A - Ballast, Contamination Flashcards
1
Q
What are the causes of ballast fouling? (4)
A
Dirt is deposited on the track from the atmosphere
Breakdown of material in areas of high stresses, between the ballast and sleeper
Tamping
If the sub-grade is not properly separated from the ballast then mixing could occur
2
Q
What is the main problem with fouled ballast?
A
Clean ballast is all single sized and has around 25% air voids in which means it is permeable and so water has no effect
However, when it is fouled it is much less permeable and so water cannot escape and remains for long periods of time reducing strength
3
Q
What can reduce ballast fouling?
A
Hard rock - good crushing resistance
Angular stones - high shear strength
Frictional stones - high stiffness of aggregate
