Ballast is the select crushed granular material placed as the top layer of the substructure in which the sleepers are embedded.
Functions of Ballast
1.Resist vertical, lateral and longitudinal forces
2.Provide resiliency and absorb energy
3.Provide large voids 4.Facilitate track geometry correction
5.Facilitate track drainage 6. .Reduce formation pressure
Requirements of Ballast
1. Tough & wear resistant
2. Hard without getting crushed under moving loads
3. Generally cubical having sharp edge
4. Non-porus & non-absorbent of water 5.Resist Attrition
6. Durable & should not get pulverized under weather condition
7. Good drainage of water 8.Cheap & economical in price
Ballast may be subdivided into following zones:
8. Top Ballast
4. Bottom Ballast
Various types of ballast used in Indian Railways are as follows
1. Sand Ballast.
2. Morrum Ballast.
3. Coal Ash Ballast.
4. Stone Ballast
Size and Classification of Ballast
In track hard ballast of 65 mm are used. From the point of view of shape it should have following qualities:
1. Detention on 65mm sieve should not be more than 5%.
2. Detention on 40mm sieve should be between 40% to 60%.
3. Detention on 20mm sieve should not be less than 98% for machine crushed ballast and 95% for hand broken ballast.
4. Detention on 65mm sieve should not be more than 5%.
5. Detention on 40mm sieve should be between 40% to 60%.
6. Detention on 20mm sieve should not be less than 98% for machine crushed ballast and 95% for hand broken ballast.
Minimum depths of Ballast Cushion –
( a ) The minimum depth of the ballast below the bottom of the sleepers at the rail seat should be as under –
( I ) Broad Gauge –