The Reality of Restrained Pipe

[EngWade]

I've thoroughly reviewed and experimented with the Ductile Iron Pipe Research Association's Thrust Restraint Design for Ductile Iron Pipe program (their program for determining required lengths of restrained pipe joints). It's clear to me that DIPRA would want to err on the side of conservativeness (and in fact, they have a factor of safety built right into their program). However, my questions is - how much conservativeness is built in? My hope is to find someone on here who has experiece with restrained pipe installations, and hopefully DIPRA's program, and how far apart the two are (or aren't). Adjusting the input parameters (within a reasonable range) results in quite significant differences in pipe lengths.

For example:

8"x8"x8" tee, with 20' between the first joints
Type 5 laying condition
Sand Silt soil classification
5' of cover
Tested at 150 psi
Factor of Safety of 1.5
Restrained pipe length (bare) = 12'
Restrained pipe length (polywrapped) = 17'

Now, say the contractor wanted to test the system prior to fully backfilling and compacting the system. Assume he conducts the test with only 2' of backfill.

8"x8"x8" tee, with 20' between the first joints
Type 5 laying condition
Sand Silt soil classification
2' of cover
Tested at 150 psi
Factor of Safety of 1.5
Restrained pipe length (bare) = 73'
Restrained pipe length (polywrapped) = 104'(!!!)

Slight, justifiable alterations of the other input parameters result in a similar effect to the pipe lengths.

My client's day job happens to be as a contractor, so he is quite experienced in construction and installation of DIP water mains. However, he has always used thrust blocking (as have I), and the liability will ultimately rest with me anyway, and so I'm concerned with the uncertainty in the program. Perhaps the lesson here is to force the contractor to complete all his backfilling/compaction prior to testing?

Any experience out there?

Thanks in advance.

 

[rconner]

It is my understanding the basic ductile iron restrained joint/length approach now espoused by DIPRA has been used in the industry since the early 1980’s, and incidentally was adopted into consensus AWWA Manual M41 also in 1996. A dearth of problems in extensive application for more than a quarter century, despite probably some field variations as you suspect, indicates to me that the method probably has a great deal of general conservatism built into it. Nevertheless, ANSI/AWWA standard C600 for the installation of ductile iron pipe does provide the following guidance in section 4.3.5.3 of same:

“Partial backfilling during testing. Newly installed pipelines are normally tested after backfilling. When unusual conditions require that pressure and leakage testing be performed before completion of backfilling or with pipe joints accessible for examination, sufficient backfill material shall be placed over the pipe barrel between the joints to prevent movement, with consideration given to restraining thrust forces during the testing. In particular, restrained-joint systems, which derive their stability from the interaction of the pipe and soil, should be backfilled prior to testing.”

While soil in most cases of any meaningful backfill probably provides far more resistance to movement than the DIPRA method assumes, it probably behooves all of us to follow at least the AWWA recommendations as best we can. If a pipeline for whatever reason is to be tested with lesser than normal (e.g. “two feet”) of cover, I believe a suitable restrained joint length, or supplementary external restraint (permanent or temporary buttressing etc.), should be provided with consideration for same. In this regard, there is incidentally nothing necessarily wrong with “thrust blocks”, particularly if you don’t have the prerequisite restraint available. ;>)