Buried PVC pipeline degradation 6

[BrianBT]

Chaps,

My company is supplied with Potable water through a system which includes 6" pvc pipe line of a few 100m or so which is burried in reclaim land so the soil is quite sandy.At either ends the PVC pipe is connected to Stainless steel manifolds and branch sections.

I asked about inspection regimes and the response was when they get a hole in it they fix it. I have concern that contaminants may get into this system unrecognised through small perforations, the main cause of perforations currently are small stones working their way through the substrate to contact the pipe and then abraid there way through. I am also concerned that there could be catastrophic failure of their system due to degradation leading to loss of this essential supply

Can anybody assist me with this or point me in the right direction as to where I might find the necessary information. I have read a few reports indicating minimum expectation sof 100 years or more but should there be some form of empirical assessment as an assurance process

Brian

 

[Duwe6]

Since PVC pipe hasn't been around for 100 years, the lifespan is a guess. Same for LED bulbs that are not over-volted.

Other than using geophone to 'listen' for leaks, there really isn't a good way to inspect buried PVC that I know of. If I had your concerns, I would buy a digital pressure meter that measured in 1/10's or 1/100's of a psi and shut off the line on both ends. A small but consistent pressure drop is evidence of a leak(s) - now you have to determine if it is from the pipe or from your boundary valves. If there is a way to "Double-block & bleed" then you can see your valve leakage. If not, you will need to put in some kind of assured shutoff, like a pipe 'pancake blind' aka paddle blind, hydro blind, skillet blind. [google it]

 

[BrianBT]

Many thanks for this, I did talk to them about pressure testing ( and vacuum testing although prefer former for obvious reasons). The system has multiple off loading points all of which are easily accessible.

I will see if i can convince them of the need for this

again, many thanks

Brian

 

[LittleInch]

Your issues seem to be the following.

Pipe buried in re-claimed land should be OK, but all plastic type pipe (PE and PVC) is susceptible to leaching of volatile organic compounds if the land is actually contaminated instead of just "reclaimed". I know there are PE pipes which incorporate a thin metal foil in the pipe to prevent this - google protectaline - PVC seems a bit better than PE, but still not impermeable to any nasty chemicals it comes across.

Assuming you don't have that, then how exactly are these small stones going to work their way through the ground?? Only if the stone is there in the first place due to the contractor putting it there instead of nice sand or fine soil will it end up next to the pipe. Even if it is there, there is no real movement of the pipe once buried in potable water systems. I have seen fibre glass pipe with holes "drilled" in due to thermal expansion back and forth, but potable water systems normally change temperature only a small amount and hence don't move.

PVC pipes essentially don't corrode and if you keep them out of sunlight and away from heat sources are incredibly stable compounds, hence I think your concern over catastrophic failure is mis placed. The biggest concern is normally someone digging it up for which no type of pipe is immune.

Of course your supplier / contractor should undertake a pressure test following completion, but be aware that these materials can creep under high pressure and hence show a pressure drop off which is actually acceptable.

If there is pressure in your system and any sort of small leak, how would anything leak in?

If you really have a critical process needing constant supply of water then maybe you need two separate connection pipes?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

degradation is generally not caused by "small stones working their way through the substrate to contact the pipe and then abraid there way through". more often, degradation is caused by a) cyclic stress caused by pressure variations such as pump cycling b) settlement which puts stress on the pipe joints c) stress at the above ground connections to your manifolds caused by temperature changes, pressure surges or mechanical impacts or even by exposure to UV radiation. Since the line is pressurized, little chance of contaminants getting in. Chlorination of the water takes care of most contaminants. Have a sample analyzed to convince yourself that it is safe.

 

[bimr]

PVC pipe is typically installed on gravel bedding so you don't need to worry about small stones causing a leak.

Not sure what you are calling reclaimed land. But as to contaminants, LittleInch is correct in stating that it is possible to have contaminants such as gasoline leach through the PVC pipe. If the land is contaminated with solvents, then a better piping choice is ductile iron pipe material with gaskets that are suitable for the particular contaminant.

 

[BrianBT]

Thats brilliant, many thanks chaps.

I am trying to get agreement from them for a wirtten instruction for actions in the event the system circulation pump fails they maintain a minimum line pressure using the static head in the storage tanks.

 

[rconner]

While I would agree with most all that is said here, I will add a comment with regard to the concern about hard stone getting against the softer plastic pipe wall. While I don't know about your specific "reclaimed" jobsite conditions, it is my understanding, and also some direct observation, that in general plastic pipelines have not faired as well in rocky soil areas as they have in others. If you do a good web search I believe you will independently find some direct, and much more indirect support for this case.
In a similar vein, so to speak (and without getting in to potential mechanisms, where much less is publicized about this), I don't think I would make an assumption that rocks, stones or cobbles don't move or migrate in or on the ground next to plastic pipelines, or for that matter vice versa. Notice e.g. the caveat on the second page of the document from reportedly "the world's largest manufacturer" of pvc pipe at

http://www.jmeagle.com/pdfs/Technical Bulletins/TB02PVCPipeTrench.pdf

that states,
"For the final backfill zone, particle size should be limited to three inch size or less..." (then notice that as pictured on the first page the "final" backfill does not begin until a goodly distance above the pipe and immediately surrounding "embedment" envelope.)
Notice the similar guidance from another rather long time manufacturer of pvc piping under the heading "Final Backfill" in the right column at

http://www.northamericanpipe.com/downloads/installation/PVC_Pressure_Installation.pdf

,
"In the final backfill, avoid using rocks over 4 inches (100 mm), clumps of frozen soil, rubble and other such material. (then notice right above that that the lower "Initial backfill" below that is specifically defined there as , "...over the crown of the pipe to a height of 6 to 12 inches (150 to 300 mm) is the initial backfill.")
Finally, see the advisory in the first column on this page,
​"Avoid using angular rock larger than ¾ inch (20 mm) or rounded rock larger than 1½ inches (40 mm) for embedment."

While I will not comment directly​ ​here ​on the practicality of achieving all this on many jobsites of our world, I will say with the size of excavator buckets and production rates in many places, it might take well-staffed and quite sharp-eyed and forceful inspectors and/or quite compliant installers indeed watching every step of the way to make sure no greater than 3/4" particles of rock or rubble are in an underground pipe trench! [A more natural tendency is instead towards "talus", and other disorder!] Another thing is for sure, if/when? a pipeline fails due to a rock impingement or gouge down the road, these manufacturers don't want blamed for the failure. While I guess when failures occur specifying Owners or Engineers can also with such copy as above attempt to blame an original installer for non-compliance with the plastic pipe manufacturer's or specification requirements, when same occurs beyond say an installers one year warrantee period this may be little consolation to the tax/rate payers, as well as inconvenienced Customers and Public.

All have a good weekend.

 

[DerbyLoco]

I had to replace my domestic water feed pipe with MDPE and the Seven Trent inspector had me laid the pipe on/in a bed of soft builders sand as the sharp sand I had original used would, he assurred me, eventually abrade the pipe and cause leaks.

 

[bimr]

PVC pipe is a flexible pipe that is typically considered to be resistant to abrasion.

"PVC sewer pipes in Europe, some of which had been in service for 28 years, were
uncovered in locations where installation had been done poorly or in conditions where
the pipe had been overloaded. Thus, most samples were pipes that had been performing
beyond design limits. The deflections were measured as high as 22% at test sites. In most
cases these extreme deflections were the result of large rocks within the backfill, which
had acted as point loads on the pipe. Test results verified that even though the majority
of pipes had been loaded in such a severe manner, none had sustained structural damage
or experienced failure. The tests performed on the samples included pipe stiffness, strain
ability, abrasion, and joint performance."

http://www.uni-bell.org/Chapter7_53442b75b3517.pdf

The Severn Trent gentlemen should read the directions for installing PE pipe:

Where the trench bottom soil can be cut and graded without difficulty, pressure
pipe may be installed directly on the prepared trench bottom. For pressure pipe,
the trench bottom may undulate, but must support the pipe smoothly and be free of
ridges, hollows, and lumps. In other situations, bedding may be prepared from the
excavated material if it is rock free and well broken up during excavation. The trench
bottom should be relatively smooth and free of rock. When rocks, boulders, or large
stones are encountered which may cause point loading on the pipe, they should
be removed and the trench bottom padded with 4 to 6 inches of tamped bedding
material. Bedding should consist of free-flowing material such as gravel, sand, silty
sand, or clayey sand that is free of stones or hard particles larger than one-half inch.

Pipe Embedment
The embedment material should be a coarse grained soil, such as gravel or sand, or a
coarse grained soil containing fines, such as a silty sand or clayey sand. The particle
size should not exceed one-half inch for 2 to 4-inch pipe, three-quarter inch for 6

to 8-inch pipe and one inch for all other sizes. Where the embedment is angular,
crushed stone may be placed around the pipe by dumping and slicing with a shovel.
Where the embedment is naturally occurring gravels, sands and mixtures with
fines, the embedment should be placed in lifts, not exceeding 6 inches in thickness,
and then tamped. Tamping should be accomplished by using a mechanical tamper.
Compact to at least 85 percent Standard Proctor density as defined in ASTM D698,
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using
Standard Effort, (12 400 ft-lbf/ft3 (600 kN-m/m3)).” Under streets and roads, increase
compaction to 95 percent Standard Proctor density.

https://plasticpipe.org/pdf/chapter07.pdf