Question

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Of the four main subballast functions listed below that you think are the most important and please give good explanations for choosing..

1. (1) provide drainage out of the track,
2. (2) help reduce applied stress to the subgrade,
3. (3) provide separation between the ballast and subgrade, and
4. (4) help to provide frost protection

Answer 1

Answer for this question is

1.The sub-ballast is normally made of granular material and is provided between the formation and the ballast in order to distribute the load evenly over the formation.

2.The sub-ballast functions is

• Provide of track drainage can be defined as the interception,collection and disposal of water from upon or under the track It is accomplished by installing a proper surface drainage and sub surface drainage system.
• 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.
• 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 rain-water 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.

These are the functions can involved in sub-ballast.