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Ballast is a granular material which is placed and packed below and around the railway sleepers. Different types of ballast materials used are broken stone, sand, gravel, moorum, brickbats etc. The main purpose of ballast is to transmit the load from sleepers to the formation(consolidated track bed) and to provide drainage facilities to the track.
Functions of Ballast
The functions of ballast are as follows :
- It provides levelled bed or support for the railway sleepers.
- It transfers the load from sleepers to subgrade and distributes the load uniformly on subgrade.
- It holds the sleepers in a firm position while the trains pass by.
- It prevents the longitudinal and lateral movement of sleepers.
- It offers good drainage to the track.
Types of Ballast
1. Broken stone Ballast
Broken stone is a widely used ballast in railways. It is obtained by crushing hard stones like granite, hard trap, quartzite etc. In lieu of broken stones, limestone and sandstone can also be used. It is suitable for high-speed railway tracks. The broken stone selected as ballast should be hard, tough and non-porous. It should stay strong against inclement weather conditions.
Benefits of Broken Stone Ballast
- Broken stones are hard, tough and durable.
- Hold the sleepers in a strong position and provide stability to the track.
- Suitable for heavy traffic tracks and for high-speed tracks.
- Economical with respect to their durability.
- Require less maintenance.
Drawbacks of Broken Stone Ballast
- Since broken stones are not easily available, their initial cost is a little high.
- Produce noise when the train is moving on the track.
- They are sharp and angular and hence wooden sleepers may be liable to damage by these broken stones.
2. Sand Ballast
Sand can also be used as a ballast material. It is well suitable under cast iron sleepers and can be seen in desert railway tracks where plenty of sand gets accrued on the track. Coarse sand is best suitable as ballast than fine sand.
Benefits of Sand Ballast
- It provides excellent drainage facilities to the track.
- Well suitable for Cast iron sleepers and does not produce any noise while the train is moving on track.
- Cheap and abundantly available material.
Drawbacks of Sand Ballast
- Sand may blow off easily due to vibrations produced by train or due to high winds. So, a frequent renewal is required.
- Excessive wear of sleepers and moving parts can occur due to friction developed by sand.
3. Gravel Ballast
Gravel is a naturally occurring material formed by the erosion of rocks. They are suitable for all types of sleepers and are usually round and smooth and can be obtained from river beds, gravel pits etc.
Benefits of Gravel Ballast
- It occurs naturally and hence is cheap and easily available.
- Properly cleaned gravel offers excellent drainage facilities to the track.
- Well packed gravel requires less maintenance and has high durability.
Drawbacks of Gravel Ballast
- Because of their smoothness and roundness, they may get separated from the bed under vibrations.
- Since it occurs naturally, it may contain some amount of earth or clay which should be cleaned. If not cleaned, the drainage properties of gravel may get affected.
- Sieving should be done to eliminate small size gravel particles otherwise they may affect the drainage properties.
- Produce noise when the train is moving on the track.
4. Moorum Ballast
Moorum is formed by the decomposition of laterite. It is available mostly in red color and, sometimes, in yellow. If the track is to be laid on black cotton soil, moorum can be used as a blanketing material or sub-ballast since it prevents permeability of water into the subgrade or formation.
Benefits of Moorum Ballast
- Moorum is good as a sub-ballast especially in the case of weak soil subgrades.
- Provides good aesthetics to the track.
Drawbacks of Moorum Ballast
- It is very soft and when subjected to vibrations gets converted into a powdered form and blows away.
- It requires frequent maintenance.
- Not recommended unless there is no other material available.
5. Coal Ash or Cinder Ballast
Coal ash also called cinder is the by-product of coal-fired power plants and railway locomotives. It can be used as a ballast material since it is cheaply available and also possesses good drainage properties. It is used as a ballast especially for station yards and as initial ballast for newly constructed tracks.
Benefits of Coal Ash Ballast
- It is economical and abundantly available.
- It has excellent drainage properties.
- It can be handled with ease and is light in weight.
Drawbacks of Coal Ash Ballast
- Turns into dust when subjected to loads.
- Makes the track dirty and complicates the maintenance procedure.
- It is not recommended when steel sleepers are used because of its corrosive action.
- The rails may also get affected by the corrosive action of coal ash.
6. Brickbat Ballast
Brickbats are nothing but crushed pieces of bricks which are generally over-burnt. Under-burnt brickbats are not suitable since they are not as porous as over-burnt brickbats.
Benefits of Brickbat Ballast
- Porous brickbats have good drainage properties.
- Brickbats are useless products of brick industries and hence can be bought at cheap prices.
Drawbacks of Brickbat Ballast
- When subjected to loads they turn into a powder which can be easily blown away by the wind.
- The brick dust makes the track dirty and demands frequent maintenance.