HDPE Under Full Vacuum - Constrained Pipe Resistance to Collapse 2

[ubique345]

Hello,

I hope someone with experience can clear something up for me.

I'm trying to size required DR for a water pipeline under full vacuum. The pipeline will be buried 3m, and I intend to achieve a soil reaction - E' = 2000-3000 psi through material type and high compaction.

Now, when calculating the DR, if the pipe is considered constrained for the entire length a DR 21 pipe will be suitable and achieve a factor of safety of ~2.5-3 for vacuum / soil prism / live loads relative to critical collapse pressure.

However, if the pipe is considered unconstrained, the external pressure of the vacuum exceeds the critical collapse pressure. In the case of unconstrained pipe, DR 13 would be the suitable choice.

My question is, is it safe to assume through diligent supervision of the construction, and tight specifications for the embedment,and primary / secondary fill, that the DR 21 will be sufficient?

Is it common practice to only consider the pipe under constrained conditions for buried pipelines?

Thank you for any help you can provide.

 

[MikeHalloran]

I don't know what a DR xx is.

... But you seem to be saying that the pipe will collapse in air, with nothing around it, and you are thinking that you can somehow cover it with dirt in such a way as to prevent its collapse?

That sounds improbable to me.


Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

It is common practice to consider the pipe as constrained when buried, except when soils have very weak cohesion, or liquified, or when pipe is near horizontal or vertical bends and other places where axial pipe forces can become significantly misaligned, or where there are significant changes in wall thicknesses that could cause force imbalance even if the pipe is straight, or near points where the pipe rises above ground and soil forces along the intial buried section are not yet balanced.

 

[MikeHalloran]

Wow.

Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

Not finished yet.

Problem is that is in the axial direction. The collapse mechanism in vacuum is first ovalization in the transverse direction, then local buckling of the pipe wall, so constrained refers to the ability of the soil to prevent local pipe wall buckling, which is dependent on compaction of the soil around the pipe ring itself. If compaction is sufficient to prevent pipe ovalization, vacuum collapse is resisted, however as a means to design to some minimal collapse force there is usually some amount of initial ovalization assumed with construction placement of the pipe itself.

 

[cvg]

Is it common practice to only consider the pipe under constrained conditions for buried pipelines?

if the pipe is not buried, than nothing is constraining it? I would try and avoid full vacuum conditions in the pipeline.

 

[BigInch]

Actally if you cannot avoid vacuum, it is better to select a very conservative pipe wall adequate for full vacuum, rather than to count on any soil preventing the ovalization and collapse. Soil gets wet, gets dry, separates from the pipe wall at times. Plus if it does begin to collapse under vacuum, a very significant length may continue to collapse well past the point of initialization. Best to avoid such things.

 

[MikeHalloran]

I am familiar with collapse of stainless steel tubing under external pressure, and the severe effect that even a little bit of ovalization has on the collapse pressure.
( ... and BTW the noise that the collapse makes. )

I am also familiar with HDPE. I've never seen an HDPE tube that was actually round, and I'd expect an HDPE pipe to be further ovalized during installation, just by supporting it with straps and allowing it to bend under its own weight.

I'd accept that you could stiffen it a bit and limit lateral expansion by encasing it in concrete and making sure it stayed relatively cold, but compacted fill?

This part: "diligent supervision of the construction, and tight specifications for the embedment,and primary / secondary fill" scares me.

I've never seen 'diligent supervision' of _any_ construction.
How do you get that?


Mike Halloran
Pembroke Pines, FL, USA

 

[ubique345]

BigInch,

Thanks a lot for your reply. Yes, I did assume initial ovalality at install in the calculation.

We were planning to used crushed rock for the primary / secondary backfill ( Haunch and crown), and would cover with geotextile should ground water be present to prevent any soil migration... but there is always risk.

I have read different design briefs which contradict one another on this approach. Some say be conservative and use unconstrained calculation as your method for selection, and others suggest soil reaction forces are adequate to resist collapse under full vacuum.

I guess it comes down to experience, with which I have little in this particular scenario. The increased wall size for the conservative approach is significantly more expensive though.

 

[MikeHalloran]

Look at it this way:
If you overdesign, you might get yelled at for the cost.
If you underdesign, you won't be congratulated for the cost savings,
which will someday surely be more than offset by the cost of replacing the pipe,
and you'll get some really angry phone calls,
and maybe worse.

You decide.



Mike Halloran
Pembroke Pines, FL, USA

 

[ubique345]

Mike,

Yes, I know its counter intuitive regarding the soil pressure.

In regards to over designing: Yes, I agree with you and this is how I would typically do it... Never been congratulated after working hard to save money...but i'd really like to try for this one. Its likely just not worth it though.

 

[BigInch]

Sounds like you're trying to balance the cost of a piece of pipe today with that of maintaining your future reputation. Tough one.
Is it really that much money???

 

[LittleInch]

The design guide I use a lot states that a negative differential of 1 bar of unsupported PE 100 is Ok for DR 17 up to 1 year and 0.9 bar differential for 50 years.

PE80 is I think a lot worse than that. I have a little chart somewhere which proves the point, but can't lay my hands on it at a weekend.

3% initial ovalisation sounds a lot to me and might be a significant conservatism.

I can't really see backfill providing sufficient resistence.

How often and how long does this negative pressure apply? Thee are significant time issues with PE - my guide has DR21 being good for 1 bar differential ofr a day, but not for a month.

Most issues come from pipes laid in water which are empty or gas filled, but at a pressure < water pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ubique345]

Littleinch,

I'm familiar with the table you are referring to, or at least I have one that is similar, but it is based on unconstrained conditions I believe.

I didnt mention this earlier, but the system will be in -7 PSI gauge always as its a syphon at the lowest pressure zone, but I have considered it as full vacuum to be conservative. One thing I failed to mention as well is that this pipeline will be buried 3m underground.

I didn't explain the mechanics of this very well in my original post as I was seeking information from someone with experience for such a design and they wouldn't require it ( As Biginch demonstrated)

As explained by Biginch, in the 5th post down, collapse occurs first through ovalization. Once the pipe has vertically deflected enough, it will dimple inward and collapse. Now, the major axis of the ellipse (oval) must expand outward for this to occur, but the soil resists this movement. The amount of soil resistance can be increased through high compaction and material selection of the primary / second fill (fill around the pipe).

If I use crushed gravel or coarse-grained soils with no fines and compact this to 95% SPD it will provide a soil reaction modulus of 2000-3000 psi. At the 3000 psi soil reaction the calculations tell me that I have a about a 4 factor of safety under full vacuum and soil load (initial ovalization not accounted for yet), using DR 21( Recall i'm only at ~ -7 psi and this would actually be 5.8).

It is not a matter of will soil provide collapse resistance against this is or not. It absolutely will. I guess it comes down to reliability of the pipe encasement. Can I trust it given all the conservation I add afterwards?

So using this method, how much soil support do I need before collapse will occur? Well If i consider full vacuum at 100 years design interval, the soil support would have to drop from 3000 psi (design) to 200 psi.

Perhaps if I used a light cement / sand mixture around my pipe I could ensure this would not happen? I dont know.. thats why i'm here. I was really hoping to find someone who has enough experience in this area to tell me that yes, you can do this if you ensure X,X+1,.... or for someone to tell me i'm completely mad. So far, the people who have told me that this will not work dont actually have experience in the design of flexible pipelines, and are unaware of the failure mechanism in collapse nor the resistance soil provides to such failure.

I mean no disrespect here as three years ago I would have agreed with the statement below.

"... But you seem to be saying that the pipe will collapse in air, with nothing around it, and you are thinking that you can somehow cover it with dirt in such a way as to prevent its collapse?
That sounds improbable to me."

but i'm not going to base any decision on opinion.

Another important point, the area where we are building will never encounter any seismic activity which could compromise the soil enclosure.

 

[LittleInch]

Have you seen this?

http://www.vinidex.com.au/technical/pvc-pressure-pipe/buckling/

Have you considered additional constraints such as straps or sections of thicker pipe in between thinner pipe?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

ubique,

BI has given a good explanation of the reality of life in buried pipes - in many locations neither fully restrained or unrestrained. Most people like to think that a buried pipeline just sits there and doesn't move. In many places that is not correct....

I personally think what you're proposing will be sufficient, especially as you have a factor of safety of 2 before you even start if you're considering full vacuum , but actually only 7 psi difference.

This issue therefore is what level of satisfaction / factor of safety are you after??

A lot also depends on the particular circumstances / locations where you're getting this low pressure.

Is the pipe subject to other stress and forces such thermal expansion or contraction. elastic bending, any formed bends??

If the pipe section you're looking at is flat, straight, long and has no or little thermal expansion then I think you can look at fully restrained.

Anything else and the chances are you will generate some gap over time around the pipe to the extent that it has room to move.

Hence my suggestion that you look at some sections of thicker pipe or add some re-inforcement in the way of metal bands or straps?

Let us know how it goes

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Has anyone mentioned vacuum breaker so the line can't go under vacuum?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ubique345]

I didn't notice this before, but literally laughed out loud as this is essentially what I'm saying in my post. But don't we all to some degree balance our reputation with design cost? FYI: ~2.5x the cost.

LI & BI,

Thank you both for your posts, you have been very helpful.

Yes, I was going to use thicker pipe around the two major bends and at crossings where soil support is unavailable due to auger bore installation. The pipeline is mostly straight flat, and no significant thermal expansion.

I do like the idea of band reinforcement though, particularly to both reinforce and isolate the two sections where relatively high pressure drops exist, and at the interface of pipe thickness transition. This will likely be much cheaper and easier than concrete encasement.

I consulted with an old professor today regarding the issue. He suggested it would be reasonable to include soil support as long as I really know my soil and ensure consistent support throughout in addition to some of the advise which you both provided.

Looks like its a go, I'll be sure to update once complete. Thanks again.

 

[ubique345]

Artisi,

Its a siphon.

 

[BigInch]

Wow. Good one. That siphon answer short-sheeted Artisi pretty quickly.

Comeon, it's not $2.5MM right. You've got to be practical sometimes.
You're heart's in the right place, not sure about your head, or is it the reverse?

Reaction to change doesn't stop it