PVC Expansion - underground

[Htracey]

Interesting situation we had last week. We are running 20’ lengths of 4” sch 40 pvc in trenches with chairs and EMT tie down stakes. We have different configurations with 4 or more conduits. Most of the runs are roughly 300’ long and tie into concrete man holes at either end. The manholes have cast in place terminators that we glue into. Because we have a set point at either end, we have to use a slip coupling some where in the run. That is glued in place after the pipe is cut to length.

Last week we had some pretty warm weather for April in Michigan (80s and sun during the day, 50’s at night) we finished some runs in the afternoon, hottest time of the day. When we came back in the morning the pipe had contracted back 4-5” and pulled out of the slip couplings. Must be the weakest link. We repaired in the morning (coolest part of the day) and monitored other runs and locations.

In the afternoon sun that same run heated up and expanded again and the only place it could go was up so. Some of the chairs and stakes broke to allow for expansion as the pipes moved in the trench. In some runs there are bends and those seem to take the expansion/contraction better. Most runs are straight pipe without much flexibility. With these temperature changes pvc is expected to change in length about 1.4” per 100’ which it definitely is doing.

We are installing thousands of feet per day and unfortunately we sometimes have to wait days for concrete or backfill. With those temperature swings this will continue to be a problem.

I’ve never heard of an underground rated expansion join. Is there?

Has anyone dealt with this before? We have hundreds of thousands of feet to go working in every weather situation imaginable. How can I control this or prevent damage?

Your experiences, expertise and advice is greatly appreciated.

 

[winnie]

I have no direct experience with this scale of work.

I know that I've seen piping systems where bends are intentionally added to runs specifically to permit some flex during expansion/contraction. But electrical conduit isn't plumbing pipe.

Is it possible that your conduit runs are being made 'too straight' and are tied down too tightly? A little bit of meander would allow for the system to flex with temperature changes. Once the fill is in place no one will see that the pipes aren't perfectly straight, and this isn't a gravity wave observatory.

I'd suggest contacting an expansion fitting supplier for their advice.

Per https://carlonsales.com/techinfo/installationtraining/IT-ISEXPJT.pdf

they explain that expansion joints are generally not used underground or encased in concrete, but also identify conduit expansion joints used where the concrete itself has an expansion joint. This suggests to me that their expansion joints can certainly be used very close to an embedded situation.

Of course you are not dealing with expansion and contraction during use, but rather temporarily for the time period between laying conduit and the backfill/concrete. Once the concrete has set, it doesn't matter if the expansion fitting is rendered locked in place; only that it permit movement during the temperature cycles until the concrete/fill is placed. A 'taped' expansion fitting might be perfectly acceptable in this situation.

-

 

[ptonsparky]

Not to you scale of project at all. Dairy barns with 80 degree temp change.
We have placed back to back expansions that were secured in place, say midway in run. Each side of that would have the 4" range of the exp coupling.

 

[synchro]

Last week we had some pretty warm weather for April in Michigan (80s and sun during the day, 50’s at night) we finished some runs in the afternoon, hottest time of the day. When we came back in the morning the pipe had contracted back 4-5” and pulled out of the slip couplings.

And if they backfilled or poured concrete when it was hot, it would've pulled out with bad consequences without being noticed.

Would it be practical to use slip couplings at both ends, but don't glue them up until most of the backfill is done except near the ends of the run to allow them to be glued then? That would equalize the temperature of the conduit relatively quickly close to a suitable value.

At a minimum, I think the temperature should be taken into account when cutting lengths for the slip couplings. So if it's hot it should be pushed together so it's shorter and there's more range left for contraction. And if cold it should be pulled apart more to allow more expansion.

 

[Tulsa Electrician]

The issue is the cast in place termination joint at the man holes.

We let the pipe float into man holes. This will resolve you issue until back fill etc than you can trim for wire pull. The pipes can be sealed with expansion seals. Something along the lines of link seals.
Install with link seal loose than when stabilized tighten up.

For cast in place a sleeve would be put into cast then pipe slid into sleeve than when ready install seal.

For you install I think a lay and cover is warranted. Then do final tie and fill with a slip joint at temp and cover to reduce the total amount of expansion and contraction to the last 20 foot. The set you slip fitting depth based on install temp.

Sounds like a pain and a reason we use slip and seal.

 

[Tulsa Electrician]

Pulled this pic from there site as an example

 

[drcampbell]

... Once the fill is in place no one will see that the pipes aren't perfectly straight, and this isn't a gravity-wave observatory. ...

The professor speaks: It's an application where perfection not only isn't necessary, it might be undesirable.

Another wrinkle: Concrete evolves heat as it cures. If it's in direct contact with the PVC, the PVC will heat up and expand while the concrete cures. It's difficult to predict what will happen as they both cool and the PVC tries to contract a lot while the concrete contracts only a little.

Reflective silver tarps over the trenches will reduce the effect of insolation (and nighttime deep-sky radiation) while waiting for backfill & concrete.

 

[Rick 0920]

The issue is the cast in place termination joint at the man holes.

We let the pipe float into man holes. This will resolve you issue until back fill etc than you can trim for wire pull. The pipes can be sealed with expansion seals. Something along the lines of link seals.
Install with link seal loose than when stabilized tighten up.

For cast in place a sleeve would be put into cast then pipe slid into sleeve than when ready install seal.

For you install I think a lay and cover is warranted. Then do final tie and fill with a slip joint at temp and cover to reduce the total amount of expansion and contraction to the last 20 foot. The set you slip fitting depth based on install temp.

Sounds like a pain and a reason we use slip and seal.

Those link seals are great for keeping water out. But I think his precast manholes are made for the conduit to fit tight going into them. when using link seals, the holes need to be considerably larger than the conduit. Great idea though.

 

[Htracey]

Yes the manhole terminators are sized for the 4” and pipe glued in. The 2” pipe glued into a reducer and the terminator. I agree that’s a big part of the problem, but can change it now. Most of the 300 vaults are already produced.

Hoping the conduit doesn’t contract much after concrete is laid, that would be a disaster.

Our plan for now is to work with the weather changes and wait to make up the slip coupling and cut the pipe to length until closer to concrete being poured. Going to monitor the movement tomorrow during concrete and see what happens.

I talked to Cantex today and they don’t know of an UG rated expansion fitting. Their suggestion was cover the conduit quickly or wait to cut to length until close to backfill. We are also installing the tie downs looser to hopefully allow the conduit to move better.

 

[don_resqcapt19]

....
I talked to Cantex today and they don’t know of an UG rated expansion fitting. Their suggestion was cover the conduit quickly or wait to cut to length until close to backfill. We are also installing the tie downs looser to hopefully allow the conduit to move better.

While not rated for that use, the expansion fitting installed when the conduit is installed will account for the length changes before the concrete is poured, and will be locked in place after the concrete sets. It would even account for the movement from the heat of the curing concrete until the concrete sets enough to grip the conduit.

 

[wwhitney]

The OP is about dealing with thermal expansion and contraction during the installation process. I'm curious if there is ever an issue with thermal changes in underground PVC after installation.

With PVC conduit embedded in concrete, I take it that after curing the concrete will generally continuously restrain the PVC. So if there is a temperature change, and the PVC and the concrete have different coefficients of thermal expansion, the PVC will only expand/contract the same amount as the concrete.

With PVC conduit embedded in soil, does the same thing happen? Or can the PVC slip relative to the soil, and then differential thermal expansion/contraction could be an issue?

E.g. suppose you install a very straight long run of PVC in an open trench on a hot summer day and backfill it while the air and PVC are at 100F. If you dig it up six months later in the middle of winter, when the ground is near freezing, might you find that the PVC sections have separated due to thermal contraction underground?

Cheers, Wayne

 

[Rick 0920]

The OP is about dealing with thermal expansion and contraction during the installation process. I'm curious if there is ever an issue with thermal changes in underground PVC after installation.

With PVC conduit embedded in concrete, I take it that after curing the concrete will generally continuously restrain the PVC. So if there is a temperature change, and the PVC and the concrete have different coefficients of thermal expansion, the PVC will only expand/contract the same amount as the concrete.

With PVC conduit embedded in soil, does the same thing happen? Or can the PVC slip relative to the soil, and then differential thermal expansion/contraction could be an issue?

E.g. suppose you install a very straight long run of PVC in an open trench on a hot summer day and backfill it while the air and PVC are at 100F. If you dig it up six months later in the middle of winter, when the ground is near freezing, might you find that the PVC sections have separated due to thermal contraction underground?

Cheers, Wayne

I have heard that our required burial depths for PVC are below the frost level. I've never used expansion couplings in underground installations nor heard of them to be required because the change in temperature isn't anywhere near what the change would be above ground. I've never had any issues other than "user error" with the conduits coming apart either. The OP has a different situation where he has to wait a significant amount of time before backfill and concrete. I really don't know what to do about that.

 

[drcampbell]

... With PVC conduit embedded in concrete, I take it that after curing the concrete will generally continuously restrain the PVC. So if there is a temperature change, and the PVC and the concrete have different coefficients of thermal expansion, the PVC will only expand/contract the same amount as the concrete. ...

Yes, but there could be some monumental internal stresses in the PVC as it attempts to expand or contract while constrained.
In the grand scheme of things, it probably doesn't matter. If this does become a problem, you'll just end up with a concrete conduit lined with fractured PVC.

... suppose you install a very straight long run of PVC in an open trench on a hot summer day and backfill it while the air and PVC are at 100°F. If you dig it up six months later in the middle of winter, when the ground is near freezing, might you find that the PVC sections have separated due to thermal contraction underground?

Ah, but it won't be dug up unless something fails. Since failures won't be visible and impending failures can't be prevented with a minor adjustment, there needs to be a suitable installation method that assures that it won't fail.

 

[wwhitney]

Ah, but it won't be dug up unless something fails.

Yes, that's a practical answer, but I'm interested in the theory. Obviously if the conduit does separate, that increases the risk of a future failure.

In a sense, my example is a variation on the OP's issue, in that it still depends on a starting configuration that is outside the realm of any operating conditions (the soil temperature at 18"+ depth will presumably never get to 100F). So a less extreme example involves installing the PVC at a temperature that is plausibly within the operating soil temperature range. E.g. I expect there are places in the US where the soil temperature at a 18"+ of burial depth varies from 30F in the winter to 70F in the summer, or a 40F range. Per Table 352.44, that would give a thermal length change of 1.6" per 100 ft.

So the basic question remains, will the soil in a backfilled trench continuously restrain the PVC along its length, or will the PVC be apt to slip relative to the soil? In the latter case it clearly would be appropriate to always install PVC conduit in a bit of a wavy path to allow for some extra length for future temperature contraction.

Cheers, Wayne