Do You Know About Ballastless Track?

Ballastless track (nonballasted-track) refers to a track structure that uses concrete, asphalt mixture and other integral foundations to replace the granular gravel roadbed. It is also called ballastless track and is the most advanced track technology in the world today.

Compared with ballasted track, ballastless track avoids splashing ballast, has good smoothness, good stability, long service life, good durability, less maintenance work, and the train can run at a speed of more than 350 kilometers.

Introduction Ballastless track uses concrete or asphalt roadbed with good stability to replace ballasted roadbed to transmit dynamic and static loads during driving, and the elastic deformation required during driving is mainly provided by the precisely defined unit materials set under the rails or fasteners. The structural design of ballastless track requires it to have sufficient anti-freezing safety, especially the subsequent settlement deformation of its substructure after track laying is very strict. Therefore, the long-term stability of ballastless track lines is good, especially under high-speed driving conditions. It is a superstructure that rarely needs maintenance under normal circumstances.

According to the type of substructure, ballastless track can be divided into three categories: ballastless track on roadbed, ballastless track in tunnel and ballastless track on bridge. According to the following five parameters, ballastless track can be divided into different structural types:

1) According to the rail support method, it can be divided into point type and continuous type;

2) According to the support fastener method, it can be divided into sleeper and sleeperless type;

3) According to the sleeper support method, it can be divided into buried type, embedded type and supported type;

4) According to the roadbed material, it can be divided into concrete and asphalt;

5) According to the roadbed construction method, it can be divided into prefabricated and cast-in-place.

Regardless of the form of ballastless track structure, due to the use of a rigid bonding hardening material as the ballast bed plate, on the one hand, the load transfer and diffusion function of the system is significantly improved, and on the other hand, its ability to adapt to the settlement and deformation of the lower structure is greatly reduced. Therefore, the requirements of various ballastless track structures for the foundation or subgrade are the same in principle. A large number of field test comparisons show that when other conditions are the same, the dynamic loads in the subgrade and subgrade under ballastless track and ballasted track are very different during driving, such as dynamic stress and vibration velocity. In comparison, the difference in dynamic loads in the subgrade under different ballastless track structures is relatively small. Starting from these two points, focusing on the design of ballastless track subgrade, this chapter representatively takes the German sleeper embedded ballastless track Rheda system as an example to introduce its basic principles and design methods, and on this basis, introduces its requirements for the functionality, durability and smoothness of the subgrade structure.

The ballastless track test section of the Ji-Su-Yu Railway is undergoing actual vehicle tests. According to the news released by Chengdu Railway Bureau, my country's first ballastless railway track has completed comprehensive testing on the evening of January 10. The test results show that the EMU speed reaches 232 kilometers per hour, its stability and comfort are excellent, and all test data are within the safety standards.

In September 2004, the Ministry of Railways decided to build my country's first ballastless track test section on the Suining (Sichuan Suining)-Chongqing (Chongqing) Railway, with a total length of 13.16 kilometers.

On January 3, 2007, the ballastless track test section of the Suining-Chongqing Railway began comprehensive testing.

On December 26, 2009, the Wuhan-Guangzhou High-Speed ​​Railway was put into operation. The line uses the RHEDA 2000 double-block ballastless track technology introduced from Germany's RAIL.ONE.

Beijing-Shanghai High-speed Railway, Beijing-Shijiazhuang High-speed Railway, Shijiazhuang-Wuhan High-speed Railway, Guangzhou-Shenzhen-Hong Kong High-speed Railway, Beijing-Shenyang High-speed Railway, Harbin-Dalian High-speed Railway, and Shanghai-Nanjing Intercity Railway all use CRTSⅠ or CRTSⅡ slab ballastless track technology.

In April 2015, the first Zhengzhou-Xuzhou Passenger Dedicated Line using CRTSⅢ track slabs began to be laid.

With the opening and operation of the Beijing-Tianjin Intercity High-speed Railway, Wuhan-Guangzhou High-speed Railway, Shanghai-Hangzhou High-speed Railway, and Beijing-Shanghai High-speed Railway, my country's high-speed railway ballastless track technology has gradually achieved serialization, modernization, and standardization. The ballastless track structure mainly includes CRTSⅠ double-block ballastless track, CRTSⅡ double-block ballastless track, CRTSⅠ slab ballastless track, CRTSⅡ slab ballastless track, and CRTSⅢ slab ballastless track. In the turnout section, there are mainly long-sleeper buried ballastless track and slab ballastless track, as shown in the figure below.

The ballastless track structure of high-speed railway is the same as that of ordinary track structure, which consists of rails, sleepers, fasteners, roadbed, turnouts and other parts. These materials with completely different mechanical properties bear the forces from the train wheels, and their work is closely related. Any change in the performance, strength and structure of any track component will affect the working conditions of other components and have a direct impact on the quality of train operation. Therefore, the track structure is a system and should be studied from the perspective and method of system theory. The rails directly bear the huge power transmitted from the locomotive and vehicle and transmit it to the sleepers; the sleepers bear the vertical force, lateral and longitudinal horizontal force transmitted from the rails and then distribute it to the roadbed, and maintain the normal geometric position of the rails; various forces between the wheels and rails are transmitted to the roadbed through the vibration isolation, vibration reduction and attenuation of the sleepers and fasteners, and the forces are diffused and transmitted to the roadbed. Since the increase in train speed is proportional to the force and speed of the track structure, the track of high-speed railway must have higher safety, reliability and smoothness than ordinary lines. In order to ensure these requirements of the track structure, the mechanical properties, performance and performance of each track component are much higher than those of ordinary track components. As a railway infrastructure, the track structure is a huge system project. Its stress state is extremely complex. Any change in operating conditions will directly cause changes in the stress state. The state and performance of the bridge and roadbed as the foundation of the track structure have a decisive influence on the track structure. Therefore, as a track structure for high-speed railways and high-speed railways, it is particularly important to have a good foundation and operate under normal stress conditions. High-speed railways generally use 60kg/m rails, 2.6 m long sleepers, elastic fasteners, ballastless track structures, large-number turnouts, straight turnout speeds consistent with the section main line, lateral turnout speeds consistent with the connecting line, use standard trains to calculate bridge loads, specify uniform train speeds and axle weights, and all use three-dimensional intersections.

Design calculation parameters According to the characteristics of the plate ballastless track structure, basic calculation parameters are selected.

In order to obtain the optimal track structure, the finite element beam-plate model was used to study the influence of the main parameters on the mechanical response of each component of the track structure. If there is no special explanation, the load acts on the plate, the elastic modulus of CA mortar is 300MPa, and the other basic parameters, the track plate or base bending moment in the calculation results are the bending moment values ​​per meter range, and the unit is KN·m/m.

According to the trial calculation, the track structure is subjected to the most unfavorable force when the load acts on the plate and the plate end, so these two working conditions are selected for research. It can be seen from Table 2 that when the load acts on the plate, the longitudinal positive bending moment of the track plate and the longitudinal and transverse negative bending moment of the base are large; when the load acts on the plate end, the longitudinal negative bending moment of the track plate, the transverse positive and negative bending moment of the track plate, the maximum reaction force of the CA mortar, and the transverse longitudinal and transverse positive bending moment of the base are large. In the design, the maximum values ​​under these two load conditions should be comprehensively considered.

Fastener stiffness The fastener stiffness was analyzed using 20KN/mm, 40KN/mm, 60KN/mm, and 80KN/mm. The bending moment of the track plate and the base and the maximum reaction force of the CA mortar increased with the increase of the fastener stiffness. However, when the fastener stiffness was greater than 40KN/mm, the bending moment of the track plate and the base slowed down with the increase of the fastener stiffness, and the lateral negative bending moment of the base decreased when the fastener stiffness was greater than 60KN/mm.

The track plate width was analyzed using 2.0m, 2.2m, 2.4m, 2.6m, and 2.8m.

As the track slab width increases, the track slab longitudinal bending moment gradually decreases; the track slab transverse positive bending moment increases with the track slab width when the track slab width is less than 2.4m, and decreases with the track slab width when the track slab width is greater than 2.4m; the track slab transverse negative bending moment decreases with the track slab width when the track slab width is less than 2.2m, and increases with the track slab width when the track slab width is greater than 2.2m; the CA mortar reaction force decreases with the track slab width when the track slab width is less than 2.4m, and does not change significantly when the track slab width is greater than 2.4m; as the track slab width increases, the base longitudinal and transverse positive bending moments gradually decrease, and the longitudinal and transverse negative bending moments do not change significantly.

When the track slab width is 2.0m, the individual mechanical indicators are obviously too large, indicating that the track slab should not be too narrow. At the same time, it can be seen that the track slab width of 2.2~2.4m is a turning point in the change of mechanical indicators. Therefore, combined with mechanical calculation and structural design, from a comprehensive analysis of technical and economic perspectives, the track slab width of 2.2~2.4m is appropriate.

CA mortar CA mortar elastic modulus is analyzed using 100MPa, 300MPa, 500MPa, and 1000MPa respectively.

With the increase of CA mortar elastic modulus, the track slab bending moment decreases, the reaction force of CA mortar itself increases, and the base bending moment increases. Among them, the longitudinal negative bending moment of the track slab and the longitudinal and transverse negative bending moment of the base do not change significantly.

When the CA mortar elastic modulus is greater than 300MPa, the changes of various mechanical indicators slow down. The maximum value can be 300MPa during calculation. At the same time, considering the discreteness of CA mortar elastic modulus and the most unfavorable situation of track slab stress, the minimum value is 100MPa.

The foundation elastic coefficient adopts K30, and is analyzed at 50MPa/m, 190MPa/m, 500MPa/m, and 1000MPa/m.

As shown in Table 6, with the increase of foundation elastic coefficient, except for the increase of lateral negative bending moment of track plate, other bending moments of track plate decrease, the reaction force of CA mortar does not change significantly, and the bending moment of base decreases. It can be seen that the foundation stiffness of tunnel and bridge sections is greater than that of soil roadbed, which is beneficial to the overall stress of track structure.

The basic parameters for calculating the most unfavorable bending moment of plate track under vertical train load are taken, and the influence of load position and discreteness of CA mortar elastic modulus on calculation results are considered to calculate the most unfavorable bending moment of plate track under vertical train load.

In the mechanical calculation of slab track, the values ​​of basic parameters such as load position, fastener stiffness, track slab width, CA mortar elastic modulus and foundation elastic coefficient are the main factors affecting the correctness of the calculation results. Only when the basic parameters are reasonable can the accuracy of the calculation results be guaranteed and provide a basis for structural design.

When calculating the most unfavorable bending moment of the track slab and base under the vertical load of the train, the load position should be considered in the middle of the slab and at the end of the slab; the CA mortar elastic modulus should consider discreteness and be calculated at 100MPa and 300MPa respectively.

When the foundation elastic coefficient of the roadbed section is K30, taking 190MPa/m is the most unfavorable situation, and the calculation result is larger than that of the tunnel and bridge sections.

Features and advantages Advantages of RHEDACITY ballastless track slabs:

Simple and transparent system structure, perfect track positioning, integration with street architecture, the use of cross sleepers ensures the geometric accuracy of track moments and tracks, and the track disc adopts a friction locking fixture. Since heat can fully enter the track span, the phenomenon of insufficient pouring of the track frame can be eliminated. The optimized track system is designed with excellent bonding quality, which can be constructed in one piece, and the elasticity of the track is ensured by the use of pre-assembled components, elastic support or continuous support of the track, and the gauge connecting rod is removed. It is extremely safe and has a long service life. It meets the requirements of electrical insulation and has the ability to "put into use while being built".

Disadvantages Ballastless track has the advantages of high stability, less maintenance, and long service life, and has been widely used in foreign railways. The "Track Overview" published in Germany in 2005 summarized the disadvantages of ballastless track as follows:

1) Rheda investment is more than 1 times that of ballasted track. The budget for the Cologne-Frankfurt line was 4.6 billion euros, but the actual cost was about 5 billion euros, an increase of about 30%. Such a high initial investment includes huge capital costs. The cost of ballasted track is 350 euros/m, while the minimum cost of ballastless track is 500 euros/m and the maximum is 750-800 euros/m. Even if the construction method is optimized and the construction length is increased, the cost factor is still reached.

The economic benefits of ballastless track relative to ballasted track can only be calculated from the increased maintenance costs required for ballasted track. The maintenance of existing ballasted track has been mechanized and automated to a large extent, which is cheaper than manual work and can maintain the track geometry for a long time; ballastless track also needs maintenance, and the workload of rail grinding is increased relative to ballasted track. As the use time of ballastless track increases, the damage will increase, and the repair work of ballastless track is relatively complicated and requires a lot of cost and time. Once the damage causes the closure of the line, the switching will be quite large, which is also impossible to calculate or predict in the early stage.

Ballastless tracks in tunnels have good economic benefits compared to ballasted tracks. However, ballastless tracks on bridges and roadbeds often have poor economic benefits. The maintenance required to limit the long-term settlement of the foundation is twice as much as that of ballasted tracks.

2) Concrete ballastless tracks are rigid bearing layers. When the bearing strength limit is reached, they will break and cause sudden changes in the track geometry and unforeseen deterioration.

3) In general, the construction and maintenance of ballastless tracks have not reached the level of automation. The quality of ballastless tracks requires high-level maintenance measures to ensure it. This means that additional costs and time will be added to the construction process and quality control. Quality defects during construction will leave hidden dangers throughout the service life and require high costs to make up for.

4) As a rigid structure, ballastless tracks are only allowed to be improved in a small amount in the later operation stage, such as improving the track geometry.

5) Ballastless tracks are difficult to lay in deep clay cuttings, soft soil embankments or earthquake areas.

6) There are no particularly effective measures for serious damage caused by derailment or other reasons, and repair is required. The curing and hardening of concrete takes a long time. In other words, a serious accident will result in a relatively long line closure, which has a greater impact on transportation.

7) The most serious disadvantage of ballastless track is that the possibility of improvement is limited.

8) Another disadvantage of ballastless track is that when laying on the roadbed, an antifreeze layer (at least 70cm thick) must be laid in any case. To extend the life cycle of ballastless track, the thickness of the hydraulic material layer can hardly be reduced. The depth of roadbed treatment is also deeper than that of ballasted track.

9) Most economic studies do not take into account the high cost of reconstruction of ballastless track after its life cycle.