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

[Bill <wguest@xtra.co.nz>]

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..

Ballast cleaning is a very expensive process, and should only be undertaken
when it is not possible to take a lift and put fresh ballast under the
sleepers. Typical locations where cleaning is necessary are tunnels (better
not to lift the track closer to the roof), near bridges that have fixed
track elevation, and on steep embankments where the formation is narrow and
cannot be widened. (There are some further situations that call for cleaning
also.)

I note your comment about doing this work a relatively short time after
concrete sleepers were put in. This could be due to poor techniques the
first time, but to be fair, it could also have been because the cleaner was
not available and the track renewal job simply had to proceed. In other
words, a scheduling problem rather than an error.

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.

Bill

Jonathan Lau wrote:

> There was trackwork on the North Shore line last weekend and I saw them
> digging out the Down shore line at the new St. Leonards station.
>
> The track in that area is relatively new (i.e concrete sleepers) and I'm
> sure they did the same thing when they put in concrete sleepers in about
> 12 months ago.
>
> A large section of track was removed and old ballast and ground
> underneath the track was then dug out and placed onto the waiting works
> train hauled by 8177/4874.
>
> Why do they need to do this?
>
> Jon Lau