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skier59 (Civil/Environmental) (OP)
18 Jun 03 13:06
Hi - I'm checking opinions.  I'm designing a C900 PVC pipeline (12") for potable water distribution that is going across open ground in a straight line, and then crossing a highway in a bore that will have a jacked steel casing.  The pipe within the casing will be mounted on centering skids.  My questions are: (1) Do I need restrained joints within the casing (pipe outside the casing has enought straight laying to not need restraint).  (2)  Is PVC the appropriate material within the steel casing?  (3a) Do I need to fill the annular space within the casing?  (3b) With grout?  (3c) Blown sand?

Thanks
dicksewerrat (Civil/Environmental)
18 Jun 03 20:44
Is the pipe laying on the ground? Have you looked at thermal expansion? I wouldn't worry about the pipe in the casing, I'd worry about the rest of it. UV light damages all plastic pipes.
KRSServices (Civil/Environmental)
19 Jun 03 0:10
Keith,

No you do not have to have restrained joints.  You do however have to affix spacers to the C-900 and ensure they fit into the casing.  Match OD of spacer to ID of casing.  Also, ensure your casing is large enough to accomodate the OD of the bell on the pipe, very important.  PVC is absolutely fine and the approach you are taking is absolutely correct.

When case boring, make sure the contractor is aware of the huge undertaking and space required to do the case bore, particularly a large area to stockpile the augered soil with a loader.  Determine who is to pay for the crane as well (one contractor I knew had no clue about the operation and it got very ugly).

KRS Services
www.krs-services.com

skier59 (Civil/Environmental) (OP)
19 Jun 03 10:39
dicksewerrat,

The pipe will be burried - by "going across open ground," I meant to imply that I had enough straight run and soil friction outside the casing that restrained joints weren't necessary.  Perhaps I should learn to write what I meant to imply.

Keith
Helpful Member!  TVE (Civil/Environmental)
19 Jun 03 10:55
Okay, in order...

1) Restrained joints are generally not needed in the casing.

2) PVC pipe in a steel casing is fine. See http://www.ipexinc.com/download/pdf/pvcinstallguide.pdf for a guideline on casing sizing and the required spacers. For spacers, you can always strap some 1"x2" to the pipe (assuming of course that the strapping will extend out past the bell of the pipes so the pipe is resting on the strapping) if you don't want to buy the manufactured spacers.

3) As for filling the anular spacing, you need to ask a couple questions...

a) Why is there a casing in the first place? If the casing is to prevent the water from eroding the road in case of a watermain break, you want the casing empty so that the water can flow to either end and discharge somewhere else.

b) Does the road authority have the requirement "Loads shall not be transfered to the carrier pipe."? If they do, then again, you can't fill the space.

c) You will need to do something at the ends of course. In the past, we have just used a few cans of spray foam sealer and sealed off the last 12" of the pipe. It doesn't transfer load and if there is a break, the water pressure will pretty much blow an opening in it. The only concern ever expressed was whether the foam would react with the PVC pipe but we got a statement from the foam manufacturer that their product was inert and wouldn't react with the pipe.

David Dietrich
Thames Valley Engineering
Zambo (Civil/Environmental)
19 Jun 03 13:27
Interesting thread because it raises several subjects. Firstly UV light (why are we discussing this for an underground installation) for PVC yes UV can affect, for HDPE if black the carbon black will protect.

Next do you need a steel casing or just a casing. The reason I ask is that if just a casing is required (by any road regulations) then possibly a plastic (HDPE?) casing could be used.

I suggest that an HDPE casing is used, installed by HDD and after an HDPE pipe is passed through.

Zambo

EricC (Civil/Environmental)
19 Jun 03 18:25
I've always seen the annular space filled, since it minimizes movement of the carrier pipe and helps reduce the potential for casing pipe collapse.  The casing should be designed to withstand the load from the roadway, so transferring the forces isn't an issue.  Differential settlement of the casing pipe versus the carrier pipe outside of the casing may be an issue, but that's why joint deflection is a good thing.  

As for whether to fill with sand or grout, either is probably fine.  There are arguments either way:  sand can be removed if there is a leak, but if it's grouted, will it ever actually leak?  Personally, I like sand as it allows for repairs if necessary.  Some situation will always arise that none of us thought of and its better to maintain flexibility.
dicksewerrat (Civil/Environmental)
20 Jun 03 21:31
UV light came up because the "pipe went across open ground" . Sorry but I have assumed things in the past that really made a difference in the answer, so I take everything here literally.
assandhu (Civil/Environmental)
22 Jun 03 1:20
What type of jointing are you adopting.
If it is bell and socket, think again, have you considered infiltration due to negative pressure.
If the pipeline is changing its alingnment before and after entering the casing, provide Thrust blocks.
Amar.
Markmcad (Civil/Environmental)
12 Jul 03 11:24
Hope I am not too late with a response for your pipeline questions. Joint restraint is recommended by some pipe manufacturers to prevent over-belling of the joint, which could be caused by pushing the pipe through a casing. Some pipe manufacturers will recommend PVC pipe installed by pulling the pipe through the casing rather than pushing. The material properties of PVC pipe and variables during the installation process create a situation where pulling is preferred over pushing. The problems caused by over-belling the pipe may not show up until the pipe is pressure tested.  If you pull the pipe, then joint restraint hardware would be used; but from your description joint restraint is not necessary.

PVC pipe is fine for casing applications, as long as the pipe meets your system requirements.

Fill the annular space. Grout is generally preferred. Recommend using mechanical casing spacers. Wood that is creosote treated will damage the pipe. Good luck.
yogibear1 (Mechanical)
14 Jul 03 9:02
No one has mentioned whether or not the pipe has any valves at either side of the road crossing, a prudent operational feature in case of piping failure.  It is also an easy place to locate a valve. If there are valves (or bends) then reaction forces forces have to be considered with regard to a sufficent number of restrained joints to keep the pipe from blowing apart, as when a valve is closed.
Ag93 (Mechanical)
18 Jul 03 15:33
If you use HDPE pipe inside the casing or for the casing itself, you will not need to restrain any joints at all.  (Except for the PVC to HDPE connections, of course)  Installation may be easier with HDPE as well.

check out www.driscoplex.com, good luck.  
HB2U (Civil/Environmental)
22 Jul 03 9:26
Valves need to be installed at either end of the pipe where it passes into the casing pipe. They need to be placed back far enough for pipe removal and replacement activities in the event of a future break within the casing pipe.  Do not fill the annular space - the field guys will be cursing you when they have to remove it to repair a leak -  just use spacers set according to the manufacturer's specs.

Include vents at each end of the casig pipe.  We construct ours out of 2 inch PVC and put a goosneck with insect screen on the top of each vent pipe.  This way, if there's a leak it'll surface in a visible way.

Your casing pipe should be installed so that it can drain to one end.  Condensation can build up in the casing pipe and cause corrosion over time if it can't drain out of the pipe.

Good luck with this project (which may be complete now??)

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