Re: Cleaning Ballast - Why do they need to do this?

[cmgord@ecr.mu.oz.au (Christopher_Martin GORDON)]

: Ballast cleaning (as distinct from simply putting new ballast under the
: sleepers) is done to remove some of the fine and dirty material from under
: the sleepers, thus lowering the track. By dirty material, I mean silts and
: organic soil; by fines I mean degraded ballast that is less than 25 mm. Fine
: and dirty material does not support the track well, and encourages the
: growth of weeds that hasten the degradation of ballast and timber sleepers..

: Placing new ballast directly on to soft clays or other fine deformable soils
: is bad technique. Under the axle loads of trains, and especially with the
: assistance of rain, the fine soil will quickly work though the ballast. It
: will act like a lubricant between the stones, allowing movement to occur.
: The line and top will rapidly deteriorate. Ballast must be placed on a finer
: stone material that can (1) carry load without excessive deformation, and
: (2) will not break down or weaken under normal rainfall.

I am not sure about NSW but it Victroia when they re-lay track like station
pits in the suburban area, they take out all the old ballast, and clean back to
level dirt, and before the first ballast is put down, something called 
Geofrabic, which basically stops the dirt/mud from coming up through the
ballast.  Then (just to keep this complete) a layer of ballast is put down,
then the concrete sleepers, then tracks, then ballast to the top of the
sleepers, then tamped, and then more ballast to the top of the sleepers again
(since the tamping will put almost all the ballast under the sleepers) and the
final grading, tamping.



--

Chris Gordon
http://www.ecr.mu.oz.au/~cmgord