Wall footing 6" over concrete sewer pipe 2

[vollEngineer]

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?

 

[cvg]

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

 

[vollEngineer]

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

 

[civilperson]

Is the sewer crossing or parallel to the footing?

 

[vollEngineer]

crossing

 

[civilperson]

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

 

[vollEngineer]

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.

 

[rconner]

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).

 

[Mickney]

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.

 

[vollEngineer]

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.

 

[hokie66]

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

 

[vollEngineer]

Have you used straw before?

 

[hokie66]

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.

 

[vollEngineer]

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

 

[hokie66]

You are welcome. Good luck with your project.

 

[DRC1]

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.

 

[hokie66]

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

 

[DRC1]

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.

 

[hokie66]

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.

 

[msquared48]

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