PANDOSCOPE® Rail Formation & Ballast Condition Assessment

The need for better asset condition data is driving change within the UK rail industry in terms of embracing new technology. With good quantitative data, we can better analyse asset condition and deterioration rates which will improve decision making on maintenance interventions and track renewals specifications. This is particularly true for track bed investigations. The Pandoscope is a simple and quick non-intrusive test that produces quantitative data to make site specific decisions. It can replace the time-consuming trial holes and the automatic ballast sampler, generally used in the UK. A case study is described in this paper to highlight the difference in approaches between the UK and France. The outputs of both methods are compared and the benefits of using the Pandoscope are demonstrated by a reduced track bed specification. Finally, proposals are made with the quantitative outputs with limits for potential changes to Network Rail standards.

KEYWORDS: Track bed investigation, sub-ballast layers, data-driven decisions, quantitative data

Ballast characterization is a fundamental task to optimize decision-making in maintenance or renewal programs of railway tracks. For that purpose, be able to evaluate the fouling and the deterioration of the ballast are the major problem. This study aims to characterize the fouled ballast using a Lightweight dynamic cone penetrometer (LDCP) and g´ eoendoscope test. The methodology presented consists in analyzing the resistance properties of the ballast layer by means of dynamic cone resistance (qd) and then performing a geoendoscopy test to obtain borehole images for the material characterization. Statistical analysis of cone resistance and statistical image parameters (texture and colometry) shows their dependence on the degree of fouling as well as on its nature. As ballast fouling increases, the average cone resistance initially increases and then decreases. Images features also depend on the proportion of fouling but especially allow to distinguish different contents of fine particles in the fouling material. However, each of the measurements taken separately is not sufficient to determine a ballast fouling amount. The final results indicate that the method can accurately determine the fouling index with minimal variations.

Keywords: Laboratory test, Penetrometer, Geoendoscopy, Railway, Ballasted track, Fouling Index, Statistical analysis, Data fusion, Grain size

The French rail network comprises 30,000 km of lines including 2,600 km of high-speed lines. Thanks to specific equipment monitoring the track geometry, infrastructure managers can adapt maintenance operations like tamping and build track renewal programs (rail, sleepers, ballast). With the increase of traffic and the aging of track, maintenance operations to restore the track level are quite frequent. In this framework the renewal program (rail-sleeper/ballast or ballast) of the first European high-speed line built in the early ‘80s between Paris and Lyon is planned in the coming years. To guarantee the durability of these renewal works, a multi-domain approach to analyse the complete system: track, platform and environment is proposed. First an interactive database integrating the parameters related to the description, use, state and environment of the network has been created. Second an analysis of the database parameters affecting the durability of the railway structure and a methodology of prioritization on sections requiring finer analysis have been developed. The first results indicate that about 12% of the tracks require additional evaluation of the sub-ballast layers and / or hydraulic system. This methodology has been validated using investigations and in-situ observations and is now spread-out on other French high-speed lines.

KEYWORDS: railway, maintenance, geotechnics, drainage, hydrogeology, holistic, assessment approach, high-speed lines

Dynamic penetrometer tests are widely used in geotechnical studies for soils characterization but their implementation tends to be difficult. The light penetrometer test is able to give information about a cone resistance useful in the field of geotechnics and recently validated as a parameter for the case of coarse granular materials. In order to characterize directly the railway ballast on track and sublayers of ballast, a huge test campaign has been carried out for more than 5 years in order to build up a database composed of 19,000 penetration tests including endoscopic video record on the French railway network. The main objective of this work is to give a first statistical analysis of cone resistance in the coarse granular layer which represents a major component of railway track: the ballast. The results show that the cone resistance (qd) increases with depth and presents strong variations corresponding to layers of different natures identified using the endoscopic records. In the first zone corresponding to the top 30cm, (qd) increases linearly with a slope of around 1MPa/cm for fresh ballast and fouled ballast. In the second zone below 30cm deep, (qd) increases more slowly with a slope of around 0,3MPa/cm and decreases below 50cm. These results show that there is no clear difference between fresh and fouled ballast. Hence, the (qd) sensitivity is important and increases with depth. The (qd) distribution for a set of tests does not follow a normal distribution. In the upper 30cm layer of ballast of track, data statistical treatment shows that train load and speed do not have any significant impact on the (qd) distribution for clean ballast; they increase by 50% the average value of (qd) for fouled ballast and increase the thickness as well. Below the 30cm upper layer, train load and speed have a clear impact on the (qd) distribution.

The principal issues of the asset manager are how to prioritize maintenance/renewal works and how to provide engineering teams with reliable geotechnical data for track design on revenue service lines. Indeed, if we can master rail and sleeper specifications, railway natural trackbed remain very variable and difficult to characterize.

As a matter of fact, the knowledge of mechanical and physical properties of existing subgrade and sub ballast layers is very important for the future track design. Such data can be acquired through geotechnical tests. However, the majority of classical geotechnical tests can be difficult to carry out on revenue service lines because of existing railway constraints (limited possession times, track access, no destabilization of the track…).

Hence, this article presents a new methodology for railway track characterization using light and cost effective tests, based on the coupled use of the PANDA® dynamic penetration tests and geoendoscopy. The goal of this methodology is to provide to the asset manager key point indicator helping with the optimization of the maintenance and renewal strategy

By means of a detailed parametric study of the settlement of ballast material performed on a full-scale track model, a predicting model for ballast settlement under cyclic loading has been developed. The model is based on track parameters: axle load, train speed and the initial mechanical state of ballast. A light dynamic penetrometer allows the characterization of the latter. Several loading tests, were performed, together with a characterization of the initial state of ballast close to the sleepers. Three hundred and sixty curves of settlement were obtained. The results show a settlement evolution in three stages (short, medium and long term settlement) depending on the initial conditions of the material and the intensity of the vibration. The settlement models reviewed in literature (Shenton, Hettler or Tom and Okley model) are not able to describe the entire evolution of the settlement curves. By developing a model based on the Chicago’s density relaxation law, it is possible to estimate the evolution of settlement from the loading parameters and the initial mechanical state of the ballast material. From the database and studies of the Chicago experiments two governing parameters, can be identified, which represent the intensity of loading and cone resistance in coarse granular material. From these parameters it is possible to identify three functions which give the three parameters of the proposed model of the settlement. The proposed model describes the three stages in the settlement evolution. This model, si selected, for the results obtained in the global analysis of error by the MSE method. With this model, we obtain a prediction of settlement evolution with an error less than 10% for almost all experimental data. As a conclusion a method is proposed which has been tested on a track and which allows one to estimate the settlement of the sleepers by considering train traffic and cone penetration resistance in order to obtain an average settlement curve and a probability to reach a threshold value. This is a first step towards a general framework for the evaluation of the geometric potential degradation of railway tracks, which is a major issue for the cost reduction of maintenance operations on railway tracks.

Keywords: ballast settlement, full-scale track model, settlement model, light penetrometer test.

The subgrade is an integral component of the track structure and its performance properties must be considered in order to effectively assess its influence on subsequent track quality. European and Asian railways are particularly advanced in implementing subgrade performance indicators into their track designs and assessments. As train speed and tonnage increase in the U.S., the evaluation and influence of subgrade performance will become even more paramount.

There are numerous means of measuring and predicting subgrade performance. Both laboratory and in-situ test methods have been used. A review of available testing methods is presented herein in the context of railroad subgrade assessment. Discussion on the applicability of each test to the American railroad industry is also included. In-situ tests likely provide the greater advantage in railway engineering because results can typically be obtained quickly, more cost effectively, and with a larger data set. Newer rail-bound, continuous testing devices, while not testing the subgrade directly, are extremely convenient and will likely become more common in the future.

Ballast is a major railway component whose behaviour is still not sufficiently controlled. The identification of mechanisms leading to track ageing is difficult to achieve as the process occurs over several years at particle scale.

Models have been proposed to take into account ballast characteristics and provide a description of geometrical and structural modifications of ballast particles through time. To be relevant, these models must be supplied with reliable and realistic input data such as on-site density and stiffness modulus.

This article presents results that could provide these parameters, starting with on-site tests that link them with cone penetration energy.

Vertical track stiffness is an important parameter in railway track engineering, both from a design and maintenance point of view. This guideline presents important aspects of track stiffness as well as different measurement methods to gather stiffness information of the track.

A method called Panda, for determining local track stiffness has been used and developed. Panda is a lightweight penetrometer which determines the cone-resistance of the layers of the track substructure rapidly.

Railway tracks, like all civil works, experience degradation after being put into service, thus losing their initial design conditions. This degradation manifests itself mainly through misalignment and specific deterioration mechanisms of each component, which reduces safety and comfort during the passage of trains. In order to maintain their operational capabilities or prolong their service life, continuous monitoring, diagnosis and maintenance processes are carried out. These actions aim to restore the physical, mechanical and geometrical properties necessary for track operation.

In practice, rail managers often employ hegemonic techniques such as geometric measurements to detect defects and routine maintenance to realign, known as tamping. Although these practices have proven to be effective, in some cases they do not technically and economically competently solve the root problem. For this reason, diagnosis should be multidisciplinary and incorporate various disciplines associated with the track components.

This paper aims to highlight the importance and impact of geotechnical diagnosis in the management of railway networks. To achieve this, the Article has the following structure: (a) Railway track components: the main elements of the track are described and the role of geotechnical components in the stability of the infrastructure is highlighted, (b) Track degradation: The main deterioration of the track, such as loss of geometry, and its relation to the wear of the geotechnical elements are addressed, (c) Monitoring and monitoring: the most commonly used geotechnical inspection methods and their effectiveness in the analysis of granular material are presented, (d) Geotechnical diagnosis: Current maintenance management is analyzed and guidelines for a comprehensive diagnosis are suggested, highlighting cases where poor assessment has generated problems, (e) Maintenance techniques: Preservation methods are reviewed, stressing the importance of accurate diagnosis for their effectiveness, and (f) Future perspectives: Trends such as the use of artificial intelligence and advanced data management in the railway sector are explored.

https://www.sciencedirect.com/science/article/pii/S2666691X24000678?via%3Dihub