For larger diameter buried waterlines that we need to remove from service (16" diameter and larger), we specify that a flowable fill should be used to fill the pipe.
For this particular application, the soil bearing capacity is very low (N60 blowcounts of less than 8).  The project manager is concerned that our waterline, once full of the flowable fill, will sink in the loose soil.
I've suggested we use aerated concrete, which can be mixed at much lower densities than flowable fill.If 90lbs extra per foot of pipe would cause it to sink, then the rail line itself would sink under the live load of the trains.  If the trains don't sink ordinarily, then I wouldn't worry about it.  My experience in this area is probably far inferior to many other posters in this thread/forum though.
Which then begs the question of what caused the blowouts.  Was the pipe in good condition, or did it have leaks and whatnot?
If you're struggling with cost issues, is there a way to use flowable fill in most of the pipe with Multi-stage pumps, and aerated concrete under the rail lines?  I've never been in the field to see how this is done, so I don't know how easy/hard it is.  Might not save you any money by the time you're done figuring out how to do it.

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