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?

 

[garrettk]

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.

 

[vollEngineer]

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.

 

[oldestguy]

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.

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[msquared48]

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

 

[Hagops]

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.