16-06-2014, 10:48 AM
SLAB TRACK STRUCTURE
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INTRODUCTION
Ballast has been used since the beginning of railways in order to serve as the transition element between the sleepers and the soil, providing compliance, and vibration damping, as well as surfacing and draining capabilities to the track. The increased maintenance costs and reduced life cycle of the track associated with higher transportation speeds, axle loads and traffic densities led to the appearance of the slab track in the 1960s. The slab track is the concept of the ballastless track where rails are supported on the stiff slab laid down on the soil without the layer of the ballast. The idea of this concept is that slab has to replace both sleepers and the ballast. From different analysis the slab track has been proven to provide distinctive superiority far greater than conventional ballast tracks in terms of safety and comfort during high speed travel, which is also more favoured in economic aspects than the ballast track. At present, construction of new railways widely use slab tracks. Most slab track systems that are of significant age in many cases have been proved better than ballasted tracks. Concrete slab track is constructed somewhat like a concrete highway with rails fastened on top. With the concrete slab and its sub base and sub grade support designed to withstand rail road loading. While there are several types of slab track, the main difference among them is the manner in which the rails are supported by fastened to the concrete slab
1 GENERAL
Slab track is a concrete surface that is replacing the standard ballasted track. Concrete is the prevailing material in slab track applications throughout the world. Only in very special occasions asphalt has been used as material for slab track constructions, and this is due to its high construction demands.
The whole slab track structure is mainly composed of five layers ubgrade or subsoil, frost protective layer, hydraulically bonded bearing layer, Concrete/Asphalt bearing layer, and the rail
.1SUBGRADE OR SUBSOIL
The slab track requires stable subsoil basically free of settlements in order to perform adequately. This is why most times slab track is found in tunnels and bridges. It is a fact that the adjustments to the track geometry after construction are very limited, hence special preparation of the subsoil before construction is essential.
The substructure requirements are the following:
• The substructure of slab track must be secured down to a depth of 2.5m below the bearing plate by special earthwork.
• The ballast in slab track construction is replaced by a concrete or asphalt bearing layer.
• In case of soft cohesive or organic soils the safer solution is to exchange them at a depth not less than 4m from the upper edge of the track .
• A frost protective layer not less than 70cm thick should be applied to keep frost away from the bearing layers
HYDRAULICALLY BONDED BEARING LAYER
A hydraulically bonded bearing layer is a mix of aggregates with a bonding agent placed under the concrete or bearing layer and contributes to an increase in the total bearing capacity of the entire system. This layer is lying on the frost protecting layer and its average compressive strength after twenty-eight days is 15 ?/?2.
Some key features of this layer are the following:
• The laying of the hydraulically bonded bearing layer is carried out by a road finisher.
• A mix of mineral aggregates is used like sandstone, crushed sand and stone chips. The maximum grain size should not exceed 32 mm.
• Portland cement is used as bonding agent, and its content is around 110 Kg/m3.
• The minimum width of the layer is 3.8 m and the deviations of thickness from the anticipated level should be ≤+0.5 ? and ≤−1.5 ?, where + is upwards and – downwards
FROST PROTECTIVE LAYER
This layer is protecting the upper layers from frost; it can also compensate the differences in stiffness of the various layers towards the subsoil and leads the surface water away rapidly. It is resistant to weathering and frost and is consisted of fine gravel to prevent water from rising from the subsoil. This layer should have very low permeability values,1×10−5 or 1×10−4 ?/? to serve adequately. The upper part of this layer is laid with materials similar to the above hydraulically bonded bearing layer
BASIC PRINCIPLES OF SLAB TRACK STRUCTURE
• Elasticity of track bed in slab track is obtained by inserting single/double elastic rubber pad between base plate and concrete slab.
• The first elastic level in the form of rubber pad , thickness generally 6mm is placed directly under the rail, and therefore it is under heavy pressure, which can only partially attenuate vibrations transmitted to floor in general above 500HZ
• To improve elasticity further a second elastic layer in the form of rubber pad ,thickness generally 10mm or so, but relatively softer than first layer is placed between bearing plate/concrete block/concrete sleeper & concrete floor
IMPORTANT TYPES OF BALLASTLESS TRACK ON WORLD RAILWAYS
1PACTIt is laid by casting of slab , with paving machine.It is prepared when long stretch of track is involved.It is suitable for higher speeds 200kmph and above.The PACT system provides good longitudinal continuity and stiffness resulting axle loads getting transmitted to longer length of formation. Thus the pressure transmitted to the ground is reduced. This can be special advantage where heavy axels are expected.As the system without expansion joints will crack due to shrinkage and thermal movements. Therefore this system is not recommended where the gradients does not exist, causing water ponding. The grade of concrete to be used is M30.Slip form concrete is used in the construction. The rail is resting directly on this concrete floor with rubber pad in between , acting as elastic medium.
HAND LAID REINFORCED CONCRETE SLAB WITH BASE PLATE
This is an alternative to the PACT system, where the construction length is too short where paving machine cannot work due to site difficulties.It is also known as resilient base plate system. Compared to the PACT system , this is more expensive, less accurate and more costly to maintain. Due to the low accuracy in the surface level, a base plate having a facility for vertical and lateral adjustments of the rail is used. The hand laid slabs may be constructed either as a continuously reinforced slab or by construction in bays with joints. However, the top surface should be cast plain and to the required cross level of track. After the concrete has set in the inserts are checked for accuracy and adjustments are done if required
4 STEDEFF SYSTEM
This system has twin block sleepers with two elastic layers. The first elastic layer is 4.5mm/9mm ribbed rubber pad between rail and sleeper block. The second elastic layer is 1mm rubber pad provided under the sleeper block which is finally kept in position by second pour concrete.
For further control of vibration especially in tunnels one or more concrete slab is introduced between concrete bock and second pour concrete and there is three elastic layers, it is called modified STEDEFF system
SLAB INSTALLATION METHODS
1 PRE-CAST CONCRETE SLAB INSTALLATION METHOD
The slabs will be accurately lined and levelled on site and grouted or concreted into position. The grout is a quick setting formula giving the required strength in less than 1 hour. The most significant advantage of this approach is the absence of curing time. As soon as the rails are installed traffic can run.
The advantage being reduced precision of the pre-cast slab placement and no pre-build. This can provide more flexibility and offer a potential time saving to the installer.
.1 STAGES OF WORK FOR A PRE-CAST CONCRETE SLAB
The emphasis is on installing the greatest amount of track in the shortest possible period of time, and also minimising the on-site plant activities and specialist work.
The stages of work for a pre-cast slab solution are:
I. Excavate and/or prepare formation depending on ground conditions
II. Lay slabs
III. Jack, align and grout slabs into final position
IV. Join slabs
V. Distribute rails
VI. Weld and install rails
CAST IN-SITU CONCRETE SLAB INSTALLATION METHOD
This is a well established process and does not require any specialised plant, tools or equipment. An attraction of this approach is its simplicity. Once the shuttering has been set up and forms placed to create the slots for the rail, the concrete can be pumped or delivered.In this case the concrete can be cast to civil engineering tolerances and then mechanical tolerances applied during the installation of the shell and clipped lid
.1 STAGES OF WORK FOR A PRE-CAST CONCRETE SLAB
I. Excavate and/or prepare formation depending on ground conditions
II. Place lean mix base for slab if required by design
III. Place reinforcement
IV. Erect shuttering
V. Pour concrete
VI. Strike formwork
DESIGN PRINCIPLES
BASIC OBJECTIVE IN DESIGN[/b]
• The track structure should be stable and provide safe track
• It should be economical to construct and operate
• Quick and simple to install and it is possible to carry repairs.
• Possible to repair in short time
• Noise level is within permissible limits
Mainly there are three ways of applying a slab track
• Using a slab with reinforcement at the neutral line. Since the bending stiffness of such slab is very poor massive soil improvements are required which makes such slab structure financially less attractive. This requirement also covers the condition that no differential settlements may occur. Slabs designed in this way only contain reinforcement in the neutral axis to control crack width in the concrete. On engineering structures like bridges and tunnels, providing rigid supporting conditions, this is a logical solution. However, when slabs are built on subgrades often massive soil improvements are required, which make slab tracks financially unattractive
SLAB TRACK COSTS
SLAB TRACK COSTS
Certain feasibility studies have proved that the slab track is profitable only if the construction process will not cost 30% more than the construction cost for the ballasted track. The slab track costs approximately 1.5-2 times more than the ballasted track design. Despite that it is widely used due to the long-term experience with this type . Taking into consideration the experience of the German network, the total costs in average is 20% to 40% more expensive than the cost of that of the ballasted track, and due to the almost zero need for maintenance it tends to be lower in the future. Few cost developments in the past showed that the estimated cost for maintenance after the construction was higher in a range of 100 years for tunnels and 40 years in earth structures. The advantage of slab tracks to perform for longer time without significant amount of maintenance comparing to conventional ballasted track has long been understood. Results from recent evaluations in this subject propose that slab track is in a long-term perspective, more economically efficient
APPLICATIONS OF SLAB TRACK ON INDIAN RAILWAY
7.1 SLAB TRACK IN CALCUTTA METRO RAILWAY
M-1 assembly: In this deign rails are fixed to RCC sleeper block which is placed on concrete base. At every third sleeper one gauge tie angle is fixed between the 2 rails to maintain the 2mm tight gauge. After bringing the track to correct position & geometry the second poured concrete is done.
M-7 assembly: In this design 10mm rubber pad and pandrol clip are held by malleable cast iron inserts which is embedded in concrete to hold down the rails on the rubber pad. While laying concrete. Great care and precision is required especially inn positioning the inserts and also maintaining the levels particularly in curve. Any error caused in the level,will result either in increasing the rubber of thickness or it may even result in dismantling the excessively laid concrete.
M-6 assembly
SLAB TRACK IN KONKAN RAILWAY
Most slab track constructions take place in tunnels where the ground is stiff and stable. The application of slab track in tunnels is very efficient in terms of construction, durability, strength and economy. The slab track can be built directly on the tunnel base and the thickness of the slab in many cases can be reduced compared to the slab track in earth structures. In addition,when slab track is applied in tunnels the drainage requirements, as well as vehicle access in case of calamities and safety issues, must be guaranteed. Indirect fixation system STEDEF with elastic boot has been used in almost all the tunnels with hand rock base. The Konkan Railway corporation has adopted the conventional PSC monolock sleeper as the main component
ADVANTAGES OF SLAB TRACK
The reasons that may lead to the construction of a slab track system instead a ballasted one are the following:
Lower maintenance need during its life cycle. No need for tamping, ballast cleaning and track lining results to a reduced cost approximately 20-30% for repairs comparing to that in ballasted track.
Higher life cycle, around 50-60 years compared to ballasted track which is only having 30-40 years.
No ballast or solid particles are whirled up on slab track.
Higher safety against lateral forces and accommodation of higher axle loads.
The eddy current brake can be applied without problems any time. This is an advantage against ballasted track in certain places such as signals or at station entrances
LIMITATIONS OF SLAB TRACK
Higher construction cost.
The danger to have wrongly selected this design because of its lower maintenance cost due to the influence of the operational organization which is fully responsible for the railways maintenance after the construction. In many railways a part or the whole construction costs of a track are usually covered by governmental financial subsidies for sums invested in infrastructure. The total cost must be assessed and carefully examined in a fair way in order to select the most suitable track design.
Slab track has an estimated life cycle of 50-60 years. Of course this is valid only if the presupposition that the expected acceptable settlements will occur. In case of a derailment or any other unforeseeable events which could cause greater damage than the expected one can result to long term and expensive track closures. Unfortunately due to the short age of slab track there is not enough information on the actual performance during its life time in order to assess and examine this issue with high validity.
The nature of the slab track does not allow for easy adjustments and repairs after its construction. It means that its quality during the construction must be checked and reassured carefully because any defect on its quality would either remain for the entire life cycle either high costly measures should be taken in order to eliminate it
CASE STUDY
The mission of the Next Generation High Speed Rail Program was to look to the future in anticipation of the implementation of more high speed rail service and to explore technologies which could assist with high speed rail operations. Recognizing that limited right-of-way availability in urban areas may require high speed rail passenger trains to share the same track with freight trains, a track structure would be needed to retain the tight geometry requirements for higher speed passenger trains while sustaining the heavier loads imposed by freight service. To address these needs, Federal Railroad Administration and Portland Cement Associationproposed a demonstration of slab track to provide the required performance. A successful demonstration would also help in validating the slab track design process
CONCLUSION
The construction costs of ballastless systems maybe higher, but the reduced need for maintenance combined with the high structural stability suggests that in many cases the use of slab track construction is more feasible.
The need of maintenance and the life cycle of a slab track would be considerably less and considerably higher respectively.
Slab track structures on tunnels will offer the best and economic solution for Indian environment.
The demand for slab track use is rising, and these must be clearly recognised in order to make future improvements and select the most suitable slab track according to the needs of each project.
Further studies in slab track performance in terms of structural stability, maintenance need and overall cost analysis in order to better understand the limitations of the slab track concept. Updates of the most successful slab track designs must be implemented as the knowledge about their performance increases.
The slab track designs gain more and more popularity in railway projects as the railway speed increases. There are many newly updated designs constructed recently around the world and much more to be built in the near future. This fact alone show that in the following years the knowledge about the slab track systems will increase rapidly allowing for much more accurate comparisons with the ballasted systems. Slab tracks may be proved to be the dominant design in the future high-speed railways