Frost Line and ductbank

[faresos]

I'm working on a project in Iowa that has a frost line depth of 82". I'll be running 4-3" conduit from the utility transformer to the building in concrete duct bank, the top of the duct bank is at 36" below grade. That applies also the same for the primary conduit. Do I need to be below the frost lines or I could stay at 36" since it's concrete encased. Any thought will be appreciated.

Thanks,

Sam

 

[RUWired]

I never heard of any requirements about frost lines, utility or nec. Article 300.5(J) requires to leave room for movement.

That would be a heck of a dig if you had to go below your frost line.

(J) Earth Movement. Where direct-buried conductors, raceways, or cables are subject to movement by settlement or frost, direct-buried conductors, raceways, or cables shall be arranged so as to prevent damage to the enclosed conductors or to equipment connected to the raceways.
FPN: This section recognizes ?S? loops in underground direct burial to raceway transitions, expansion fittings in raceway risers to fixed equipment, and, generally, the provision of flexible connections to equipment subject to settlement or frost heaves.

 

[faresos]

I never heard of any requirements about frost lines, utility or nec. Article 300.5(J) requires to leave room for movement.

That would be a heck of a dig if you had to go below your frost line.

Thanks for the reply, it looks like article 300.5(J) talks about direct buried conduits but in my case it's encased in concrete ductbank.

thanks again,

 

[Smart $]

Thanks for the reply, it looks like article 300.5(J) talks about direct buried conduits but in my case it's encased in concrete ductbank.

thanks again,

FWIW, in the design of concrete-encased duct banks, it is the depth of the bottom that is of the most concern... not the top. Soil frost heave above the bottom of the concrete will not affect the ductbank. Only frost heave beneath the duct bank will.

The 82" depth is likely the maximum frost depth over a hundred year record period. I couldn't find anything official through a web search, but it appears the average frost depth is about 4'... but then again it likely depends on what part of the state this installation is located.

Where did you get the 82" figure?

At what depth are building foundations in the local area?

Will the concrete encasement be rebar reinforced to prevent shifts at stress cracks in the concrete?

 

[steve66]

The 82" depth is likely the maximum frost depth over a hundred year record period. I couldn't find anything official through a web search, but it appears the average frost depth is about 4'... but then again it likely depends on what part of the state this installation is located.

Where did you get the 82" figure?

I was also thinking that 82" sounds really deep, but I couldn't find anything to support that.

I did find that in places with a "permafrost", they have a "thaw depth" instead of a "frost depth". I wonder what the heck someone would do there?

Steve

 

[Hendrix]

I was also thinking that 82" sounds really deep, but I couldn't find anything to support that.

I did find that in places with a "permafrost", they have a "thaw depth" instead of a "frost depth". I wonder what the heck someone would do there?

Steve

Move the hell away!!!!!!!

 

[wwhitney]

II did find that in places with a "permafrost", they have a "thaw depth" instead of a "frost depth". I wonder what the heck someone would do there?

Dig their foundations deep enough to get to permanently frozen soil? The problem frost presents comes from the annual transitions between frozen and unfrozen, it doesn't matter if the soil you're building on is permanently frozen or unfrozen, just that it doesn't change.

Cheers, Wayne