Pipe Expansion Containment of Buried Pipe

I'm the one who raised the issue, based on experience with open-air piping. This may not be a problem underground, but it would be interesting to compare the lengthwise compressive strength of these pipes with the frictional forces exerted by the ground.

Insulated pipe might conceivably slide within the insulation, even if the insulation does not move w.r.t. the ground. I'm sort of thinking out loud, but the issue may justify a close look.

Great reference at http://books.google.com/books?id=G1tcLwWKtOQC&pg=PA496&lpg=PA496&dq=buried+pipe+expansion&source=bl&ots=gFhRx0k_Do&s . The ground stabilizes the changes in temperature in unheated or uncooled piping. For piping that has changing temperatures there are expansion and contraction concerns.

The kind of feedback that is still outstanding but would be very helpful is the field experience or improvised methods to contain expansion for the listed cases.

Be more specific Depth of pipe. length of of underground piping. soil type? Some factors to base a solution on.

Depth of Pipe 3 feet.

Straight Lenght of pipes sections 1500 feet that have no ells or u bends (that in above ground pipes could potentially take pipe expansion if designed with fixed and sliding points).

Total pipe lenghts of the networks is 9000 feet.

The soil is not rocky but hard setteled and stable soil like regular earth material.

The PVC and metal pipes will be laid in properly prepared trenches with a choice of pipe bedding of about 9 inches of fine white sand or granular backfill material. Then backfilled by a same material 9 inches above the pipe crown and then backfilled with soil and compacted as per rules.

Thanks

Check with a local licensed fire protection sprinkler contractor.Their regs require anchoring at specific distances.He can probably quote chapter and verse on requirements.Strangely enough, PVC pipe stretches length-wise under pressure, and contracts in diameter as a result.This can result in joints creeping apart and causing a blowout.

I gleaned this information while watching a blowout being repaired, and talking with the contractor doing the repair.

Contrary to first impression, pvc pipe will actually shrink lengthwise when pressurized, due to expansion in diameter.Repeated cycling of pressure over time will cause loosening and wear so the pipe must be restrained(anchored) at proper intervals.Consult a local code expert for relevant local requirements,which do vary.

You have not mentioned anything about expected temperature variations in the various cases, nor if the lines will have shut-downs etc. There can be significant temperature changes in some pipelines, e.g. carrying surface water in temperate zones, and otherwise caused by climate and other effects, without intentionally applying or extracting heat. The opposite of "expansion" is of course contraction, and with any thermal and pressure cycling, this in turn means there will be a tendency towards back and forth, or rebating motion of the pipes (most pronounced with polymeric pipes). Lastly, I guess if it were literally true that "...the provision for underground pipe is not as vital as above ground pipe.", then I guess us folks in temperate zones should not have to worry about our pipes breaking apart in cold weather!

Plant shutdowns, maintenance and a host of other situations as well as ambient temperature fluctuations would cause the type of cycling you have stated. It is precisely these changes that will try to expand or contract the pipe. The original question was that would the gripping action of soil allow the pipe to expand and contract freely as above ground pipe does. If yes, what are the types of expansion joints or methods that are suitable for underground applications for the cases listed in the original question?

If you were to place a shard or coupon from a pvc or whatever pipe wall on a sizable bed of soil (or rock, or like anything else that has migrated through the soil), then place a surcharge or normal weight on top of that shard, and finally apply an increasing pulling or pushing shear load to the assembly, I believe you would find that friction between the contact surfaces would in effect prevent movement, but only up to a point. At that point, the surfaces would move (or grind) relative to each other. While I am not going to profess I have all the expertise in this matter, it would appear to me that the answer to this, like so many other general or open-ended questions with little information or background provided, is most accurately, "It depends…" (on magnitude of anticipated thermal and pressure etc. variations and excursions, as well as cycles of same, and probably other factors etc.) I have heard that some folks in particularly the plastic pipe industry have apparently claimed that there is no movement, but I do not know of nor have I seen what analysis or testing has been offered up that supports such a claim. At least ignoring soil, calculations reveal a surprising amount of long-term movement in some cases with polymeric-type pipes. I have also read that perhaps non-obvious axial movement may even occur with even very short duration higher pressure spikes(in a published article from a manager of the Johns-Manville research lab many years ago that also noted some alarming surge events measured in pvc piping systems, as mentioned also in the prior thread on another blog at http://eng-tips.com/viewthread.cfm?qid=227976 ), "Since PVC pipe moves axially as well as circumferentially in response to pressure surges ("Poisson's ratio effect"), pipe may undergo scratching of the outside when buried and in contact with sharp stones in the backfill. This area requires further study."