Soil strengthening of age-old-designed railway tracks - Research Paper Example

? Soil Strengthening of age-old-designed Railway Tracks + Railway is a significant sector of Australian Transportation Network. History of Australian Railways goes back to 1850. Australian Railway network comprises of more than 40,000 kilometers. However, a major section of this network needs rehabilitation work of the track bed. Rail track bed is subjected to heavy cyclic loading and different environmental effects. As a result of it, both ballast and sub-grade, in the course of time get deteriorated. Moreover, today’s consumer demands push for speed and heavier load. It is obvious that a significant segment of the existing railway network, which was built about 60 to 100 years ago, requires strengthening of the track bed, especially the sub-grade. The issue is not something new and unfamiliar. Rehabilitation of existing rail track is a part of routine work in the USA, Canada, and European countries. That is why; methods and techniques are known and have been in practice for many years. However, many conventional methods require dismantling of a major section of railway transportation network. This is not an option for Australian Transportation Department. This research proposes a technique that has been in use for the last three decades in countries like USA, Canada, Western European countries, and India. This new technique is cost effective, technically proven and does not require entire track dismantling. Table of Content Index Page Introduction 4 Literature Review 5 Summary 6 Methodology 7 Results and Findings 8 Discussion 10 Conclusion and Recommendation 13 Reference List 16 Soil Strengthening of age-old-designed Railway Tracks Introduction In Australia, the railway system is a major infrastructure used in transporting goods and passengers. The railway lines are 60 to 100 years old, and do not meet modern railway traffic requirement. Demands for faster passenger travel and heavier freight trains are growing. Old railway structure can experience stability problems in complying with the consumers’ new demands. It is, therefore, necessary to improve the substructure of the existing railway foundations applicable to new traffic conditions. A conventional railway track consists of superstructure and substructure. Superstructure consists of rails, fastening, and sleepers, and sub-structure consists of ballast, sub-ballast, and sub-grade. This research assignment studies the functions of substructure in track operation. Railroad substructure receives dynamic load from train movement and behaves according to the principles and laws of soil mechanics. Soil performance under loading in soil mechanics is governed by two characteristics: strength and deformation (STRATIGRAPHICS n.d.). Strength refers to the shear strength properties, and deformation refers to settlement. The proper functioning of substructure is characterized by the bearing capacity which implies that shear stresses caused by the cyclic loading has to be lower than the soil’s undrained shear strength value, and at the same time settlement will not develop plastic behavior in the soil. After a number of years of operation, instability of both super and sub structures is caused by the shear failure and plastic settlement of sub-grade. There are many factors that may contribute to the loss of soil-strength. Among them, poor drainage, and trapped water in ballast pockets play significant roles. According to Harry Cedergreen, drainage represents a significant issue for railroad construction and maintenance; stability and low maintenance cost can only exist when adequate drainage is provided (Cedergren 1989, p. 364). Sub-grade failure in railroad jargon is called “soft track” (Australian Rail Track Corporation a 2001). Soft track includes ballast failure, top formation failure, shallow sub-grade failure, embankment failure, and landslide failure. Water is one of the several contributors that cause these failures. The scope of this assignment is to study methods of sub-grade improvement of the existing Australian railway tracks. Literature Review The effective track design ensures safe support of dynamic load by its sub-layer: ballast, sub-ballast, and sub-grade. The sub-grade of rail track structure is naturally deposited soil, fill material, or a combination of both (Indraratna et.al. 2006). The sub-grade serves as a foundation of rail track superstructure, and stability of the entire system in certain extent depends on the stability of the sub-grade. Sub-grade material shows different response to moisture; for example, fine-grained materials are badly affected by moisture (Das 2011). Sub-grade with high volume of plastic clays can sustain additional pore pressure during cyclic loading, which drastically reduces bearing capacity of subsoil causing overall track failure (Indraratna a 2006). The laboratory test and field measurement showed when natural water content approximates the optimal water content, increase in dynamic stress and load frequency does not impair the stability of the silt sub-grade. However, with the increase of degree of saturation of the silt sub-grade layer, thresholds of elastic deformation of sub-grade and resilient modulus abruptly decreases and can cause shear failure. Increase in water content in the substructure layers can cause the following problems (Das 2011): Loss of strength; Softening of clay sub soils can reduce resilient modulus resulting in plastic failure, and ballast attrition; Can create mud pumping through the migration of subsoil soil into the overlying ballast; Can cause volume change of soils. Water to the railway track foundation system comes from the following sources (Dingqing Li 1994): Precipitation, Surface flow, Rising ground water, and Capillary water. Summary Wet ballast causes reduction of shearing strength. Water trapped in ballast pocket is a major contributor to many soft track situations (Australian Rail Track Corporation b n.d.). Drainage of railway track system is a key factor to endure track performance. The key to reducing the soft track phenomenon is drainage. The ballast must be able to drain the track system. Methodology This research uses positivist and interpretive categories of qualitative approach. “Positivists assume that reality is objectively given and can be described by measurable properties, which are, independent of the observer (researcher) and his or her instruments” (Mayers 2013, p.1). A positivist research can use different methods; however, we adopted case studies from Internet public domain. A case study method is characterized as an empirical inquiry that investigates a contemporary phenomenon within its real life context (Adhikari 2013), which suits the purpose of this research. The concept of case study for this research is the set of views and analysis expressed in publications. Interpretive researchers start out with the assumptions that access to reality is only through social constructions such as language, consciousness and shared meanings. Interpretive studies seek to understand the phenomenon through the meanings that people attribute to them. This research accepts information as authors assigned to them. Research information does not have dependent and independent variables. The research uses informal narrative style and qualitative terms. Predefined set up procedure for conducting research is the use of reading materials available in public domain. Reading materials will be used to obtain empirical information in railway track foundation design, its stability, and rehabilitation methods through soil strengthening. Data will be collected in textural description with the combination of figures and numbers. This research selected descriptive case studies related to track foundation design concepts and principles, principles of soil mechanics, and use of geosynthatic materials in track rehabilitation design. The author of this research does not take authenticity of the validity of qualitative and quantitative data used in this analysis. The data is obtained from the respected public domain, and its validity belongs to the publisher of the domain. Results and Findings In general, drainage systems are designed for surface water, gravitational water, and Capillary water. From geotechnical viewpoint, there are two separate water flows: seepage and permeability. One of the design requirements of railway track is to meet seepage and permeability characteristics in preventing failure of the entire structure. A conventional railway track section consists of ballast, sub-ballast, and sub-grade. Ballast bed is formed using coarse stones, and its primary tasks are to absorb dynamic load, distribute the load from sleepers, increase lateral load, and allow draining condition (Indraratna a et. al. 2006). Sub-ballast is an intermediate layer between ballast and sub-grade. It prevents mutual penetration of ballast and sub-grade. In general, sand and gravel serve as sub-ballast material, however, any material that meet filtering requirement can be used as sub-ballast material. It is virtually impossible preventing water to ballast bed, and that is why; it is essential to push the water rapidly. In engineering design and construction, water movement into or through a soil mass is a powerful phenomenon; this phenomenon could possibly be attributed to the sole significant cause of soil failures for many civil constructions including the railway track transportation structure. Capillarity phenomenon, which is water’s ability to flow against gravity through the porous material, may cause infiltration of water from free water surface through the cracks into the sub-grade under the railway track. Accumulated water may then significantly reduce bearing capacity of sub-grade soil, allowing failure under wheel loads if engineering design and construction failed to take necessary precautions (University of Twente n.d.). Seepage flow may be responsible for failure of many earth designs such as open cut slope, embankment, and railway track foundations. Seepage reduces shearing strength and allows swelling of the soil. This is true for sub-grade that comprised of fine grade material or a coarse soil with a substantial amount of fine plastic particles. Different approaches are taken to avoid failure from seepage. One of them is to excavate the material that accelerate Capillary action of water and replace it with granular material (University of Twente n.d.). However, this is not a realistic choice for the existing Australian railway transportation system. Rail track construction also recommend to include a layer that stays unaffected by Capillary water in taking care of problems associated with Capillary water. This layer interrupts the flow of Capillary moisture. However, under certain circumstances the base of the layer itself has to be drained to remove Capillary water (University of Twente n.d.). This method is not an acceptable option for the existing railway transportation route in Australia. Use of side ditches is also recommended in order to reduce the water table. However, this method experiences difficulty in finding drainage outlets in flat areas. Another recommended approach is to use of a layer known as sand blanket or a geotextile fabric preventing intrusion of Capillary moisture into the base course (University of Twente n.d.). Permeability is a property that explains flow of water through the soil. The function of each layer such as ballast, sub-ballast, and sub-grade in track design is to provide adequate drainage following rainfall. However, in the course of time, ballast permeability reduces due to its deterioration; wear of sleepers, and migration of fine particles from sub-grade (Gataora & Rushton Ken 2012). Experimental observations have shown that, lack of necessary permeability of ballast traps water, which remains in contact with the sub-grade for an extended period (Gataora & Rushton Ken 2012). In addition to it, the upper layer of sub-grade gets adversely affected during cycling loading. In such a situation, not only soft clay, even stiff sub-grade becomes softer (Gataora & Rushton Ken 2012). This results in creation of soft track mentioned earlier. Discussion Way before from now, in 1889, from geotechnical viewpoint it was recognized that, the worst enemy of railway track foundation system is water, and the further it can be kept away from the track or sooner it can be diverted from it, the better the track will be protected. The approach to solve this problem over a century ago was (AREMA-American Railway Engineering n.d., p.8-9) exchange the sub-grade material itself, or improve it by mixing it with enforcement substances, or protect the sub-grade with a protective layer. Studies have noted that train induced (Burrow et.al. p 1) high cyclic stress causes the soil to be sheared and remolded. Ballast starts penetrating into the sub-grade creating pockets in the underlying soils, and collection of water in the pockets weakens the sub-grade causing excessive settlement of foundation and mud pumping. To address this problem, use of sand blanket has been advised by the researchers (Burrow et.al.). In this regard, geosynthatics may be considered as a blanket material. Geosynthatics had been in use for the last 30 years acting as separation, filtration, reinforcement, and drainage (Das 2011, p.14 – 20) material in different structures including rail in track foundation design. As a separation material, it divides two soil layers with different physical properties like grain size, consistency, and density. Thus, it permanently separates mixing of two materials. As a reinforcement material, it can be seen as another layer between ballast and sub-grade. It increases soil shear strength acting as a bond in the geosynthetic-soil system. As a filtration material, it retains fine particles when water passes from fine-grained to coarse-grained soil layers (Innovative Tech System 2006, p. 6-8). As a drainage material, it drains off liquid. Geosynthetics material acting as a blanket in the railway track design improves the structure’s stability and expands service life of railway track foundation. Geosynthetics is human made materials made from various types of polymers (Burrow et.al.). There are several categories of gesynthetics, most commonly used in the rail track design are geotextiles, gegrides, and geocomposite. Geotextile is a flexible, textile like fabric with a permeability parameter, and it used to provide filtration and reinforcement in soil and rock. Geogrids is plastic formed grid like material. In designing with geogrid, tensile modulus and strength are important parameters for its selection (Burrow et. al.). Geogrid may be stiff or flexible, which is also made of polymer grid like a sheet with large opening or slot used primary for reinforced material for unstable soil (Burrow et. al.). Selection of geosynthetic material for strengthening sub-grade should be done with the consideration of filtration size, in the plane and in cross plane hydraulic flow capacity, tensile strength, and modulus. From visual viewpoint, Geotextile material could be woven or nonwoven. As a soil strengthening material for old rail tracks, it should be selected to satisfy reinforcement and drainage requirements. Geotextiles in railway track foundation design is used mainly for filtration and lateral drainage. Its function as reinforcement material is still under study, and that is why it while using geotextiles, the ballast, and sub-ballast thickness should not be reduced. Geotextiles is widely used in railroad rehabilitation design where soft conditions have been created as a result of poor drainage and high impact loading (Integrated Publishing n.d.). It is normally placed between ballast and sub-grade layers, or sub-ballast and sub-grade layer if sub-ballast is used in the design. When sub-grade is wet, cycling loading causes sub-grade and ballast or sub-ballast material intermixing. As the process continues, ballast becomes fouled by excessive fine contaminants, which reduces drainage through ballast and also ballast shear resistance. As the process continues further, ballast penetrates the sub-grade and forms ballast pockets, which collect water thus reduces the strength of the sub-grade. Rehabilitation of these areas using geotextiles provides separation, filtration, and drainage and prevents occurring of pumping track. However, if geotextiles are used without allowing adequate drainage provisions, then water will be retained in the track structure, which will worsen the track instability. In rehabilitation projects, adequate drainage must be considered in the design. Train movement over the time deteriorates ballast with voids forming within the ballast structure. Visually it can be observed when track side sleepers bouncing back and forth. To reduce the voids ballast tamping operations are carried out. However, this operation can further cause particle breakage, and no longer can be used; needs replacement. Use of geogrid can solve this issue in much better way by limiting the deformation in ballast layers and providing interlock along the plane (TENAX n.d.). Conclusion and Recommendation The response to the cyclic loading of rail track is an interaction of superstructure and substructure. Ballast, sub-ballast, and sub-grade relates to substructure. Decades of operation bring structural changes of substructures. The sub-grades of the old existing rail tracks require rehabilitation to satisfy proper functioning and meet current technical operational standards. The scope of this assignment is to find the rehabilitation approach for the existing tracks of Australia. The solution must account that improvement of substructure by dismantling the railways is not an option due to the transport collapse, which is accompanied by financial losses. Rehabilitation of the existing railway is not a sheer issue that relates to Australia only; it is also a significant issue for the European countries, Canada, and USA. CSX Rail Line in Milstead, Alabama, USA faced excessive ballast settlement and soil pumping problems due to the poor sub-grade conditions. Moreover, Tallapoosa River runs parallel to the rail line. The rehabilitation of the track included raising the track, undercutting and removing the existing ballast, and use of filter fabric over the exposed sub-ballast, which was followed by a layer of Geogrid (TENSAR n.d.). Researchers in Australia conducted studies on a section of railway track of Bulli, New South Wales in order to evaluate the effectiveness of geocomposite material consisting of biaxial geogrid and non-woven polypropylene geotextile. The composite material was installed at the ballast-capping interface. A large-scale direct shear force was applied using various ballast fouling conditions. The researchers concluded, “Geosynthetic materials can play an important role in improving the efficiency and performance of a ballasted rail track” (Indraratna b et.al., p. 1). It was mentioned earlier that substructure consists of ballast and sub-grade. The regular operation of train movement causes the following problems with ballast: degradation, lateral movement, and penetration into sub-grade. At the same time, the sub-grade experiences the following problems: general sub-grade failure causing mud pump. Different remedies are offered such as tamping of ballast, excavation of sub-grade and its replacement, grouting, soil mixing, chemical stabilization of sub-grade with lime, cement, or other additives. The above options are not suitable for the rehabilitation of Australian existing railways. As a remedy, this study discussed the use of geosynthatics as a blanketing layer in the track rehabilitation design. North America has a vast experience in the use of geosynthatics (Raymond 1999, p. 213 - 230). Researchers studied durability of the material by examining the track condition and change in properties of exhumed geotextiles at different intervals (Transportation Research Board n.d.). Researchers examined properties such as soil fouling, change in permeability ratio, and change in getextile strength (Transportation Research Board n.d.). The results were compared to material’s initial function properties. The study confirmed that the material if correctly installed and meets the specification gives satisfactory results and it is cost-effective. Indian researchers conducted a study on railway sub-structure rehabilitation project in the Golahat District of Assam using nonwoven geotextile. In this project researchers studied ballast sinking problem. After the installation of geotextile, the researchers continued observation for 12 months. Researchers did not observe ballast-sinking problem and reported appreciable improvement in the performance (Thomas Jimmy et.al. 2007) of the track. Based on the conducted theoretical study within its limitation, this research recommends using of geosynthetic materials in order to strengthening substructure of rail track foundations of the age-old-designed railway tracks. Reference List Adhikari, R 2013, The Case Study as a Research Method. 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Auck;and.ac.nz., viewed 21 May 2013, http://www.qual.auckland.ac.nz/ Raymond GP 1999, ‘Railway Rehabilitation Geotextiles,’ Geotextile Geomembranes, Volume 17, Issue 4, pp. 213-230, viewed 21 May 2013, http://www.sciencedirect.com/science/article/pii/S0266114499000023 STRATIGRAPHICS n.d., Kentucky Transportation Report, straightgraphics.com., stratigraphics.com., viewed 21 May 2013, http://www.stratigraphics.com/resources/KTC_12_02_FR_136_04_6F.pdf TENSAR n.d., CSX Rail Line Rehabilitation, tensarcorp.com., viewed 21 May 2013, http://www.tensarcorp.com/~/media/Document%20Libraries/Americas/Case%20Studies/SPECTRA_CS_RCSX_5.07.pdf.ashxn TENAX n.d., TENAX Solution for railways, tenax.net., viewed 21 May 2013, http://www.tenax.net/geosynthetics/soil-stabilisation/tenax-solutions-for-railways.htm Thomas J, Purkayastha A, Upadhyay R, & Satav M 2007, Trackbed Stabilization using nonwoven Geotextiles, techfabindia.com., viewed 21 May 2013 http://www.google.com.ua/url?sa=t&rct=j&q=&esrc=s&source=web&cd=20&ved=0CE8QFjAJOAo&url=http%3A%2F%2Fwww.techfabindia.com%2Fcomponent%2Foption%2Ccom_phocadownload%2FItemid%2C71%2Fdownload%2C175%2Fid%2C50%2Fview%2Ccategory%2F&ei=WaCaUaf9KbOv4QTeqYGwDA&usg=AFQjCNFgwT5VgeuTwy3f0XJ7P-QXI0-loQ&bvm=bv.46751780,d.bGE Transportation Research Board n.d., DURABILITY OF GEOTEXTILES IN RAILWAY REHABILITATION, trb.org., viewed 21 May 2013, http://pubsindex.trb.org/view.aspx?id=412884 University of Twente n.d., Movement of Water Through Soils, itc.nl., viewed 21 May 2013, http://www.itc.nl/~rossiter/Docs/FM5-410/FM5-410_Ch7.pdf Read More