Support of Sanitary Pipe below Structural Slab

[ChrisConley]

When running a plumbing sanitary 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.**

 

[stanfea]

If the pipe is close to the slab you could just put insulation on the pipe and cast concrete around it so that the pipe is connected to the slab. Doesn't sound like a great idea however. I have never seen a pipe connected to a slab. Typically the piping is placed in a well compacted trench and carefully backfilled. Sounds to me like you should concentrate on ensuring your soil is well compacted and that appropriate frost protection has been provided (if you live in the North like me ;-). Compaction inspection is standard and is relatively easy to do.

Good luck
Stan

 

[ChrisConley]

Thank you for the post stanfea.

I agree that compaction is important, my point is that the soil under a structural slab isn't compacted and is often just loose fill.

I do live in the North, and frost protection is 'always' a major concern

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

 

[PEDARRIN2]

I specify/design piping under slabs all the time and have not seen anything like what you are describing. Maybe there is something I am missing, but why is the soil under the slab not compacted?

Please elaborate.

 

[ChrisConley]

It isn't compacted because it a a structural slab (or floating slab). Slab is poured on a void form, which dissolves over time. Slab is supported by piles.

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

 

[bvi]

We have hung pipe from a slab for the case where the slab is pile supported, and the ground was expected to settle away from the slab. Not in any other case to my recollection.

 

[stanfea]

Chris

If you are certain the soil is going to settle and you want the pipe attached to the slab. I would then do as you intend, but put a void form all around your pipe as well and hang it on stainless hangers. That way there is no heaving issues and the pipe will litterly hang from the slab. I still however say that the best way is to just compact the earth and bury it. Tell the structural guy the allowable settlement on the pipe and tell him to make it happen...

Stan