What is Ballast?
Its importance has grown with the increasing axle loads and train speeds. aggregates of rock and boulder, natural gravel, shingle and sand, etc. are used according to the requirement, availability and cost to make ballast.
Granite, quartz, trap, sandstone, limestone, etc. have been used as stone ballast in India. The shape of the ballast should be cubic and / or angular.
This would be automatically achieved if the parent rock material is non-stratified and has good compressive strength.
When ballast is tightly rammed under the sleeper for transmitting the loads, it is termed as packing. It is referred as boxing when loosely filled on slopes and thrown around the sleepers. Boxing tend to prevent the lateral and longitudinal movement Of sleepers.
FUNCTIONS OF BALLAST
Main functions of the ballast are:
1. To distribute the load uniformly over large area of formation.
2. To provide elasticity and resilience to the track. 
3. To drain off the rain water from the track.
4. To prevent lateral and longitudinal movement of sleepers.
5. To hold the sleepers in position when the train passes Over them.
6. To prevent the growth of weeds inside the track.
7. To provide a firm, level and resilient bed for the sleeper.
8. To protect formation from direct exposure to rain, frost or sun.
CHARACTERISTICS OF AN IDEAL BALLAST MATERIAL
An ideal ballast material should have characteristics given below:
1. It should allow for easy and quick drainage Of the track,
2. It should not have any chemical actions 00 rail and steel sleepers.
3. It should not be too rigid.
4. It should be sufficiently durable to resist the abrasion and weathering action.
5. It Should not be brittle and have sufficient strength to resist crushing under heavy loads Of moving trains.
6. It should be cheaply and easily available.
7. It should be capable of maintaining the depth uniformly throughout so that the loads are distributed uniformly on the formation.
8. It should maintain sufficient grip so that sleepers do not move horizontally.
9. It should have good workability, so that it can be easily laid on formation.
10. It should maintain the alignment of the track. ll. It should be easy. to clean.
TYPES OF BALLAST MATERIALS
Ballast materials are classified as follows:
A. Broken stone
B. Gravel
C. Sand
D. Moorum
E. Cinders
F. Broken bricks
G. Kankar
H. Blast furnace slag
I. Selected earth
A) Broken stone
Broken stones satisfy all the requirements of ballast. Mostly this type of ballast is used on Indian Railways. Broken stone is produced from stones of good quality like hard trap, quartzite and granite. It should be hard, tough, non-porous. It should not decompose when exposed to weathering conditions. Granite fulfills these conditions. Graded stones from 50 mm to 20 mm provide maximum stability to the railway track. Large sized stones are used for flat bottom sleepers such as wooden, concrete, C.I. (Pot type). They transfers the load uniformly. 40 mm size stones are used for metal sleepers. They fit readily into the curved profile of the sleeper. 25 mm size stones are used for points and crossings. Good quality hard stone is normally used for high-speed tracks.
Advantages: 
a) Due to its high interlocking action, it holds the track to the correct alignment and gradient.
b) It is not subjected to abrasion and hence it provides good drainage properties.
c) Low maintenance.
d) Economical in long run.
Disadvantages: 
a) Initial cost is high.
b) Difficult in procurement.
c) Angular shape may injure wooden sleepers.
B) Gravel
It is the next best material for ballast. Gravel is obtained from river beds. It consists of smooth rounded fragments. It varies in size from very small to very large pieces. Screening is done and only the specified sizes are used whereas others are discarded. Rounded pieces at broken into the required size. This will also increase interlocking action.
Advantages :
a) It has excellent drainage property.
b) It keeps the track to correct alignment and gradient.
c) It is cheaper than stone ballast. 
d) It gives good performance on soft formations.
Disadvantages :
a) It tends to loose the packing under the sleepers due to vibrations caused by the moving tram.
b) Gravel obtained from pits contain earth and hence needs to be washed.
c) Due to much variations in its Size, it needs screening before use.
C) Sand
It is a cheap material. Sand may be coarse or fine but coarse sand is preferred. Its use is recommended on unimportant lines, sidings and, marshalling yards which are light traffic sections, it has excellent drainage property if it is free from soil particles. It has no stability. It gets disturbed by the vibration. It is therefore, covered by a layer of stone ballast. Suitable for C.I. pot sleeper tacks. Not suitable for high speed tracks.
Advantages:
a) Good drainage property if it is free from vegetation and earth.
b) It provides a track free form noise.
c) It is cheap. if available in nearby locality.
Disadvantages:
a) Sand increases the maintenance cost because it gets into moving parts Of the rolling stock and causes friction which further leads to heavy wear Of vehicles.
b) It is likely to get washed away during rain and blown off due to strong winds.
D) Moorum
It is formed due to the decomposition of laterite. It has a red colour but sometimes a yellow colour. It is also not used on those sections where the traffic intensity is high because it crumbles to powder under fast moving trains. Its use is recommended where the traffic intensity is low. It prevents water from percolating into formation and is therefore used as a blanket for black cotton soil.
Moorum is used as a sub-ballast & Initial ballast for new construction.
Advantages:
a) Provides excellent drainage.
b) It can be used on newly laid tracks and will act as soling later on when Stone ballast is laid.
Disadvantages:
a) Difficult to maintain the track in proper form.
b) On being subjected to load transmitted through the sleeper the road metal gets crushed into powder resulting in the track getting dirty.
E) Cinders
Cinders are nothing but ashes which are the waste products obtained from steam locomotives and this is one of the most universal forms of ballast, since it is by-product of every railway which uses coal as fuel. These have more void spaces and therefore possess good drainage Property. It is available in abundance at a short notice. It holds the track to correct shape.
Advantages:
a) Good drainage property.
b) Ease in handling.
c) Very cheap.
d) Being cheap it is widely used in sidings and yards or as initial ballast in new construction.
e) For yards it has proved excellent ballast where it forms excellent path between tracks for 
shunting staff.
f) At the time of emergency as during floods, it is very useful for repairing formation.
Disadvantages:
a) It is not used at places provided with steel or C.I sleepers as it possesses corrosive properties.
b) It also affects the foot of the rail.
c) The use of cinders is objectionable in dry weather as it makes the track dusty being very soft. 
F) Broken bricks
Over burnt bricks or Jhamas when broken further form good ballast. Its use is generally recommended where intensity of traffic is low.
Advantages
a) Excellent drainage property
b) Does not permit growth of vegetation. 
c) It is cheap material as it is obtained from over burnt bricks unsuitable for masonry work.
Disadvantages
a) It makes the track dusty as it gets crushed under fast moving trains.
b) The rails often get corrugated on the track laid with brick ballast.
G) Selected earth
It is used on new tracks till the formation settles due to heavy loads. Over it stone ballast is placed later on after the formation has consolidated. It prevents the loss of expensive ballast 
by sinking into the formation ; which is soft in the initial stages.
Decomposed rock and indulated clay are suitable for being used under layer for harder material such as stone, brick or blast furnace slag.
H) Kankar
It is a natural material and is found in many places in the form of nodules of varying sizes from which lime is prepared. It is found in some of the clayey soils. It can be used where no Other suitable material is available. It is used where the intensity of traffic is light. 
Advantages:
a) Excellent drainage property. 
b) Where kankar quarries are available it forms cheap variety.
Disadvantages:
a) Rails gets corrosive.
b) Difficult to maintain the track due to its relative softness.
c) It gets crushed under traffic resulting in dusty track.
I) Blast furnace slag
It is obtained from blast furnaces of heavy metal plants. It is cheap and possesses good drainage properties. It holds the track to correct alignment and gauge. It fulfills all the requirements of a good ballast. It is widely used in American and English railways but not in India as it is not available in large quantities.
0 Comments