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Design of Pipe with Minimal Cover

Design of Pipe with Minimal Cover

Design of Pipe with Minimal Cover

(OP)
Hello -

I have a project where I need to get the stormwater to an existing manhole. To do so, we have a pumping station with a force main.  The problem is that there are two 54" combined sewer pipes to cross, and a 20" water main which goes under those two pipes. Either we have to go under EVERYTHING (about 16' below grade) or over everything!

At this point, over seems the most feasible.  The limiting crossing is a 16" gas line with approximately 2' of cover above it.  This is all within a roadway as well.

I have been scouring the internet for information on different types of pipe and the minimum cover.  What I would love to hear is a type of 6" pipe that will only need 6-9" of cover, even for a road way...

I would like to know the minimum covers for various materials for 6" pipes that other engineers have used in the past, or links to detailed info on the net regarding the same.

If you need any more info regarding this specific project, let me know! Thanks!  

RE: Design of Pipe with Minimal Cover

If you are in a roadway with traffic on it, you are in trouble. Six to nine inches of cover, as in nine inches from the top of the roadway?  This means that if anyone tries to repair or regrade the road, they will take out your pipe. Is there any possible way to at least push the pipe between the gas main and the combined sewers? Ductile iron pipe Class 54 would work but 6-9 inches is still a problem for the pipeline. What is the average frost depth there?

RE: Design of Pipe with Minimal Cover

In the UK and Ireland concrete pipes require a minimum cover of 24" (600mm). If the cover is less than this a 6" (150mm) insitu concrete surround to the pipe is necessary.The minimum cover to a PVC-U (Plastic) pipe is 48" (1200mm).If this cover is not achieved a reinforced concrete slab must be placed over the pipe to protect it.This slab bears on the soil either side of the pipe trench.The slab must be designed to carry all road loads. This method could be used to protect any pipe. For a 6" pipe the trench width would be quite narrow and a reasonable thin slab 4"-6" could be used.As the previous reply stated you still have the problem of making up the road depth.If concrete protection was used I would not worry too much about the road being dug up in the future as any concrete work would act to protect the pipe.  

RE: Design of Pipe with Minimal Cover

(OP)
Thanks for the information.  We are still looking into the possibility of going beneath the gas pipes.

On a related note: What is the minimum distance needed between the storm force main and the gas, if the storm goes over the gas??  Can the storm sit on the gas if concrete is placed around it?

Of course under everything would be best, and after more examination it does appear that it may be feasible, but everything is still a very tight squeeze!

Thanks!

Kate

RE: Design of Pipe with Minimal Cover

I doubt that the gas company will allow that.  and don't fool around, because high pressure gas lines are dangerous.  you will need a permit from the gas company, so you had better ask for their requirements

RE: Design of Pipe with Minimal Cover

(OP)
Yeah - I know what the standard rules are... I was just hoping someone has broken them and not had a problem ;)

We're about to send the plans to the MUA for review and comment, and we will discuss various options with them.

Thanks!

RE: Design of Pipe with Minimal Cover

I don't see how you could get away with much less than a foot of cover for your force main. Other options are a concrete cap over the main and possibly using the equivalent flow with 3 or 4 smaller pipes. You'll need a header off of your pump. SDR11 HDPE is my choice of pipe material in tough situations. You can lay that stuff right on the road and it won't break.

RE: Design of Pipe with Minimal Cover

I have run across this kind of problem before.  I would never attempt to shallow bury an active pipe 9" below a road, regardless of sturcture.  To resolve the issues you described, notwithstanding specific elevations of the pipes, I simply shot the new pipe under and over existing utility pipes.  In those cases, the torpedo was subjected to sands, silts and clays.  If granular in nature, then augering ma be an option, albiet expensive and tricky.  I had to be very precise and accurate, but all were successful.  If you wish, you could email me a simple sketch and I can determine if this is a feasible option.

KRS Services
www.krs-services.com

RE: Design of Pipe with Minimal Cover

KThomson; I agree with CVG.  Don't mess with the gas line.  any shift in your line or any ground movement, could stress the gas line and cause a problem.

RE: Design of Pipe with Minimal Cover

What is the problem with a directional boring, it is only a few grand?  Another option is a conflict manhole on the 54" combined sewerline.  I would definately keep at least 12" away for the 16" gas main.  You could go over it all, the concrete bridging over the 6" FM is feasable.  You could build up the roadway surface if you needed a few inches to make it work.  However, the directional boring is my choice.  You may want to offer the options to the contractor and let him chose.  Best of luck.....

Clifford H Laubstein
FL Certified PE #58662

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