Investigation of transition zones between bridge and balastless track for light rail transit systems

Railway transportation is a safe transportation mode for people since the 19th century. The demand of the transportation needs has dramatically increased in time. Once compared to other modes of transportation, the advantages of railway transportation stands out. Especially, increased traffic load and land use are becoming an unsolvable problem with highway transportation in growing and mega cities today. In addition, air pollution and noise are resourced substantially by highways with increasing populations. Many developed countries such as USA, Japan, Germany, France and UK had already constructed their local transportation networks and have been using effectively for decades. Their rail networks also contribute to their development and civilization level in paralel since more than a century. Usage of rail transportation has affected publics outstandingly. Today demand of rail transportaton is higher than before due to increasing population rates and longer distances in cities. Following this, growing cities also pay more attention to railway investments day by day. Accordingly, in company with new railway networks all around the world bring a lot of technical questions to engineers. How superstructure and infrastructure of railway should be designed and how it may be more sufficient and long-lasting are some of these questions. Engineering studies focus these subjects and try to find effective and innovative answers for challenges of railway with developing technology and return of experiences. Railway structure is consist of two main parts; superstructure and infrastructure. These two are depends on environmental factors, transport capacity and demands, design life, carried loads on rail. Every project is shaped by unique circumstances and restrictions. Most important point is to built safe, rapid, effective, comfortable and long lasting railway network. Scientific studies demonstrates that railway transportation is very open to innovations and developments. Some acceptions of railway design has changed completely with technological inventions. For insance; 30 years before, engineers could not foresee that one day ground-level power supply will be commonly used but today this kind of innovative solutions spread fastly. Every problem is an opportunity for a creative solution and also biggest motive power to solve. In parallel with this, railway engineering experienced challenges and focuses solutions of special problems. Main focus of this thesis study is transition zones between track and bridge for railways. In order to explain resources of transition zone problems so many studies have been done in railway engineering literature. Once these studies analyzed, it is seen that main issue for transiton zones between bridge decks and rail tracks is abrupt stiffness change of different zones. Rail track suffers early due to sudden changes and then different settlements are triggered. Different structural behaviour of conventional track and bridge causes accelerated degradation and shortened component life of rail track. In order to understand methods of improvements for transition zones, this study firstly focuses railway structure and its components. Rails, fastenings, sleepers, ballast and concrete slab built main frame of track structure. On the other hand success of rail structure as a whole topic which requires; durable subgrade and foundation of the track to carry repeated traffic loads and to be resilient to environmental impacts. Hence in first section of the study obtains railway structure as a complete subject. Infrastructe elements, geotechnical classification methods and analyses, formation layers, protection layers of railways and geosynthetics for railways are investigated. The platform layer is bearing layer for wheel loads. Superstructure supports and distrubutes wheel loads and needs periodical maintenance of the renovation during time.To ensure safe operation of rails, fastenings and sleepers should work in harmony to carry loads and guide vehicles smoothly. The ballast including crushed stone and help damping of train vibrations. Also it ensures fast drainage and proper load distribution. Track is seperated two different types: ballasted and ballastles track. Conventional track design is consist of ballastles track. In the last 40 years an increase in train speed and axle load and other challenges in the conventional ballasted track system modified to ballastless railway track system. Two systems have different advantages and disadvantages, although ballastles track usage has become more common type regarding long life and low maintenance. However intercity rail networks still prefers ballasted track in so many countries all around the world due to low initial capital investment of ballasted track. All aspects should be evaluated during project design phase and selection of track types should be done accordingly. In the second section, local railway system types are explained and in order to understand differencies, useful figures of railway classification regarding load capacity and investment costs are used. The main criteria to classify local rail network is load carrying capacity of rail system. LRT and Metro systems have higher carrying capacity than tramway systems. On the other hand tramway lines are still popular for short distances of central areas. Investment cost of Metro is exactly higher than other systems as 40-90 Million US $. In the third section, thesis more focuses to main subject; "Transition zones between bridges and open tracks of railway". In this section resources of problems and solution methods are detailed specifically. Main resource of settlements is sudden change of stiffness. Two main improvement methods are used for transition areas. These are "modifying substructure and subgrade elements" and "modifying superstructure elements". Track having vertical settlements due to subgrades plastic behaviour. In this situation, higher quality soils can be used to ensure durable infrastructure. Also in order to increase rigidity of track; additional rails, increasing ballast stability, wider sleepers may be used superstructure methods. Moreover combine methods can have higher rate of success in transition zones of track because in the same time settlements decrease and rigidity can be higher on track structure. This means; rails can have smoother and comfortable drive and higher maintenance costs can be reduced. During design phase, natural soil of transition areas should be studied and according to this the best improvement method should be choosen accordingly. A case study has been carried out for LRT track in transition zone between bridge and ballasted track in the last section in order to understand track structure behaviour with and without modifying substructure. Therefore, four different modeles have been created and analysed with using HyperWorks, finite element analysis software. For models, natural soil is chosen as good quality limestone material in order to have precise analyse with linear elastic acception. All settlement analyses are investigated in conclusion part. Open track without any improvement and transition track to bridge approach models demonstrates settlements. Hovewer, it is observed that both used improvement methods (technical block and approach slab) have good performances in order to prevent different settlement of ballastless track while the techical block application having best transition performance.