PANDOSCOPE® Research Papers

Railway track-bed materials are mostly in unsaturated state, and their hydro-mechanical properties depend strongly on their water contents or suctions. In France, problematic soils such as collapsible loess and swelling marl can be origin of instability problems for new lines for high speed trains, while the hydro-mechanical behaviour of interlayer soil formed mainly by interpenetration of ballast and subgrade soil is of concern for conventional lines. In the latter case, the main challenge relies in the large variability of the interlayer soils owing to the variability of natural subgrade soils involved in the railway network.

In this paper, a study on interlayer soils is presented, that covers a large spectrum of aspects: geotechnical/geophysical site characterisation, laboratory investigation into the hydraulic behaviour and mechanical behaviour as well as the mud pumping/interlayer creation phenomena, and field monitoring. The combined effect of water content and fines content were emphasized in the laboratory investigation, whilst interaction between atmosphere and track was focused on in the field monitoring. The results show clearly that it is of paramount importance to consider the unsaturated aspect of track-bed materials when analysing the overall behaviour of tracks, in particular when clay is involved in fine fraction.

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.

A comparison of two methods of determining the stiffness of track-bed materials (dynamic penetration and dynamic plate load) comparing the PANDA® / PANDOSCOPE® and the Light Weight Deflectometer.

An advanced technology for measuring track substructure conditions has been developed by Sol Solution in France. This technology, referred to as the PANDA® and Geo-Endoscopy® technique, comprises driving a variable energy cone penetration device into the track substructure using an instrumented anvil to collect strength (and modulus by correlation) profile with depth. Condition monitoring of the track substructure layers is accomplished through insertion of a camera into the same hole, also called Geoendoscopy®. The tests are commonly faster than Dynamic Cone Penetration (DCP) testing and cause negligible damage to the track substructure with the use of light and portable devices.

This paper presents data from PANDA® and Geo-Endoscopy® testing efforts recently carried out under the scope of an ongoing research study at the University of Illinois. The primary objective was to relate the substructure layer properties to plastic layer deformations measured through advanced geotechnical instrumentation. Combined analysis of the PANDA®, Geo-Endoscopy®, and field instrumentation data has been used to highlight the reliability of this innovative technique towards improved evaluation of track substructure layer conditions.

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.

Use of unconventional aggregate materials, such as primary crusher run and concrete demolition waste, have become viable for the construction of pavement working platforms over very weak and often wet subgrade soils. To this end, a research study was undertaken at the Illinois Center for Transportation to evaluate the adequacy and field performances of such large-sized aggregate materials and validate new material specifications.

A state-of-the-art image analysis technique was utilized to characterize the size and morphological properties, e.g. shape, texture and angularity of two large-sized aggregates, referred to herein as primary crusher run and crushed concrete. For the field evaluation, full-scale test sections were constructed with these large-sized aggregate materials over a very weak engineered subgrade and subjected to accelerated pavement testing. Construction quality control was achieved through in-place density and modulus measurements on conventional aggregate capping surface layers using nuclear gauge, lightweight deflectometer and soil stiffness gauge type devices. Periodic rut measurements were carried out on the pavement surface throughout the accelerated loading process using an Accelerated Transportation Loading Assembly (ATLAS). Contributions of the underlying pavement layers to the total rut accumulation was evaluated through innovative applications of ground penetrating radar (GPR), a light weight penetrometer device, known as the French Panda, as well as a geo- endoscopy probe. Layer intermixing and material migration at the aggregate subgrade and subgrade interface was found to improve the layer stiffness and pavement performance results significantly.

Segregation remains one of the major problems for traditional and self-compacting concrete. The consequences of this pathology are numerous and may affect the long-term properties of the structures. In order to ensure the expected characteristics of the concrete, it is essential to be able to check its homogeneity.

Some tests allow the checking of the fresh concrete properties at the concrete mixing plant, but there is at the present time no method to assess concrete segregation on site.

The development of a quick and low disturbance method allowing quantification of the segregation phenomenon automatically within structures constitutes an advance in the pathology detection area. The method presented here relies on the use of geoendoscopy and automatic image processing techniques. After a short presentation of the tools and the auscultation methodology, the image processing techniques developed in order to measure the concrete homogeneity and to control the concrete particle size distribution are exposed.

Results obtained with this methodology in laboratory experiments are then compared with those obtained with the traditional video counting technology. Finally, the last part is devoted to the application of this method to a real self-compacting concrete structure.

Given the issues surrounding the long­term future of ageing underground structures, agencies responsible for these struc­tures need to optimize their maintenance policy. This involves a better evaluation of deterioration in structures and their ability to perform their function over time.

Based on geophysical testing and non­aggressive probes, this methodo­logy offers quantitative evaluation of the condition of masonry, the state of contact between the structure and the surrounding terrain, and a description of the surrounding terrain and its variability. Based on properties obtained using this new ins­pection method, a scoring system for structures was defined in order to prioritize maintenance works.

At the same time, mo­delling work incorporating this data was performed in order to study the behaviour of the structures once the deterioration and properties measured on site had been integrated. Development and validation of this research was performed using actual structures on the RATP metro network in Paris. This paper pre­sents the whole of the fully­ developed diagnosis methodology for these structures, and its application to an actual structure.