Concrete Support Structure over Old Pipe
Concrete Support Structure over Old Pipe
(OP)
Working on a project involving some new rail lines. There is an existing old wastewater line (60" diameter) about 15 feet below grade. The pipe is in questionable condition. The city (owner of the WW line) wants to avoid having any new loads translate down onto the pipe. I intend to design a short isolation structure over the top of the pipe. The structure will have a span of about 10-12 feet and will be supported by drilled piers. The structure will be cast on void forms. However, my concern is what happens when the train axles leave the structure. The loads will translate through the soil at a 45 degree (or some other debatable angle) and still impose load onto the top of the pipe. I originally thought of using sheet piling behind the drilled piers on each side to cut off the load, but I'm wondering how close to the back of the piers I can get with the sheet piling. Perhaps a better idea would be to use several closely spaced piers. If so, I wonder if they need to touch, or if I can leave a one-foot (or preferably bigger) gap between them.
We cannot excavate down to case the pipe because there are existing rail lines right next to these proposed rails. The existing WW line was cased beneath the existing rail lines, but it didn't extend out far enough to protect the pipe beneath the proposed rails. See the attached sketch.
We cannot excavate down to case the pipe because there are existing rail lines right next to these proposed rails. The existing WW line was cased beneath the existing rail lines, but it didn't extend out far enough to protect the pipe beneath the proposed rails. See the attached sketch.






RE: Concrete Support Structure over Old Pipe
It will give you some idea of forces without your structure.
Can you make the flat bit of the structure wider?
At that depth, the extra load on the pipe even from a railroad waggon is not that high so with your structure I certainly wouldn't be concerned about some load appearing at 45 degrees, and I'm a pipeline man.
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RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
Could you consider just a long (say 30') heavy structural slab-on-grade instead? With significant reinforcing steel, a thickness of 2' to 3' should do a good job of dissipating the load over a large area. Probably lower cost, too. There could still be some differential settlement issues, but not as dramatic.
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RE: Concrete Support Structure over Old Pipe
Then install your 'bridge' and sheet piles etc. If you could take the line out of service and jack a new casing to isolate it that would help. My guess a 60 inch pipe would require a large bypass and may not be able to be lined and or cut and replaced with the casing.
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
The differential settlement issue is real, we had to deal with it on an ongoing basis at the coal railcar unloaders at electric generating stations. Here is a typical photo, I marked where rail road bed maintenance has been performed for years:
Suggest not ruling out a slab-on-grade without running the numbers. On a small scale (say the 30' length I mentioned) the axles are spread further apart than intuition suggests. While doing design work on one unloader I was surprised how well distributed the axle loads are. After thinking about it, they have to be. Any given railroad bed can support only so much.
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RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
Assume the load is 80 kips per axle (Cooper E80), not 80 kips per wheel?
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RE: Concrete Support Structure over Old Pipe
RE: Concrete Support Structure over Old Pipe
Keep in mind two things:
1. The 30' length is my first-order approximation, if a little extra length may be needed. If 35' feet is insufficient, extra length beyond that probably won't help.
2. The rail load on the adjacent track will apply some of its load to the uncased pipe length. Not much, but some. The load distribution footprint thru the soil takes place not only along the length of the track, but also perpendicular to the track.
Edit:
If you want to try avoid both loading the pipe and installing sheeting try modeling this. Be sure the axle spacing is realistic:
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RE: Concrete Support Structure over Old Pipe
We are also evaluating a 12-inch sewer in a steel casing that crosses under a RR. The sewer appears to have settled about 6-inches since it was originally installed the early 1980's.