Ballast Specification for High Axle Load & High Speed ≥ 250kmph – Seminar

Ballast Specification For High Axle Load & High Speed ≥ 250kmph

submitted by PONAMALA RAVI KUMAR

Introduction

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.

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 sub-grade).

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
• Apart 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 Behavior

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

1. 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.
2. Uniform support to the super structure and distribution of track load to the sub grade.
3. 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
4. Uniform support to the super structure and distribution of track load to the sub grade.
5. Ease in maintainability of the track parameters like, alignment, cross level and grade. It should allow retention of the track parameters
6. 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
7.  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.
8. 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.
9. Ballast should have resistance against Ballast Pick Up phenomena.
10. 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
11.  under pressure just behind the front or the rear of the train
12.  vibrations due to train passage that reduce the friction among the rocks and make them lift easier.
13. 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 Zimmermann’s theory and by applying a Dynamic Amplification Factor due the speed of the Rolling stock as per Eisenmann’s 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

 CONCLUSION

1. Based on the above discussion, it can be concluded that for high axle loads (32.5 T) and high speed ( 250 kmph )
2.  Ballasted Track on PSC sleepers can be adopted
3.  Depth of Ballast of the order of 300 mm. is adequate
4.  Higher size of the ballast is preferred
5.  Ballast material should be Granite/Basalt only
6.  About 150 mm thick Sub-ballast layer preferably of Bituminous ballast is necessary
7.  The shoulder ballast may be increased to 500-700 mm.
8.  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