Underground Piping Supports 2

[DGrayPPD]

Hello all,

I will use my previous post about thrust blocks to transition into another similar topic that I am now researching: Underground supports for welded steel piping.

To put it simply, are they needed?

It is my understanding that all buried piping in a process plant rests evenly on the soil/bedding. Are there situations where support of an underground pipe is needed? The particular support in this situation is just a regular square concrete block but my question pertains to all types of underground supports.

Why would underground supports be needed?
Would this not be a more complicated installation due to the fact that there are span limitations and proper backfilling would become more challenging?
Would this create additional stresses on the piping to have it resting on steel or concrete supports at various points rather than having it supported continuously by the soil?
Wouldn't an even distributed load upon the soil/bedding be a more economic approach?

One thought may be differential settlement of the piping in the soil due to poor soil properties. My thought on this, and please let me know if I am wrong, is that if there are issues with poor soil, then why wouldn't the entire plant be at jeopardy long before a buried pipeline? Seems that there would be much greater loads coming from equipment and structures than from the piping itself.

Looking forward to hearing back from you all.

Thanks,
CPPD

 

[saplanti]

Buried liquid welded steel pipelines which are back-filled with soil do not require additional supports unless the pipeline crossing water ways from my experience. In case the pipelines transporting gas or light liquids, you need to be careful with buoyancy of the pipe which needs to be checked.

Sometimes trenches are not adequately drained or there is a chance of saturated soil in and around the pipe trench this may cause the pipeline move, so additional supports may be required by geotechnical/civil engineer for the pipeline or surrounding of the trench.

Sometimes adding large concrete block to support the pipeline cause more harm to the pipe since the settlement under the block becomes more critical.

Since your question is broad, I hope, this is sufficient. If you need more detail information, I suggest, you should look at American Lifeline Alliance/ASCE Guideline.

 

[rconner]

The drawing attached to the 1st post of the OP earlier showed what appeared to be a welded steel pipeline connecting two structures. There appeared however to be four potential sort of "anchor points", one at each structure as well as piers supporting the line aboveground several feet outside both structures, before same dove straight underground with vertical risers to the underground portion of the pipeline. Some perhaps minor discomfort I had with this layout when I first saw same (knowing really nothing about the geotechnical aspects nor location of the project), was not that there were no "thrust blocks" provided within this system, but it appeared there was no "flexibility" other than that of the all-welded pipe string itself provided near these structures.
I was aware e.g. AWWA standards e.g for water tanks require that sufficient flexibility/flexible joints be provided for water piping connections to accomodate relative settlements and seismic movements, and protect the connections. If there will be no significant movements, there won't be any damage due to localized stress from same, and I guess my concern would in that case be unfounded.

 

[LittleInch]

In nearly all circumstances, underground supports for buried pipes / pipelines is not required and in many places is a positively bad idea as it concentrates bending and shear if the ground compacts outside of the supports and the pipe ends up holding up the ground between the more solid supports. Differential settlement is a pipe killer made morse by supports.

In a plant setting it is very difficult to see how the ground can be as bad that the pipe needs support or that the compacted ground in the bottom of a trench could not support it.

You get more difficult choices where you have something very solid, say a tank supported on piles down to bedrock and then very soft ground coming from it. Laying pipes in muskeg and peat bogs are equally difficult, but even then, your best bet by far is to compact the trench bottom and then just lay pipe.



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

 

[dbill74]

Why would underground supports be needed?

Bad Soil. LittleInch is right though, it is very very rarely needed. But as I learned a couple years ago, there are times when it is. I have project that requires the underground support located outsite New Orleans, LA. If you have a project in this area, check with the structural and geotech guys to verify the situation.

Would this create additional stresses on the piping to have it resting on steel or concrete supports at various points rather than having it supported continuously by the soil?

No more than supporting the pipe above grade.

The particular support in this situation is just a regular square concrete block

One thought may be differential settlement of the piping in the soil due to poor soil properties.

This is a common problem around New Orleans. The solution engineers in the area use is to "suspend" the piping from the floor slab using hangers. Using a "regular square concrete block" in this situation would be useless as the block would fall away with the soil. To address the concern of soil resting on the pipe, a "shield" is used to keep soil off the pipe.

if there are issues with poor soil, then why wouldn't the entire plant be at jeopardy long before a buried pipeline?

Structural engineers are more aware of poor soil conditions than we plumbing engineers and will design their structures accordingly. For my project in New Orleans, the slab was a "structural slab"; constructed basically the same way as an upper floor not needing to rest on soil for support.

I've mentioned New Orleans having problems with bad soil, I don't know of any other areas for certain. I would imagine though parts of southern California might need/want to support underground piping due to the threat of earthquakes and soil liquifaction.

 

[DGrayPPD]

saplanti, rconner, and LittleInch,

Thank you for your responses.

rconner, in the previous post the piping was connected to a fire water storage tank and the other end that appears to be floating in space is connected to the fire water main coming into, and circling the plant. I will attach a picture of the supports that are used outside the vertical risers.

LittleInch, my thinking seems to match your views the most. I don't necessarily understand why some people believe that you need pipe supports when you should just lay the pipe in the trench. Seems to me like it is a waste of money, a more difficult and timely installation, and a potential hazard for the piping. If the 330,000 gallon fire water storage tank resting on a ring wall foundation with no piles is only expected to settle 1-1/4" to 1-1/2" over the course of 5 years, then why would people think a 6" carbon steel line is going to settle?

 

[DGrayPPD]

dbill74,

Thank you for your response. I am from Baton Rouge, so I definitely know and understand what you're talking about in New Orleans. We are a small engineering firm and most of our projects are based out of Texas around the Houston area.

 

[Gator]

In northern Alberta, virtually everything sits on piles due to muskeg/ground freezing. I'm not certain, but I seem to recall a lot of plant surface area backfilling, not sure how deep. Not a lot of buried piping.

I'll post if I get some further info.

 

[DGrayPPD]

Thank you Gator