Frost heave protection for branch circuits

[greenspark1]

Hello,
We're working on a project in the NE where frost heaves are definitely a concern. We have a lot of circuits we're running to a mechanical yard, planning to run rigid sweeps and PVC underground (at least 24" deep). Previously we have used expansion couplings on the verticals in order to protect against frost heaves and meet NEC 300.5(J). Curious how others account for this earth movement? Expansion joints aren't really intended for this type of movement, more thermal expansion/contraction of the conduit. Thanks for the feedback.

 

[iwire]

We use expansion fittings.

But are you coming up through a slab of just dirt?

 

[greenspark1]

Not dirt, we'll have concrete slabs for the equipment. Expansion joint on each end of the circuit conduit (two per run)?

 

[kwired]

Bury your conduit below expected frost line. If you do that, how is frost heave going to have much significant impact on a vertical only run of RMC through the frost zone? For it to move it will have to pull the horizontal portion with it, and if it is below the frost line it is not moving, if the horizontal portion is in the frost zone the entire run will move with the frost expansion.

 

[jim dungar]

Sleeve your conduits where they pass through a slab.
Frost heaves cause the stuff in and on top of the frost to move, the stuff below the frost line stays put.

The frost heave in this area regulary exceeds 2", very few expansion fittings can handle that much movement.

 

[iwire]

If the slab is done right it should not move.

As in the organic material should be removed and replaced with easy draining fill such as crushed stone etc.

 

[jim dungar]

If the slab is done right it should not move.

It depends on the slab and the frost depth.

Equipment slabs, such as for padmount transformers, commonly move as they tend to sit ontop of the frost.
Floor slabs on footings, which are below the frost line, do not move.
Driveways and road beds all most never stay in place even though they sit on gravel that has been designed to drain.

 

[texie]

Sleeve your conduits where they pass through a slab.
Frost heaves cause the stuff in and on top of the frost to move, the stuff below the frost line stays put.

The frost heave in this area regulary exceeds 2", very few expansion fittings can handle that much movement.

This reminds me of something I see way to often. The EC dutifully puts an expansion joint in the raceway between a meter socket and the ground and then proceeds to put a conduit anchor to the wall between the ground and the expansion joint. Then to add insult to injury, the GC comes along and pours a concrete slab around the conduit.

 

[jim dungar]

This reminds me of something I see way to often. The EC dutifully puts an expansion joint in the raceway between a meter socket and the ground and then proceeds to put a conduit anchor to the wall between the ground and the expansion joint. Then to add insult to injury, the GC comes along and pours a concrete slab around the conduit.

O yeah.:happyyes:

 

[greenspark1]

Sleeve your conduits where they pass through a slab.
Frost heaves cause the stuff in and on top of the frost to move, the stuff below the frost line stays put.

The frost heave in this area regulary exceeds 2", very few expansion fittings can handle that much movement.

I was looking at these typical expansion fittings which have 4" or 8" travel length:
http://www.cooperindustries.com/con...inds/commercial-products/catalog-pdfs/xjg.pdf
By itself, will these solve the problem?

Burying below the frost line would be at least 48", much deeper than anyone would want to dig. And the equipment slabs are not below the frost line either, so they will move around. Definitely appreciate the feedback, sounds like a variety of methods have been used but no consensus.

 

[steve66]

I was looking at these typical expansion fittings which have 4" or 8" travel length:
http://www.cooperindustries.com/con...inds/commercial-products/catalog-pdfs/xjg.pdf
By itself, will these solve the problem?

Burying below the frost line would be at least 48", much deeper than anyone would want to dig. And the equipment slabs are not below the frost line either, so they will move around. Definitely appreciate the feedback, sounds like a variety of methods have been used but no consensus.

At some point, it seems like it might be better to just install USE cable without conduit, and just sleeve the last few feet before it comes up out of the ground.

 

[iwire]

It depends on the slab and the frost depth.

Equipment slabs, such as for padmount transformers, commonly move as they tend to sit ontop of the frost.

I would not call those slabs 'done right'.

Here a pad mount transformer either sits on a slab on crushed stone that goes below the frost line or is on a precast concrete pad that goes down below the frost.

As far as equipment slabs for say pumps and things the same things apply, if the slab is done in a way that it can move there are issues for the pipe fitters etc.

Driveways and road beds all most never stay in place even though they sit on gravel that has been designed to drain.

I doubt he is installing the stub ups in either of those ...

 

[kwired]

Never seen a transformer pad with deep enough footings to be below frost lines. Concrete transformer pads are nearly non existent around here anymore, POCO's are using a fibreglass base to set them on - saves time not having to form, pour, finish, and wait for concrete to cure, for most installations they have the thing up and running the same day they show up to install it. Those bases are in the ground some but not below the frost line, and the entire underside of the base is accessible anyway so the frost line is likely a little deeper in the vicinity of that base. For larger padmounts they usually do put crushed rock in the hole before setting the base, but I don't know that it would prevent much frost movement the way it is done. Leaving some slack in conductors should allow for the transformer to move with frost changes and that is usually done from what I have seen.

 

[jim dungar]

I would not call those slabs 'done right'.[/quote[
Your opinion.
In this area we just assume it heaving will happen.
Right now our frost depth is beginning to reach 6' deep in some places. The good part is frost heave seems to only affect the stuff in the top couple of feet rather than footings, probably due to the freeze thaw issue that occur at the surface.

I doubt he is installing the stub ups in either of those...

Texie gave an example. Again in this area most parking areas and driveways extend right up to the conduits mounted to the side of buildings.