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Wall footing 6" over concrete sewer pipe
2

Wall footing 6" over concrete sewer pipe

Wall footing 6" over concrete sewer pipe

(OP)
One of my colleagues asked me this: School wants to put a retaining wall footing 6 inches above an existing sewer pipe.  He already tried the "move the wall" recommendation.  They are expanding the footing to reduce pressure on the pipe.  Currently, they have 5-7 stone over the pipe.  Colleague wonders if 6 inches of loose-ish sand will protect the pipe.  Anyone have a cheap (school) way of protecting that pipe?  Anyone used any other materials?  Styrofoam?   

RE: Wall footing 6" over concrete sewer pipe

put the sewer in a steel sleeve and then bridge the footing over the top

RE: Wall footing 6" over concrete sewer pipe

(OP)
Oh, seriously, that's what I said!  Thanks for that!

RE: Wall footing 6" over concrete sewer pipe

Is the sewer crossing or parallel to the footing?

RE: Wall footing 6" over concrete sewer pipe

(OP)
crossing

RE: Wall footing 6" over concrete sewer pipe

Leave a gap in the footing over the sewer. A foot either side should be plenty.  Additional width can compensate for the missing area.

RE: Wall footing 6" over concrete sewer pipe

(OP)
That's an interesting idea.  And now I wish I had answered you more completely. The pipe crosses at the end of the footing.  I will pass the thought along, but I think they need that small portion of the footing.   

RE: Wall footing 6" over concrete sewer pipe

Was just curious what is the size of the sewer pipe?  In any case, (while I don't know the practicality of this with your specific structure) maybe another thought might be piles/piers to support the footing and span over the existing pipeline, or piles in general as an alternative to somehow support the wall and keep the surcharge load/settlement off the pipeline (instead of just a spread footing).

RE: Wall footing 6" over concrete sewer pipe

What is the foundation load?  

If the load is significant and applies pressure to the bearing soils below the pipe, I would check the settlement potential under the pipe.

RE: Wall footing 6" over concrete sewer pipe

(OP)
Unfortunately, they're not going to want to spend money on the piles in this case.  And I understand that they have increased the footing size to negate the settlement issue.  I'll let you know what they decide.  Thanks, everyone for the input.

RE: Wall footing 6" over concrete sewer pipe

Just thicken the footing each side of the pipe so that it bears below the pipe.  Put 6" of straw over the pipe.

RE: Wall footing 6" over concrete sewer pipe

(OP)
Have you used straw before?

RE: Wall footing 6" over concrete sewer pipe

Yes, but you could use some other compressible material.  Strong enough to support the fresh concrete, but weak enough to create the void you want.  Cardboard void former would be better, but I was just trying to suggest something that is readily available everywhere.

RE: Wall footing 6" over concrete sewer pipe

(OP)
That's exactly the info I was looking for.  Thanks very much!

RE: Wall footing 6" over concrete sewer pipe

You are welcome.  Good luck with your project.

RE: Wall footing 6" over concrete sewer pipe

Why are you staying above the pipe? Typically the footing is dropped to invert or below on each side of pipe to avoid footing loads on the pipe.

RE: Wall footing 6" over concrete sewer pipe

If he stepped the footing down under the pipe, he would still have to isolate the pipe from the wall.

RE: Wall footing 6" over concrete sewer pipe

But that is not difficult. A couple inches of blue foam and loosely comacted backfill should be sufficent as long as the pipe is below frost line. The trick is to get the footing loads for the wall on either side below the pipe so they dont load the pipe. I would think AIA Graphical Standards would have a detail of such a condition. Older copies are usually available in public libraries.

RE: Wall footing 6" over concrete sewer pipe

I think we are actually on the same page here.  If the footing goes under the pipe, isolate the pipe from the wall by wrapping it.  If the footing crosses over the pipe, deepen the footing each side adjacent to the pipe for bearing, with compressible material over the pipe.

RE: Wall footing 6" over concrete sewer pipe

I would use styrofoam over the top and 6" to either side, extending 1' to either side of the footing, then add extra steel in the footing over the pipe for 3 feet to either side.

Mike McCann
MMC Engineering

RE: Wall footing 6" over concrete sewer pipe

It appears that most of the discussion as been directed at the footing of the retaining wall.

Is the retaining wall going to be backfilled to a new elevation with fill material?  How high is the fill material?  Has anyone evaluated the earth pressures resulting from the new fill (if there is new fill) on the pipe?

If the wall is simply to retain existing soils there is no major issue.   

RE: Wall footing 6" over concrete sewer pipe

(OP)
Styrofoam is surprisingly rigid.  I think my co-worker is going with something more compressible like straw.  

I didn't get into the results of the backfill with him.  (I know him from a project on which all we do is look at minor structures experiencing new fill, so I think he has a handle on that already.)  But I will, nonetheless, mention that YOU GUYS wondered if he had remembered to check it. :)

 

RE: Wall footing 6" over concrete sewer pipe

The compressible fill o the pipe solution works fine.

This actually was proposed many years ago (maybe 100) by Marston, an Iowa State professor and engineer.  He called it the "imperfect trench" purposely filled with leaves or corn stalks for a short distance above the pipe to limit the loading on a pipe that otherwise would be over loaded.

It was used by the Wisconsin DOT when they wanted to place a high embankment on an active sewage outfall pipe at Eau Claire for Interstate 94, which could not be taken out of service.

In addition to leaves and corn stalks there was placed a slab in the fill to help span the active sewage treated waste discharge pipe.

In Madison, WI a bank foundation was to be placed over an active sewage force main and this also was used there with good results.  Imagine what a mess would take place if a force main was ruptured.

So the recommendatons are for a system that works.

RE: Wall footing 6" over concrete sewer pipe

Don't rule out styrofoam.  There are different densities, with some very compressible.  Just have to get the right one.

Mike McCann
MMC Engineering

RE: Wall footing 6" over concrete sewer pipe

As a further thought, intermittent styrofoam pads are used under strip footings in Colorado, and other areas, to control differential uplift where there are expansive clays to deal with.  

Mike McCann
MMC Engineering

RE: Wall footing 6" over concrete sewer pipe

Use a 2" layer of the "compressible foam" that Roofers use Under Single Ply Membrane roofs that are directly applied to hard surfaces,usually referred to as "cushion board" by SPM Roofers,widen your footing either side if the pipe. Have used this methodology for frost heave, it works.    

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