Flowable fill to abandon a buried waterline

[s0eebuch]

Hey all,

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.

Has anyone ever used aerated concrete to fill a pipe for abandonment? Anything I should consider in my analysis that is unusual? Any other materials beside concrete that have been used with success?

Thanks!

 

[bimr]

You are only adding about 90 lbs/feet over the existing weight of water in the pipe. It should not be a big concern.

 

[cvg]

use a lightweight cellular concrete grout and you can reduce that load down considerably. Plus it may be easier to pump.

 

[coloeng]

And the problem with the abandoned water line sinking is?

 

[s0eebuch]

coloeng - the line to be abandoned runs under a railyard. The owner of the railyard is asking the question.

cvg - my thoughts exactly. But before I analyze the use of a cellular grout (or other lightweight fill), I wanted to know if the PMs concern was even a valid one.

 

[cvg]

you can analyze the settlement, but without better soils data (that I imagine is difficult to obtain under a railroad) you may never know precisely how much settlement to expect. Keep your grout density less than water and you can expect no additional settlement.

 

[bimr]

Would you explain how the soil can be so bad under a railroad?

Is the piping inside of a casing? Most railroads require casing.

You also should review your contract arrangement with the railroad. Some of the contracts have requirements on the method of removal. The last one that I reviewed did not allow abandonment, the piping had to be removed.

 

[s0eebuch]

bimr - LoL, I wish I could! I have boring analysis from our geotechnical engineer and the blowcounts are consistently low along the pipeline corridor. One possible theory is that the majority of the soils out there are some type of riverbed silt or sand (since the entire railyard sits on a manmade penninsula) but do not have the historical data to support that idea. Either way, does it matter why the soil is that bad? I still have to deal with it, regardless of why. And to the best of my knowledge, the existing line is not encased or sleeved.

cvg - I spoke with the project manager briefly today and gave him a progress report regarding my analysis. I told him that one of the thoughts I've had was to introduce that aerated concrete into the pipe, mixed to about 70 lbs/ft3. This is easily achieved, according to the manufacturing data I have. But I don't know the cost delta between aerated concrete and our standard flowable fill. I'm also fighting the "But we've always done it this way" mentality in my office. I was hoping someone on here had done this before with success. As far as I know, we'd be the first in the area to suggest anything but flowable fill to abandon a buried pipeline.

 

[beej67]

I'm no expert, but I think bimr was getting at this:

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, 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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