INTRODUCTION - CivilDigital

INTRODUCTION - CivilDigital

BALAST SPECIFICATION FOR HIGH AXLE LOAD (32.5T) AND HEIGH SPEED 250KMPH GUIDED BY: DR.S.K.TIWARI Associate Professor Dept. of Structural Engineering MNIT Jaipur SUBMITTED BY: PONAMALA RAVI KUMAR (2010PST107) INTRODUCTION On IR, the growth of both passengers as well as freight

traffic has been phenomenal independence Indian Railway has embarked upon ambitious modernization plan to improve the carrying capacity of the existing network besides construction of dedicated Freight Corridors . In order to achieve the objective of carrying more freight & passenger traffic, designing the infrastructure for higher axle loads and higher speeds is inevitable . It was in this context that recently, axle load was increased from 20.32 tonnes to 22.82 tonnes on some identified routes . All these were achieved/are being targeted to be achieved without much alteration/improvement to the existing infrastructure other than resorting to better monitoring and discipline

. In the long run there is need to have infrastructure designed/upgraded to run freight trains with an axle load of 32.5 tonnes and passenger trains at a speed of more than 250kmph to meet the future demand commensurate to all-round growth. require improvement in locomotives, rolling stock, track structure, bridges OPTIONS AVAILABLE To achieve the objectives of running freight trains with 32.5 tonnes axle load and passenger trains at a speed of more than 250 kmph we have a choice to go either for ballasted track or ballast less track. Each has its own advantages and disadvantages. COMPARISON OF BALLASTED TRACKAND BALLASTLESS TRACK:

S.N. 1 2 3 4 5 DESCRIPTION BALLASTED BALLASTLESS TRACK TRACK Maintenance Input. Frequent maintenance No frequent maintenance for geometry. for geometry.

Realitely Relatively low Relatively high comparison construction construction cost but Costs but high life lower life cycle cost. cycle cost Elasticity. High elasticity due Elasticity is achieved to ballast. through use of rubber pads and other artificial materials. Riding Comfort.

Good riding comfort Excellent riding comfort at speeds up to 250 Even at speeds greater 280 km ph. than 250 km ph. Life expectation. Poor Life expectation. Good Life expectation. 6 7 8 9 Lateral resistance.

Noise. Limited non-compensated lateral acceleration in curves, due to the limited lateral resistance offered by the ballast. High lateral resistance to the track which allows future increase in speeds in combination with tilting coach technology.

Relatively low noise and vibration nuisance. Churning up of Ballast can be churned up at No such damage to rails and Ballast. high speeds, wheels. causing serious damage to rails and wheels. P e r m e a b i l i t y. Relatively High noise. Reduced permeability High impermeability.

due to contamination, grindingdown of the ballast and transfer of fine particles from the sub grade. 10 Construction cost of Bridges/ Tunnels/etc. Ballast is relatively heavy, leading to an increase in the costs of building bridges and viaducts if they are to carry acontinuous ballasted track. Less cost of construction of bridges and viaducts due to

lower dead weight of the ballast-less track. 12 Construction depth 13 Availability of Material. Stability. 14 Depth of Ballasted track is relatively high, and

this has direct consequences for tunnel diameters and for access points. Limited availability of suitable ballast material. Over time, the track tends to float, in both longitudinal and lateral directions, as a result of non-linear, Behaviour of the materials. Reduced height. No problem of

material. No such problem. BALLASTED TRACK Considering extended experience and capital investment constraints, it is proposed to adopt ballasted track for running freight trains with axle loads of 32.5 tonnes and passenger trains at a speed of 250 280 kmph A typical railway track consists of superstructure (rails, fastenings and sleepers) and sub-structure (ballast, sub-ballast and formation including subgrade). Function of the ballast is to transfer the load from the super

structure to the sub grade Performance of the track system depends on the effectiveness of the ballast in providing drainage, stability, flexibility, uniform support to the super structure and distribution of the track loading to the sub grade and facilitating maintenance. Increase in axle loads, traffic density and speed increase the rate of settlement of the track. And to keep this within permissible limits, stresses in sub grade should be reduced suitably to ensure stability of track parameters two modes to achieve this- either by strengthening the track superstructure or by strengthening the track sub structure. PROPERTIES OF TRACK BALLAST ballast should be clean and graded crushed stone

aggregate with hard, dense, angular particle structure providing sharp corners and cubical fragments with a minimum of flat and elongated pieces. These qualities will provide for proper drainage of the ballast section The angular property will provide interlocking qualities which will grip the sleeper firmly to prevent movement. Excess flat and elongated particles could restrict proper consolidation of the ballast section. The ballast must have high wear and abrasive qualities to withstand the impact of traffic loads without excessive degradation Excessive abrasion loss of an aggregate will result in reduction of particle size, fouling of the ballast

section, reduction of drainage and loss of supporting strength of the ballast section. The ballast particles should have high internal shearing strength to have high stability. The ballast material should possess sufficient unit weight to provide a stable ballast section and in turn provide support and alignment stability to the track structure The ballast should provide high resistance to temperature changes, chemical attack, exhibit a high electrical resistance and low absorption properties Ballast material should be free from cementing properties. Deterioration of the ballast particles should not induce cementing together of the degraded particles.

The ballast material should have less absorption of water as excessive absorption can result in rapid deterioration during alternate wetting and drying cycles. The ballast gradation should be such that it allows development of necessary compressive strength, meet density requirements of the ballast section, uniform support, elasticity and provide necessary void space to allow proper runoff of ground water. It should reduce deformation of the ballast section from repeated track loadings FACTORS INFLUENCING DESIGN OF BALLAST AND SUB BALLAST 1.Total Static and Dynamic Loads Coming on the

Track design of the ballast and sub-ballast should be such that they are able to successfully transmit all the loads coming on the track superstructure to the sub grade without any failure of the sub grade A part of track settlement is attributed to ballast breakdown, its orientation and lateral creep. But most of the settlement is due to vertical settlement of the underlying sub grade. With increase in axle loads, stresses induced into sub grade increases proportionately which lead to increase in rate of settlement of sub grade. with increase in traffic density, stresses in sub grade do not increase but rate of settlement increases due to increased frequency of load application.

2. Speed of the Trains The speed of the trains affects the Dynamic Augment which in turn alters the magnitude of the load coming on the track the stresses do not increase with speed but higher speeds call for better maintenance standards (tolerances). With increase in speed, though dynamic augment DA increases a little, but, increase is compensated due to adoption of higher maintenance standards. Studies by ORE have shown that DA increases a little with speed up to critical speed and thereafter it decreases or remains constant but it is very much sensitive to track leveling defects. Resilience/Elasticity/Flexibility of Track Structure for Good Running Behaviour Running of trains causes vibrations which are

transmitted to the track through rail-wheel interaction vibrations influence the performance of the various track components. The ballast and sub-ballast should be such that it absorbs the vibrations and transfers minimum disturbance to the sub grade. Durability material should be such that it does not create fines that may fill the voids between the particles thereby inhibiting drainage excessive abrasion loss of an aggregate will result in reduction in particle size, fouling of the ballast section and loss of supporting strength of ballast section.

Cementing Properties Some of the powdery fines of carbonate materials have a tendency to cement together and clogging action could occur. Further, cementing reduces resiliency and provides undesirable distribution of track loads and in most instances results in permanent track deformations. Cementing also interferes with track maintenance. So, a ballast material should be free of cementing properties. Stability To provide track stability, the ballast must perform several well defined functions. The ballast must sustain and transmit

static and dynamic loads in three directions (transverse, vertical and longitudinal) and distribute these loads uniformly over the sub grade. Uniform support to the super structure and distribution of track load to the sub grade. The ballast and sub ballast material should be such that it should be possible to get well compacted ballast and sub ballast section to provide a stable and uniform areas for the distribution of the track loads throughout the ballast section Uniform support to the super structure and distribution of track load to the sub grade. Ease in maintainability of the track parameters like, alignment, cross level and grade. It should allow retention of the track parameters The sub-ballast must be sufficiently impervious to divert

most of the water falling into the track to the side drains to prevent saturation of the sub grade which could weaken/soften the sub grade and contribute to failure under load sub-ballast material should be such that it serves as a buffer or filter to prevent sub grade material from penetrating the sub-ballast section while at the same time permitting escape of capillary water or seepage of water, to prevent accumulation of water below the sub-ballast. The sub-ballast particles should be so graded that sub-ballast particles do not penetrate into the sub grade and at the same time does not allow penetration of ballast particles into the sub ballast zone. Ballast should have resistance against Ballast Pick Up phenomena.

It has been observed that at high speeds, the track ballast has a tendency to lift up/fly from the bed and thus hit the under frame of the rolling stock and even the nearby structures under pressure just behind the front or the rear of the train vibrations due to train passage that reduce the friction among the rocks and make them lift easier. Lifting of ballast can be reduced to some extent by using larger size of ballast and keeping the ballast level low as compared to the sleeper top STRESSES ON BALLAST BED Ballast bed and formation are conceived as a two-layer system for the purpose of computation of stresses Vertical forces on the ballast bed due to wheel loads will be considered as the determining stresses for the load bearing capacity of the layer system

Over loading of ballast bed due to increased axle loads causes rapid deterioration of the quality of the track when heavy axle load trains are introduced. compressive stresses that the sleepers exert on the ballast bed are considered evenly distributed for the purpose of calculation. maximum stress between the sleeper and the ballast bed under the wheel load P is expressed based on Zimmermanns theory and by applying a Dynamic Amplification Factor due the speed of the Rolling stock as per Eisenmanns model sb = { DA* Pa/2 *(U/4EI)1/4}/Asb = Fmax/Asb where, P = Effective Wheel Load (T) a = Sleeper Spacing (cm.) U = Modulus of Elasticity of Rail Support or Track Modulus

(Kg/cm/cm) E = Modulus of Elasticity of Rail Steel (Kg/sq. cm.) I = Moment of Inertia of Rail Section (cm4) Asb = Contact area between sleeper and ballast bed for half sleeper (sq. mm.) DA = Dynamic Augment Factor. DEVELOPMENTS IN SUB-BALLAST In order to ensure that there is no influx of water into the sub grade, which can lead to softening of sub grade in combination with vibration, we can use conventional granular sub-ballast of required qualities as per established practice. But, as per recent developments and current trend, Bituminous Ballast as Sub-Ballast is being widely used throughout the world

Bituminous Ballast Bituminous ballast consists of mixture of aggregate and bitumen. The mineral aggregate varies from very fine dust (filler) to a maximum particle size, which is usually around 40 mm. By varying the composition of the mixture, the ratio of the various constituents and the particle size distribution of the aggregate, the properties of the eventual mixture can be adapted to suit the specific requirements of the construction. Depending on the mix composition and the quality of the constituent bitumen and aggregates, the bituminous ballast mixture may be either stiff and ofhigh stability and almost impermeable A bituminous ballast construction may consist of one or more separate layers of possibly different composition.

Depending on the design the various layers each perform a specific role in the construction The properties of bitumen offer good opportunities to apply this type of material in railway track construction 2 Bituminous Ballast as Sub Ballast Layer The rail ballast absorbs the train weight and distributes it from the rails to the sub grade, thereby avoiding any deformation The rapid decay of the railroad level which occurs with traditional ballast construction is mainly due to the unsatisfactory "fatigue behaviour" of the ballast; this is mostly due to embankment settling. By interposing a special semi-rigid layer (the so-called "sub-ballast") in the area between the ballast and the embankment, the behaviour of the overall structure is

greatly improved. The sub-ballast is normally laid on a highly compacted embankment layer 3 The sub-ballast functions to create a working platform on which subsequent work operations, such as ballast and rail laying, are more easily undertaken; to assist in distributing the loads transmitted by passing trains to protect the embankment body from the seepage of rainwater and from seasonal thermal extremes (frost and thaw cycles to eliminate contamination of the ballast from fine material migrating up from the foundation to distribute the concentrated pressures and eliminate any "rupture" of the embankment

A railway structure with sub-ballast works almost exclusively on compression and, therefore, differs from a traditional structure This consequently eliminates fatigue cracking. Especially on high-speedtracks maintaining levels and profile is of high importance. A higher stiffness has as a consequence of better load distribution to the ballast and sub ballast material. This will prevent an early deterioration of the rail geometric. In this case the use of bituminous ballast in a sub-ballast layer can offer the solution Application of Bituminous Ballast in Railway Construction 1 Bearing Capacity application of a monolithic layer (0.1 0.2 m) of bituminous ballast, as a sub-ballast layer will increase

the stiffness of the total structure. The fact that a bituminous ballast layer is also capable of withstanding tensile forces gives an extra positive contribution to this effect Geotechnical Stability The relatively high stiffness of the bituminous ballast sub-ballast layer will make a positive contribution to the compaction of the layers on top of the bituminous ballast layer. This improves the total stability. So the bituminous ballast mix as sub-ballast contributes to keeping the railroad geometry unaltered 3 Resistance to Vertical Deformation The relatively high stiffness of the bituminous ballast layer compared to granular material will lead to less permanent vertical deformation by trainloads. The vertical loading conditions and the relatively short

loading time are relatively small, so there will be no permanent deformation in the bituminous ballast layer 4 Drainage When a layer of dense bituminous concrete is used as a sub-ballast layer, optimal drainage of the total structure will be realized. The impermeable bituminous ballast as sub-ballast layer can prevent possible contamination of the sub-structure by vertical hydraulic transport of mud and fines. 5 Durability ballast by the bituminous ballast layer, the ballast layer is strengthened and deterioration of the ballast is reduced The bituminous ballast as sub-ballast layer increases the foundation modulus, providing a more rigid foundation, with

the effect that there is a reduction of tension and shearing stress inside the ballast material, with consequently less fatigue and less degradation and wear of the individual aggregate particles Because of the low air voids in the mix (1 3%) and because the bituminous ballast layer is buried, weather effects (temperature changes, Ultra Violet radiation, oxygen) will not affect the hot mix, so no deterioration (aging) of the bitumen will take place . Even if limited deformation of the sub-soil does take place, this will not affect the bituminous ballast layer because it is capable of withstanding the deformation without loosing its integrity because of the visco-elastic properties of bitumen. 6 Noise and Vibrations mechanical properties of the bituminous ballast layer will lead

to a reduction in the vibrations and noise produced by passing trains. The use of modified bitumen (polymer modified bitumen, rubber crumb) can further improve the vibration dampening effect of the sub-ballast TYPICAL CROSS SECTIONS OF HIGH SPEED TRACKS WITH SUB-BALLAST CONCLUSION Based on the above discussion, it can be concluded that for high axle loads (32.5 T) and high speed ( 250 kmph ) Ballasted Track on PSC sleepers can be adopted Depth of Ballast of the order of 300 mm. is adequate Higher size of the ballast is preferred Ballast material should be Granite/Basalt only About 150 mm thick Sub-ballast layer preferably of Bituminous ballast is necessary

The shoulder ballast may be increased to 500-700 mm. The various design parameters should not be decided on the basis of initial cost of laying but on the basis of principles of Life Cycle Costing REFERENCES I., Alfi, S., Bruni, S., Van Leuven, A., Apostolou, M.,and Gazetas, G. _2009_. Numerical and experimental assessment ofadvanced concepts to reduce noise and vibration on urban railwayturnouts. J. Transp. Eng., 135_5_, 279 287. 2.Anastasopoulos, I., and Gazetas, G. _2007_. Analysis of failure of scissorscrossover guardrail support base-plates and the role offoundationstructureinteraction. Eng. Failure Anal., 14_5_, 765782

3.Bode, C., Hirschauer, R., and Savidis, S. _2000_. Three-dimensionaltime domain analysis of moving loads on railway tracks on layeredsoils. Proc., WAVE, 2000, N. Chouw and G. Schmid, eds., Balkema,Rotterdam, The Netherlands, 312. 4.Kaynia, A. M., Madshus, C., and Zackrisson, P. _2000_. Ground vibrationfrom high-speed trains: Prediction and countermeasure. J. Geotech.Geoenviron. Eng., 126_6_, 531537. 5.Bishop, W. A., and Henkel, D. J. (1962). The measurement of soils properties in the triaxial test. Edward Arnold Ltd., London, U.K.Bolton, M. D. (1986). "The strength and dilatancy of sand." Geotechnique,London, U.K., 13(1), 65-78.

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