Support of Sanitary Piping under Slab

[ChrisConley]

As this particular thread is not very active I'm also posting this question in Piping & Fluid Mechanics Engineering.

When running a plumbing line under a structural slab we usually specify supports to hang under the structural slab to support the pipe in the event that the uncompacted earth settles leaving our sanitary line unsupported, this is particularly bad situation with plastic sanitary lines.

I recently encountered a structural engineer, and project manager, that called for the removal of these supports with the following justification:

If the subsurface ground heaves it will buckle the hanger rod.

If the subsurface ground settles you may be okay in the short term provided your hangers are installed at a close spacing and they are not disturbed during the concrete placing operation. We believe the hanger rods will eventually corrode and fail as a mild steel rod in earth is very susceptible to corrosion.

My question is: who is right, and more importantly are there any guidelines or codes that support the correct person?

My thanks in advance.

**If you are not part of the solution, then you are part of the precipitate.**

 

[hydrae]

Chris
My solution has been to specify backfill using CDF (controlled density fill) for any time working under a slab new or existing. It is very weak concrete 150 psi or so, lots of fly ash if available. The fill surrounds the pipe preventing both issues, since the fill is flowable until it sets, getting a good compaction is easy and reliable. After it sets, it can be excavated using picks and shovels (just make sure the concrete company does not sub an old load of normal strength concrete in your mix). The CDF will also flow enough to provide support to an existing bridged structure such a tunnel under an existing foundation.
One other reason to not use the hanger rods, how do you compact around the rods? Any equipment jumping jack, plate tamper, sheeps foot or pogo stick is not going to give the same level of compaction when the operator has to avoid those #@$% rods.

Hydrae