Use of underground conduit - standards

[eeprom]

Hello,
I have been working on a project which requires PVC coated rigid conduit for all underground wiring. In the past I have used PVC conduit encased in concrete, with steel risers. I was told that PVC was not desirable because in very cold temperatures, the ground will shift and the PVC will break. The frost line is about 5' down and the PVC is installed at 36".

I have been contacting PVC manufacturers in order to discuss this possible problem. No one I have contacted seems to know anything about this. Does anyone in this forum have any experience or literature which would justify the use of PVC coated rigid underground?

thanks,
EE

 

[dpc]

Certain soils cause rapid corrosion of standard rigid galvanized steel. The PVC-coated steel should not have any low-temperature issues. Field bends are generally not allowed for PVC-coated steel.

The worst corrosion occurs at the earth-air boundary. We specify PVC-coated steel for risers. If the steel conduit is going to be concrete-encased, the soil corrosion issue is obviously not as big an issue.

The steel conduit provides additional physical protection and EMI shielding compared to rigid PVC.

If the owner has specified it and is willing to pay for it, I don't see an issue. For UG conduit, I don't think the cost is justified, but there are no technical issues that I am aware of.

 

[eeprom]

Thank you for your reply. I think I was not clear enough in my phrasing. I am trying to figure out if there are any known issues in using standard PVC coated in concrete instead of using the PVC coated steel conduit. I know that the PVC coated steel is good, I just don't believe it is necessary. I was told that standard PVC would fail at very low temperatures due to ground shifting. I had never heard of this, and I'm trying to figure out if there is an engineering reason to use PVC coated steel instead of using standard PVC encased in concrete.

 

[dpc]

It's possible that the Sched 40 PVC solvent weld fittings could fail due to frost heave and things like differential settlement. I have seen that occur. I have not seen the conduit itself fail. If it is concrete-encased, that is much less likely, especially if the duct bank is reinforced.

Sched 80 PVC is another option, but you lose a lot of fill area compared to Sched 40.