PVC Conduit bending due to Differential Settlement Tolerance

[Machiavelli999]

I am being asked a question that I have never faced before. We are installing conduit in mat slab to be routed underground in a building. The conduit is going from a main switchboard to a distribution panel and other mechanical equipment control panels. Again all the conduit is to be run in concrete slab.

The geotech report has said that we need to account for up to 1" differential settlement within this new building. Due to this, the cast iron piping for water that is also being routed in the slab is going to be wrapped in a compressible wrapping that will protect it from cracking due to this differential settlement. I am being asked to confirm this is NOT needed to for PVC conduit. Since it's PVC, I am thinking it's OK, but I don't know of any publications that list these type of tolerances for SCH40 or SCH80 PVC.

 

[texie]

In my area this is standard procedure to wrap all pipe/conduit penetrations with foam in slab on grade floors due to expansive soil conditions. And no, PVC will not tolerate this without provisions for the floor to move without breaking it or puling it out of a box, etc.

 

[David Castor]

It's probably not ok. An inch of settlement can easily pop the solvent welds and crack the PVC. You will need a method to accommodate this. Details would depend on location and nature of the settlement.

 

[electrofelon]

But if the slab settles, the entire thing is going to move so I don't see an issue for the portion IN the slab. I assume this is an open bottom switchboard on one end and if so no issue there. Perhaps the other end if it terminates in a panelboard mounted to the wall would potentially be an issue, but you could throw an expansion coupling in there.

 

[David Castor]

For conduits totally within the same structural slab, there is probably not a problem. The problems come at the transitions between slabs and between slabs and earth. Also, most conduits at grade level are run under slabs, not in them.