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Monitoring and Substructure Condition Assessment of Existing Railway Lines for Upgrading to Higher Axle Loads and Speeds

A. Smekal1, E. G. Berggren1, M. Silvast2

1Banverket, Borlänge, Sweden;

2Roadscanners Oy, Tampere, Finland

Abstract

Many of the European railways have been designed for completely different traffic conditions than those required today. Upgrading of existing railway lines for higher axle loads and speeds requires new modern methods for in situ investigation of the railway ballast and substructure. Combination of continuous measurements of track geometry quality and non-destructive methods like dynamic track stiffness and Ground Penetrating Radar (GPR) can be a good example of obtaining the important information about the conditions of existing railways. A new methodology for evaluation of all available measurements completed with results from geotechnical investigations is proposed to study problems dealing with railway structure and subgrade in case upgrading or maintenance work is required. Railway investigations using Banverket´s Rolling Stiffness Measurement Vehicle (RSMV) equipped with GPR, and a methodology of comprehensive evaluation of all relevant available information have been tested on a few railway lines in Sweden since 2002. Results from these investigations are used for assessment of the root cause of existing or possible future problems with repeated track maintenance, settlement and stability when upgrading a track for higher axle load and/or speed. The paper presents practical results of investigations and a new methodology to evaluate several types of measurements in comparison with real track – substructure conditions. Suggestions on upgrading activities are given for a case study of 25 km of track with a planned increase of axle load from 22.5 to 25 metric tons. The goal is to minimize the upgrading and maintenance cost in a Life Cycle Cost (LCC) perspective.

1 Introduction

Construction of railways in Sweden started some 150 years ago. Of course at this time the design and requirements on track, substructure and subsoil were very much different from those for modern railway operations today. Axle loads, speeds and demands on quality of railway transport have increased several times during this long period. Existing railways are going to be used even in the future for new traffic conditions. In Sweden mixed traffic is common where high speed trains (200 km/h) operate on the same track as heavy freight trains. Existing railway lines are upgraded to higher axle loads mostly, from 22.5 to 25 metric tons. The aim is not only to increase loads but even decrease maintenance costs in Life Cycle Cost perspective on many important railway lines in Sweden. It is obvious that the quality of railway tracks depends on general condition of all layers under the sleepers, including subsoil. The ballast thickness, resistance to vertical and lateral forces, ballasts fouling, track geometry quality and properties of sub-ballast and subsoil are the main characteristics that can have a great impact on the track performance. In many cases there is a need for upgrading of the substructure and subsoil of the railway track to improve the quality of existing track and decrease expensive maintenance. Traditional methods of geotechnical investigation are very slow, expensive and insufficient to provide complete information about the conditions along railway lines. Therefore many railway authorities have started to use non-destructive continuous methods for investigation of existing lines. With non-destructive methods problem spots can be detected and additional geotechnical investigations can in a better way concentrate on clarifying causes of problems and help to design mitigation of problem for particular places.

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Banverket has carried out such comprehensive investigations for upgrading existing railways to higher axle loads and speeds since 2002. Parts of this work are described in [3], [5], [6] and [7].

2 Measurement methodology components

Decision about necessary measures to upgrade an existing railway line has to be based on a comprehensive assembly of all available information. One single parameter can not sufficiently explain the substructure condition. It is not just results of measurements using RSMV and GPR but all data like recent track geometry measurements, position of all other structures (bridges, crossings, culverts, drainage) and information about maintenance that are important to evaluate when upgrading a railway line. The measurement methods used are briefly explained below. The ability to handle all measurements together in a structured way is also of great importance, since the amount of data is huge. Therefore the software RDMS (Railway Data Management System) is used. The software is also explained below. 2.1 Track geometry quality All railway infrastructure managers use measurements of track geometry quality to assure safety and ride comfort of railway operations and to plan maintenance of their tracks. In this paper only the longitudinal level (vertical irregularities) has been used, since that is the parameter most associated with substructure condition. The longitudinal level is measured with an inertial based track recording car with a flat transfer function and wavelengths between 1 – 25 meters. In this paper two measurements per year during the last three years have been used. 2.2 Geotechnical investigation When upgrading of an existing railway line is required to increase axel loads or speed, assessment of all available information including geotechnical records are necessary. Usually stability, settlement, bearing capacity, susceptibility to frost penetration and track or environmental vibrations are reviewed. Both records of old geotechnical investigations and new geotechnical surveys are used to get the best basis for decision on which parts of an existing line that will need remedy measures before the railway is opened for new operation conditions. Old geotechnical records consist of all geotechnical investigations including geotechnical profiles and in many cases results of ballast and sub-ballast surveys. Banverket uses special drilling equipment just for ballast and sub-ballast investigations where thickness of particular layers followed by laboratory assessment of the ballast fouling is performed. Classical investigation methods using drilling or excavation followed by laboratory test only give spot information and do not represent changing conditions for many kilometres of railway lines. Therefore continuous non-destructive methods like GPR or RSMV can give better information. With non-destructive methods problem spots can be detected and additional geotechnical investigation can be performed. Combination of classical and non-destructive methods is a better way of carrying out geotechnical investigations for existing railways, where causes of present and future problems can be assessed and also support a design of appropriate mitigations.

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2.3 Dynamic track stiffness During the last years Banverket has performed research on rolling dynamic track stiffness measurements, resulting in a new measurement car, called RSMV (Rolling Stiffness Measurement Vehicle) shown in figure 1.

A B Figure 1. A. Measurement equipment in the RSMV (vertically moving masses contained in steel cages, above measuring axle); B. Schematic picture of measurement principle (one side only) of RSMV. The track is dynamically excited by two oscillating masses above an ordinary wheel axle of a freight wagon. Track stiffness is calculated from measured axle box forces and accelerations as described thoroughly in [1] and [2]. Both overall measurements at higher speeds (up to 60 km/h) with 1 – 3 sinusoidal excitation frequencies or detailed investigations at lower speeds (below 10 km/h) with noise excitation up to 50 Hz can be performed. The dynamic track stiffness is a complex-valued quantity and is presented as magnitude and phase. The detailed investigation presented in the case study of chapter 3 (figure 4) is analyzed by calculating a transfer function every third meter. These transfer functions are displayed as a 2-D surface with the stiffness magnitude and phase coded in grey scale. Black shall be interpreted as a low stiffness / large phase delay and white as a high stiffness / small phase delay. The low stiffness and/or a large phase delay is most often associated with soft subsoil like clay or peat. This is important to know for further investigations of bearing capacity and stability. Transition zones between low and high stiffness can also easily be detected. 2.4 Ground penetrating radar The ground penetrating radar (GPR) uses a radio wave source to transmit a pulse of electromagnetic energy into the object. The reflected energy, originating within the object at interfaces between materials of different dielectric properties, is received and recorded for analysis. GPR data consist of changes in reflection amplitude, changes in arrival time of specific reflections and signal attenuation. Figure 2 presents an example of the GPR profile.

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Figure 2. Example of processed 400 MHz GPR antenna data from measured railway section. In order to obtain layer thickness information from GPR, the data have to be processed and interpreted after data collection. The basic processing methods are time-zero correction to surface reflection, background noise removal and horizontal filtering. After processing the data interpretation is carried out using test pit or drilling information as reference data. In railway practice, GPR can be used for detecting railway structures, determining layer thickness and subgrade soil types. This information can further be used to analyze mixing of materials and structural defects. The reflections from different railway structure layers can be seen as continuous reflectors along the profile. The GPR data helps to determine track subsurface conditions and locate frost insulation boards [4]. 2.5 RDMS – Railway Data Management System

Data visualization, interpretation and analysis are performed using a Railway Data Management System (RDMS) software [5]. RDMS is designed especially for railway structural surveys, data analysis and mitigation planning. The software enables the user to simultaneously view, interpret and analyze multiple data sets using the same co-ordinates, e.g. GPR data from different antennas, maps, digital video, railway databases and condition measurements. This kind of data combination allows the user to conduct an integrated analysis of all the available data sets on a single computer screen. In railway applications the program enables the user to present railway database information as a form of columns for several different parameters e.g. locations of all man-made structures like bridges, culverts, drainage, insulation boards etc. Also the program makes it possible to analyze track geometry quality data from different seasons and different years and include information about places of problems and performed tamping operations. The mitigation or upgrading planning, statistical calculations and cost estimations can be conducted after data analysis using the same software. An example of a screen view of RDMS is shown in figure 4.

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3 Condition assessment of existing railway lines - a case study

The Haparanda railway line consists of two parts. The old part between Boden and Kalix, and the new one between Kalix and Haparanda (not built yet). This railway will be part of “The Northern East-West freight corridor”, a freight corridor comprising a combination of sea and railway transports. The corridor links Western Asia and Eastern Europe with the USA via Finland, Sweden and the harbour in Narvik, Norway. By using the freight corridor and the harbour in Narvik, transports between North America and Moscow become considerably shorter than via Rotterdam in The Netherlands. Today capacity of the Haparanda Line is considerably restricted by the low technical standard. One example is that inferior bearing capacity limits the speed at certain times of year. At this part of the track comprehensive investigations have been done with the aim to upgrade the old track for an axle load of 25 metric tons and a speed of 100 km/h. Ground Penetration Radar (GPR) and Rolling Stiffness Measurement Vehicle (RSMV) measurements have been carried out during the summer 2005 and the results have been evaluated together with other information using the software Railway Data Management System (RDMS). Investigations and evaluations have been done in co-operation between Banverket and Roadscanners Oy. A total of 114 kilometers have been measured and evaluated. The railway line is located in a very sparsely populated area in the north of Sweden. Figure 3 is showing a picture of this railway with jointed rails fixed on wooden sleepers. Present superstructure is going to be replaced by UIC 60 rails as a CWR on concrete sleepers and 30 cm thickness of ballast.

Figure 3. Picture of a part of the Haparanda line between Boden and Kalix.

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We have chosen 25 km of this line to present condition assessment as a practical case in this paper. The main aim of our investigation is to assess the present state of this line and to detect those parts where special actions have to be carried out when with upgrading of this railway. The following issues have been in focus in the investigation: • Detection of sections with poor bearing capacity • Detection of existing frost insulations • Detection of insufficient thickness of superstructure and substructure as regards frost penetration

depth • Explanation to repeated track geometry problems • Suggestion on remedy measures when upgrading to higher axle load and speed It is obvious that this railway was constructed many years ago for completely different traffic requirements than those considered at upgrading. By just studying records of track geometry measurements one can see that there are already many sections and places with problems requiring repeated tamping. Of course these problems will be more severe in case higher axle loads will be applied.

Figure 4. View of the RDMS software. Upper left – Digital Video of the track; Lower left – Soil type map, Upper right – Interpreted result from GPR (layer thicknesses); Middle right, upper part – Database of man-made structures (in this case two areas of frost insulation boards and one area with ditch are indicated); Middle right, lower part – Magnitude and phase of dynamic track stiffness; Lower right – Two measurements of longitudinal level (track geometry quality). When using the program different measurement data can be viewed as in the example in figure 4 (showing 450 meters). The view can be automatically run showing the video and updating the data views forwards and backwards. In this particular view one can see insufficient thickness of ballast / subballast (interpreted GPR) (2.4 m according to the standard of Banverket), as well as a very soft area, probably peat (magnitude of track stiffness). Also some places with bad track geometry quality are visible at the ends of insulation and ditches.

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Investigations have detected 2987 meters of frost insulation which is about 12% of those 25 kilometers of measured section. One can see that even those sections with frost insulation boards can suffer problems that can be connected to insufficient function of insulation. Especially transitions to non-insulated parts are often places of deterioration of track geometry quality. Other places where problems usually arise are transitions close to level crossings and bridges. A total of 125 m of this line are situated on bridges and 5 of them have problems in transition zones. The same is obvious close to level crossings. Using video, 22 level crossings have been detected and 5 of them have problems in the transition areas. As explained this line is situated in the very north part of Sweden where frost penetration is very deep. The design philosophy of Banverket is based on frost-free subgrade. In this part of Sweden the thickness of the superstructure and substructure is regulated by standards and has to be at least 2.4 m. This is very much for an old existing railway but on the other hand thinner structures cause problems with frost heave. Banverket´s records show that 344 m of this line have suffered to frost heave problems. Our measurements at this section have proved that about 4800 meters, that is about 19% of the track length, have thickness of ballast and superstructure less than 1.5 meters. It is obvious that those parts can have problem with frost and frost heave that reflects into very bad track geometry requiring leveling measures. Our suggestion is to increase thickness of layers or place new frost insulation at those sections where subsoil is frost susceptible. Another important part of our investigation was detection of those areas where the railway is founded on peat or other weak subsoils. Those sections can have low bearing capacity and can cause a number of problems. A total of 12 sections of different lengths with peat have been detected. It has been calculated that 4.6 % of the track length is softer than 60 kN/mm and 3.3 % of the track is softer than 60 kN/mm and have a phase delay of at least -50ⶠ at the same time. The functions of drainage systems like ditches and culverts have a great influence on the performance of railway structures. Especially existing railways usually suffer from poor function of drainage. It can be either insufficient function of ditches or obstructions in culverts that cause accumulation of water in the railway body and many problems. Despite that the investigation was carried out during very dry summer conditions, water in ditches was observed at many places. Study of all available information including measurements has shown that ends of ditches and connection to culverts are often the places where poor drainage can cause problems. We have detected a total of 25 spots where we can explain repeated problems due to poor drainage. Since the current track is a jointed track, the main problem with track geometry quality can be associated to bad joints. 4 Conclusions The survey results have proved that GPR is a potential tool for quick non-destructive surveys of existing railway structures and subgrade soil where upgrading to higher axle load is planned. In the last years the GPR technology for structure inspection has improved to faster systems and higher frequencies. The processing and interpretation software has also been developed for better visualization of processed data. The GPR technology helps to reveal the railway structure thickness and defects without dense drillings. GPR is a powerful non-destructive testing method with several major advantages such as fast measurement speed, continuous survey lines and mapping of the different structure layers. With the GPR technique the railway structures can be investigated in an effective way down to depths of several meters with the speed of 40km/h. The results have also proved that dynamic stiffness measurements with the RSMV is a potential tool for the detection of soft soils as well as transition zones. However, in the present case study the correlation between track stiffness and track geometry quality has been low due to the jointed track and complex dependencies.

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The use of the integrated software (RDMS) is very important since the combined evaluation of several different measurements is a key to success in finding root causes to existing problems and predict future problems for new traffic conditions. All measurements and collection of other available information can be analyzed together which provides an excellent overview of the surveyed railway line. The results show the present and future places of problems in the structures and help to plan mitigation and upgrading of proper sections of the railway. Since the railway’s subsurface layers are beneath the track structure, they are difficult to properly inspect with traditional geotechnical investigation methods. The survey results clearly show that GPR and RSMV can be used to measure the thickness of railway structural layers and subgrade qualities. Further, insulation boards under the ballast layer were clearly visible with high frequency antenna and locating them within the GPR data was simple. In assessing the results of this survey, a combination of GPR and RSMV offers a promising tool for monitoring and substructure condition assessment for existing railway lines that are upgraded to higher axle loads or speeds. In future developments of this methodology, automatic analysis would be an effective tool for locating track geometry quality anomalies and correlations among GPR, RSMV and assets for other railway relevant information. Further developments and analysis of GPR and RSMV are ongoing to find ways for better understanding and correlation between these two methods.

5 References

[1] E. Berggren, Å. Jahlénius, B. E. Bengtsson, M. Berg, Simulation, Development and Field Testing of a Track Stiffness Measurement Vehicle, Proceedings of 8th International Heavy Haul Conference, Rio de Janeiro, 13-16 June 2005.

[2] E. Berggren, “Dynamic Track Stiffness Measurement – A New Tool for Condition Monitoring of

Track Substructure”, Licentiate Thesis TRITA AVE 2005:14, Royal Institute of Technology (KTH), Stockholm 2005.

[3] K. Hrubec, M. Tesaᖐ. “Ground Penetrating Radar in Railway Engineering – Feasibility Study”,

GImpuls Prague, Report for Banverket 2002. [4] T. Saarenketo, M. Silvast, J. Noukka. ”Using GPR on railways to identify frost susceptible areas”,

Proceedings from Railway Engineering London 2003. [5] M. Silvast, “Ground Penetrating Radar (GPR) and Rolling Stiffness Measurement Vehicle

(RSMV), surveys on railway line section between Boden-Kalix-Karlsborgsbruk. Status of existing railway line, STAX 25t”, Roadscanners Report for Banverket 2005.

[6] A. Smekal, E. Berggren, K. Hrubec “Track-substructure Investigations using Ground Penetrating

Radar and Track Loading Vehicle”, Proceedings from Railway Engineering, London 2003. [7] A. Smekal, E. Berggren, “Railway Investigations using Ground Penetrating Radar and Track

Loading Vehicle”, Proceedings from Nordic Geotechnical Meeting NGM, Malmö 2004.