Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance

Transcription

1 IJR International Journal of Railway Vol. 9, No. 2 / December 2016, pp The Korean Society for Railway Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance J. M. Sadeghi, J. Ali Zakeri and M. Emad Motieyan Najar Abstract In spite of recent advances in ballasted railway track, the correct choice of ballast for rail track is still considered critical because aggregates progressively deteriorate under traffic loading and environmental exposures. Various ballast requirements, functions and duties have been defined by researchers, railway companies and countries regulations even though it needs to be integrated to make following proper decision during track operation and maintenance. A proper understanding of ballast properties and suitable tests are prerequisites for minimizing maintenance costs. This paper presents a capable classification for ballast characteristics which need to be investigated beforehand to such a way, firstly to assign ballast functions, secondly need to clarify ballast requirements, thirdly to map appropriate tests to evaluate ballast characteristics and then it must be such that if ballast cannot carry out one of these duties, be able to call there is ballast defect. The paper is structured in order to achieve these objectives. Keywords: Track Ballast, Railway track maintenance, Ballast specification, Quality Assessing 1. Introduction Corresponding author: Iran University of Science and Technology-School of Railway Engineering. Narmak, Iran. m.motieyan@gmail.com cthe Korean Society for Railway Rail tracks are conventionally built on compacted granular platforms called as ballast, which are laid on natural formation. Ballast layer is main structural part of railroad where the sleepers are laid on. Ballast layer is the largest component of the track more than 1600 m 3 and 2500 metric tons by volume and weight respectively for single track in one kilometer. A common problem with this part of track is the premature degradation of ballast due to increasing traffic passage, loads and speed, so that leading to fail of high track quality, cause to maintenance works and consequently huge upkeep and downtime cost. Prior to 1970 generally there were no significant consideration on relationship between ballast type and its properties, maintenance performance and costs. The thing which was important purchases price and hauling cost (DIPI- LATO, Steinberg et al. 1983). Sources of ballast often chosen by the railway just based on the vicinity to be inexpensive and satisfied minimum specifications set forth by the client. Therefore, choosing proximity cheaper material quite resulted poor performance and that required frequent maintenance to keep track operation safe and with good quality for running. Previously some available data on performance of road and highway coarse material and concrete aggregates might be appropriate for use in ballast studies(robnett, Thompson et al. 1975, Brattli 1992). But, the performance of a railway track is highly affected by the ballast characteristics and properties in response to train loading (Hay 1982, Selig, Yoo et al. 1982, Li and Selig 1995). Over time in terms of applied stress levels and aggregate environmental exposure it was necessary to define required and quantifiable characteristics of ballast that respond to the all aspects of ballast environmental factors and traffic loading. Since 1970 two approaches small scale laboratory and in service full scale have been used to study track ballast. Engineers and researchers primarily have concentrated on such area: 1- Mechanical properties of ballast; 2- Evaluation of ballast performance in operating track; 3- Ballast behavior in track under traffic and cyclic Vol. 9, No. 2 / December

2 J. M. Sadeghi, J. Ali Zakeri and M. Emad Motieyan Najar / IJR, 9(2), 27-35, 2016 loads. Some conducted investigation (Thompson 1977, Alvahurtado and Selig 1979, Simon, Edgers et al. 1983, Johnson 1985) in America, (Dalton 1973, Raymond, Gaskin et al. 1975, Raymond 1978, Raymond and Williams 1978) in Canada, (Parkin and Adikari 1981) have been published. In Europe and under International Union of railway (UIC) some studies have been executed from 1970 to Because of long life span of railway, for ballast studies in real track it was required to spend plenty time to find out ballast behavior and responses during its services period. They had to create accelerated tests to realize ballast actions in short time to enable predicting long behavior of ballast in tracks(raymond, Gaskin et al. 1975, Raymond, Gaskin et al. 1975, Raymond and Davies 1978, Siller 1980, Alva-Hurtado and Selig 1981, Debra and Ernest 1990, Indraratna, Ionescu et al. 1997, Ionescu, Indraratna et al. 1998, Key 1998, Indraratna and Ionescu 1999, Indraratna, Lackenby et al. 2005, Aursudkij 2007, Bach and Veit 2013) Other Following, researchers and practicing engineers based on knowledge of experiences and experiments, established ballast functions and codify ballast layer characteristics and requirements to carry out its duties as a well component of track. At present, various types of tests and experiments have introduced to evaluate the ballast characteristics, however, the railway authorities worldwide do not agree on which the optimum ballast index characteristics are. Demand increasing cause to heavier traffic load and running speed to the track, which will inevitably give a more rapid degradation of the ballast material and an increase in loss of track geometry(bathurst and Raymond 1987). This can to some extent be avoided by clarification and optimizing the ballast specifications regarding reduced defects in track due ballast layer, maintenance cost and increasing track quality and services. Nowadays, in view point of economy, the life cycle costs of ballast are not governed by purchase or transport cost, but by the cost of maintenance work such as tamping and ballast cleaning to keep service quality. By shifting the focus of the maintenance strategy from meeting safety limits to obtaining cost-effective maintenance via ballast features recognizing so much better, high quality track standards can be achieved and maintained. and Waters 1994, Esveld 2001, Indraratna, Khabbaz et al. 2003, Indraratna, Khabbaz et al. 2006). The fundamental and major functions of ballast can be summarized as follows: 1. To distribute load by from the sleeper uniformly to acceptable stress level over the subgrade assisting in track stability and disperse intensity of load over underneath layers. 2. Supports track structure, anchors the track in place against lateral, vertical and longitudinal movement. 3. Provide a resilient support layer to reduce transmission of dynamic loads to under layers and assists in absorbing shock and airborne noise. 4. Provide rapid and easily drainage any moisture introduced into the track. 5. Provide a ready means for adjusting track geometry to reestablish line and grade. 6. Provide an insulating layer to limit frost penetration into the subgrade. 7. provide a cover to deter growth of plants in the track Ballast portions Ballast layer can be divided in to four zones in constructed and operating track: 1. Crib: ballast zones between the sleepers; 2. Shoulder: the sloppy zone between the end of the sleeper and down to the top of subballast; 3. Top ballast: the top portion of the ballast structure which is usually exposed to tamping; 4. Bottom ballast: the bottom and lower part of the structure which support the overall structure; depend on the quality of the sub-ballast material, loading condition, presence of water and drainage property of the structure; it is the more fouled part of the structure than the rest of the structure. Only the top and bottom ballast distributes the load transmitted from a sleeper down to the sub-ballast and further on to the subgrade. The role of crib ballast and shoulder ballast is mainly to provide minimum confinement against lateral and longitudinal movement for track stability. The contribution of each part in playing the roles of ballast function to perform duties is presented in table (1). 2. Ballast 2.1 Ballast functions Different ballast duties have been reported and interpreted by researchers and railway engineers so far in the world (Robnett, Thompson et al. 1975, Hay 1982, Selig Fig. 1 Schematic of ballast zones in railway track 28

3 Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance Table 1 Contribution of ballast portions to perform ballast duties Ballast Duties Crib Shoulder Ballast Ballast Bottom Ballast Top Ballast Vertical strength Lateral strength Longitudinal strength Resiliency Adjusting and track leveling Drainage Load distribution and dispersing Prevent growth of plant Shock absorption and noise attenuation Electrical resistivity Subgrade protection layer Fig. 2 Classification of ballast specification regarding its properties in railway track Ballast specifications Generally, ballast material should be sufficiently tough to resist breakage under impact, hard to resist abrasion due to inter particle contact, dense enough to resist lateral forces and finally holding the sleepers in place. It must be also freeze-thaw resistant which results further degradation due to weathering and chemical effect. The ballast materials should be non-void particles, when the voids filled with water, during winter it will get freeze, which will form inter particle volume change finally causes bulging of layers. In addition, it should be free of dust and dirt and resistant to cementing action (Knutson 1977, Chrismer 1986, Chanda and Krishna 2003, Indraratna, Khabbaz et al. 2003, Kerr 2003, Ionescu 2005). To perform its duties, ballast layer should meet some requirements. For appraising of requirements, need to classify the specification of granular ballast material to prepare an adequate guideline to assess its characteristics. Requirements for supply of ballast aggregate for railway track construction can be relied on mechanical, environmental, physical and profile properties after laying track down. A proposed ballast characteristic categorization is depicted in figure (2) as below. The physical behavior of ballast material can be estimated by evaluating the shape, surface texture and its angularity or roundness. The mechanical behavior of ballast in railway track can be controlled by strength and resistance to attrition, abrasion and breakdown under traffic loading. Ballast layer profile is governing track stability and resistance to vertical, longitudinal and lateral applied load by running train and thermal forces which is happened by temperature variation. Also the ballast properties should be assessed during it lifespan exposed to the climate changing, chemical alteration, wet-dry cycle, freeze-thaw cycles, and weathering impact. For measuring these specifications, various criteria and experiments are proposed by railway authorities. Ballast layer profile The thickness of the ballast should be such that the subgrade is loaded as uniformly possible. The thickness is usually measured from the lower side of the sleeper (Esveld 2001). Ballast confining pressure depends on the thickness of ballast layer especially in crib part(indraratna, Nimbalkar et al. 2009). Ballast settlement occurs in a railway subjected to long-term traffic loading depends on ballast thickness. Ballast shoulder width and side slope need to provide confining pressure and lateral strength to the track. Also longitudinal and lateral deviation of ballast layer from designed layer should be considered as irregularity of ballast layer geometry. Physical properties Grading of aggregate is the most commonly requested test within this media. The purpose of the test is to determine the varying amounts of material contained in standard size segment. Drainage and compaction facilities are one of the importance properties for ballast media to have been carried. Therefore gradation and sieve analysis is one of the main criterion for ballast particle size evaluation.(sharpe 2000) The next ballast physical characteristic is grain specific gravity, in addition to sustainability and ballast layer stability may be gross indicator for mineral composition. The voids within an aggregate particle should not be confused with the void system which makes up the space between particles in an aggregate mass. Pore size is related to the average particle size and gradation, degree of consolidation or in-place bulk density. Therefore bulk density of ballast aggregate is a function of two factors; 1- particle density and media void content. So the results should be expressed in terms of void ratio (porosity) in addition to bulk density to eliminate the variety of particle density (Selig and Waters 1994). Later part is related to shape of 29

4 J. M. Sadeghi, J. Ali Zakeri and M. Emad Motieyan Najar / IJR, 9(2), 27-35, 2016 Fig. 3 Proposed parent rock classification for selection of ballast aggregate particles called particle shape index included; angularity, flakiness and elongation indices. The particle shape of an aggregate is the percentage of the mass determined as flaky or elongated. Environmental properties Wet-Dry Resistance can be a primarily test to assess environmental properties of ballast. Such clay stone, mud stone and shale will disintegrate if subjected to dry-wetting alteration cycle. They contaminate ballast layer by friable particles and clay lump which are susceptible to break down under wet-dry action. The next water absorption test is used to determine saturated porosity. This observation can predict freeze thaw cycle resistance. It is recommended air drying instead of oven prior to immersing in water. Water absorption testing is performed in conjunction with determining the particle density, both in the dry state and in the Saturated Surface Dry condition (SSD). Soaking duration effects to absorbed water content(nålsund 2014). Freeze-thaw action can affect ballast performance in two ways: 1- The pressure caused by freeze-thaw within the aggregate water- filled voids tends to fragmentation and split, 2-Freeze-thaw cycle changes elastic and inelastic ballast layer mechanical behavior under load due to thaw softening and frost heave. Sulfate soundness test is proposed as an index to predict freeze-thaw cycle resistance of ballast aggregates. The test is thought to operate by forming salt crystal within aggregate pores which cause internal pressure similar to freezethaw experience by icing expansion. Another most important test is chemical activity potential. The rate of environmental alteration of ballast aggregate depends on many factors, including coarse mineralogy, temperature, available moisture, particle surface, particle size, season s weather condition, and chemical contaminants. Mechanical properties Mechanical tests are considered as the best and most important indicators of ballast performance in service. Raymond and Meeker examined ballast materials provided from various quarries in North America, and arrived at the conclusion that, in general, fine grained, hard-mineral and unweathered aggregates are best as ballast materials (Raymond 1979, Meeker 1990). The important and primer effective factor is parent rock source. Although there is general classification for rock properties in view point of geo-technique, in figure (3), a proposed classification is introduced for selection of rock for ballast producing and using in railway track. As shown in figure (3), from left toward right the coarser-grained igneous rocks such as granite, diorite and gabbro, along with the hardmineral grained form the best source, then Hypabyssal rock type, followed by the extrusive igneous rocks (rhyolite, andesite and basalt) the most adequate and then metamorphic rocks such as quartzite, gneiss, and finally wellcemented sedimentary (Lime stone and dolomite,..) and 30

5 Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance slag as weakest aggregate can be chosen. The experiments results for sedimentary, metamorphic and igneous rocks shows, the first two rock types are weaker in abrasion than the later (Ozcelik 2011). In ballast petrographic test, by identifying inherent physical properties of the parent rocks and minerals in ballast aggregate and estimating percentage composition, it is capable to map rock source based on the classification and mineralogy, evaluate chemical activities, and provide the clues of the susceptibility to weathering, soundness, hardness and toughness properties(johansson, Lukschová et al. 2011). However, the evaluation is mostly subjective and value of the analysis and prediction obviously depend on the skill and experience of the petrographer. Some relationships between petrographic results and identifiable rock particle properties by petrographic analysis have been studied (Watters, Klassen et al. 1987, Nålsund 2014). Hardness index is using for measuring the abrasion resistance of aggregate interaction under the loading by friction grain are being scratch by another grain. Toughness describes individual aggregate resistance to impact and is ability to absorb the compelled energy in track. It can be the fracture surface energy required to create unit new crack surface.(aursudkij 2007). 3. Ballast Examining To identify ballast testing methods which provide results reflecting the field performance of different ballast materials, various tests are recommended and performed by researchers. In the case of achieving appropriate ballast recognition regarding its characteristics needs to perform these general and well-known tests as categorized in table Table 2. Track ballast layer specification indices Ballast properties Characteristics Test method Terms of evaluation Mechanical Physical Environmental Geometrical Hardness &Toughness Los Angeles Abrasion, attrition and fragmentation Hardness Deval attrition test (dry, wet) Surface abrasion test Hardness Micro Deval Surface abrasion test Toughness Aggregate Crushing Value Fragmentation and crushing resistance Hardness Scratch (Mohs)Hardness Surface abrasion to produce fines Toughness Point load Strength Fragmentation(splitting) Toughness Aggregate impact Value Aggregate resistance to sudden shock, fracturing Hardness Dorry abrasion Surface abrasion to produce fines Hardness Mill abrasion Surface abrasion to produce fines Clue to Hardness, toughness and soundness Petrography analysis (intact particles/fines) Mineralogy, Chemical alteration Gradation Particle size distribution Compaction energy and drainage ability Gradation Fineness of material Drainage Stability Particle density Stability and indirect toughness Stability Bulk density Inter particle void, drainage and track stability Shape index Particle shape Fragmentation(splitting) and drainage Soundness Dry-wet Durability and weathering resistance Soundness Freeze and Thaw Durability and weathering resistance Soundness Water absorption Saturation and weathering resistance Soundness Aggregate Porosity Durability and weathering resistance Soundness Magnesium/Sodium sulfate Durability and weathering resistance Soundness Friable particles, clay lumps Durability and preliminary abrasion Thickness Track structural stability, resiliency and damping Cross Profile Shoulder width Track lateral stability Side slope Track structural stability Layer Deviation Unevenness, misalignment Track stability to ride comfortable 31

6 J. M. Sadeghi, J. Ali Zakeri and M. Emad Motieyan Najar / IJR, 9(2), 27-35, 2016 for ballast assessment can be divided in three stages of ballast source and particle, assembly particle and ballast constructed layer which are presented in figure (4) and (5). In the case of ballast requirements a set of standards and tests have been identified. Railway authorities throughout the world follows their own local or common standards to ensure uniform material compliance and appropriate ballast aggregate for use in track. In table (3) based on recommended classification for ballast characteristics evaluation through selected technical codes of America, Europe and Australia a compilation of corresponding items is briefly presented. 4. Conclusion Fig. 4 Assessment steps in track ballast evolution Fig. 5 Ballast characteristic controlling tests in term of ballast evolution (2) to measure needful indices in terms of evaluation of ballast specifications. These tests are concerned with establishing a quantitative estimate of the resistance to in track instability and degradation under loading. Beside mentioned tests, some supplementary test are being used to simulate actual ballast field performance in order to assess the reliability compared to using simple laboratory tests. Odeometer, ballast box test, triaxial test and full scale railway track model test are supplementary tests to find out more realistic behavior of ballast under cyclic loading; however are costly and time consuming(selig and Waters 1994, Indraratna, Khabbaz et al. 2003, Indraratna, Khabbaz et al. 2004, Indraratna, Khabbaz et al. 2006, Aursudkij 2007). Recommended control parameters and selection criteria Recently, more attention is given for both the superstructure and substructure part of the track to get good performance for heavier wheel loads, higher operating speeds and unit trains. Based on the evaluation of track ballast through required experiments, physical, mechanical, environmental and geometry of ballast profile properties according to adopting thresholds, it is possible to insure better overall performance of the track structure which requires minimum cost of maintenance. However, frequently, availability and cost have been the prime factors considered in the selection of ballast materials, but all over performance and required life span of the railway should be safe and cost effectiveness. Hence the cheapest ballast means that, track structure not having low cost all over its service period. The characteristics of ballast layer can be divided into four categories, mechanical, environmental, physical and geometry that quantify its performance. These specification data are obtained by conducting series laboratory, field tests and also evaluating the performance of the different ballast materials under existing condition on the track It is therefore very important for railway to have full information about ballast service period identifying methods of construction and they should already be laid down in its specification. This article presents a comprehensive study of ballast functions and its properties. It proposes mechanism to reach appropriate tests to improve the availability of the track for ballast services. References 1. Alva-hurtado, J. and E. Selig (1979). Static and Dynamic Properties of Railroad Ballast. 2. Alva-Hurtado, J. and E. Selig (1981). Permanent strain behavior of railroad ballast. Proceedings of The Interna- 32

7 Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance Table 3 Compiled track ballast assessment based on proposed categorization of ballast technical requirement Test zone Physical Environmental Soundness and resistance to the weathering Mechanical Ballast profile (Geometry of layer) Wet attrition Test Toughness Fragmentation And fracturing resistance Australia Europe EN13450 USA Requirements (AS ) (2009) (2003) AREMA(2012) Particle size distribution (PSD) AS EN933-1 ASTM-C136 Max % Max 1 % Max 1% (0.063mm) AS ASTM-C117 EN % material passing No. 200 sieve % Clay lumps and friable particles % Particle Density (Specific gravity) Misshapen particles Bulk Density (unit weight) Elongation Flakiness Crushed Surface (texture) Min 2.5 t/m3 (dry) Min 1.2 t/m3 (AS1141.4) NSW Min 1.4 t/m3 2:1 ratio Max 30% 3:1 possible Alt. Max 30 % AS Min 75 % by mass 2 crushed faces EN CEN ISO/TS CEN ISO/TS :1, Max 4-12% EN Length>100mm <6% Max % EN (1997) Water absorption% EN Freeze and Thaw Cycle Sulfate test Max 0.5 % ASTM-C142 Min 2.6 t/m3 ASTM-C127 Min 1.12 t/m3 C29 Max 5 % ASTM D4791 3:1 Max 5 % ASTM D4791 EN-NS (20 cycles) EN (1% NaCl-10 cycle) Clay lumps and friable particles % Micro-Deval Max 5 15% EN Mill Attrition Test Wet Attrition Value (WAV) Max 6%,8%,12 % based on class of track Los Angles Value Max 25 %, 30%, 40% based on class of track (LA) ( ) Aggregate Crushing Value Max 25%, 30%, 40% based on class of track (ACV) (AS ) Impact Value (IV) - Strength Drainage Electric Conductivity Ballast Depth underside of sleeper to the top of the finished formation Ballast shoulder width Side Slope of the ballast shoulder Wet : Min 175 KN, 150 KN, 110 KN Wet/dry variation: max 25%, 30%,40% Based on track class AS Min 60 Ohm.m Max 1-2% ASTM-C127 Max 5 % ASTM-C88 (5 cycles) Max 0.5 % ASTM-C142 AN= LAA+5MA* 25%<AN<65% - Max 12 to 24 % EN Max 30% ASTM-C535- C131-14%<IV<22% EN CEN ISO/TS , 275, 225 mm based on class of track mm (CWR, LWR***) mm Loose rail Height: horizontal 1:1.5 Point load test Dry>1200 kg Wet>800 >12 in** Main line >12 in (CWR) Main line Height: horizontal 1:2 Crib zone To the top level of sleeper *Abrasion Number is an index included Los Angeles and mill abrasion loss. **The measurement is made under the line rail in tangent track, or under inside rail in curved track, and is made with respect to the top of the sub-ballast at the center line. ***CWR: continued welded rail, LWR: long welded rail 33

8 J. M. Sadeghi, J. Ali Zakeri and M. Emad Motieyan Najar / IJR, 9(2), 27-35, 2016 tional Conference on Soil Mechanics and Foundation Engineering, 10th. 3. Aursudkij, B. (2007). A laboratory study of railway ballast behaviour under traffic loading and tamping maintenance, University of Nottingham. 4. Bach, H. and P. Veit (2013). Evaluation of attrition tests for railway ballast. 5. Bathurst, R. J. and G. P. Raymond (1987). Geogrid reinforcement of ballasted track. 6. Brattli, B. (1992). The influence of geological factors on the mechanical properties of basic igneous rocks used as road surface aggregates. Engineering Geology 33(1): Chanda, M. and R. Krishna (2003). Selection and testing of ballast stones for underground railway tracks. African Journal of science and technology 4(2): Chrismer, S. (1986). Considerations of factors affecting ballast performance. American Railway Engineering Association Dalton, C. (1973). 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9 Developing Track Ballast Characteristic Guideline In Order To Evaluate Its Performance 38. Raymond, G. P. and D. R. Williams (1978). Repeated load triaxial tests on a dolomite ballast. Journal of Geotechnical and Geoenvironmental Engineering 104(7). 39. Robnett, Q., M. Thompson, W. Hay, S. Tayabji and R. Knutson (1975). Technical data bases report. Ballast and Foundation Materials Research Program. 40. Selig, E., T. Yoo and C. Panuccio (1982). MECHANICS OF BALLAST COMPACTION. VOLUME I: TECHNICAL REVIEW OF BALLAST COMPACTION AND RELATED TOPICS. 41. Selig, E. T. and J. M. Waters (1994). Track geotechnology and substructure management, Thomas Telford. 42. Sharpe, P. (2000). Trackbed investigation. Journal and report of proceedings-permanent Way Institution, Permanent Way Institution. 43. Siller, T. (1980). Properties of railroad ballast and subballast for track performance prediction. Master's project report. Con-crete Tie Correlation Study, Department of Civil Engineering, University of Massachusetts, Amherst. 44. Simon, R., L. Edgers and J. Errico (1983). Ballast and Subgrade Requirements Study: Railroad Track Substructure- Materials Evaluation and Stabilization Practices. 45. Thompson, M. (1977). FAST Ballast and Subgrade Materials Evaluation. Ballast and Foundation Materials Research Program, University of Illinois at Urbana-Champaign. for US Federal Railroad Administration, Report No. FRA/ORD- 77/ Watters, B., M. Klassen and A. Clifton (1987). Evaluation of ballast materials using petrographic criteria. 35