up was then investigated using different multi-frequency (1000, 2000 MHz) GPR horn antenna systems. These were towed along the rail sections by means of a dedicated railway cart that allowed the antenna to be oriented with an angle of 0°, 45° and 90° with respect to the rails.
The effects of the physical conditions of the railway ballast on its EM response was estimated for each scenario using time- and frequency-domain signal processing techniques. Parallel to this, modelling was provided to estimate fouling content.
Interpretation of the results has shown viability of the GPR method in detecting signs of decay at the network level, thereby proving this technique to be worthy for implementation in asset management systems. Such a conclusion confirms previous research findings on this subject [12
]. However, the present research has pointed out key information in regard to the practical use of an air-coupled GPR system for assessing fragmentation and fouling in railway track-beds.
In this regard, use of high-frequency antenna systems was reported to be promisingly effective in detecting the fragmentation rate of the aggregates. In case of track-beds isolated from the subgrade, such as the typical modern high-speed tracks, the spectral analysis of the 2000 MHz signal is expected to provide viable information about this particular type of decay.
On the other hand, time-domain and combined time-frequency analyses of the 1000 MHz antenna system, allowed us to achieve a reliable indication of the pollution rate of voids within the aggregates. The influence of fouling on the EM response was observed to be particularly relevant for highly-wet conditions. This could imply that monitoring activities may need to be carried out in relatively-dry conditions of the track-bed in order to optimise the functionality of the process.