HDD Pipe and Casing Q's?

[sjohns4]

A municipality I'm working with to design a developer funded system is requiring a casing to be drilled under a creek then a carrier pipe to be slid through, both HDPE.

I've had many HDPE pipelines installed by HDD method, but never did a casing / carrier - it just seems overkill to me.

However, before I try to argue the point that it's overkill I'd like to hear opnions from some others in the industry. Has anyone seen a fusible joint pipe fail after passing the inital pressure test?

Thanks,

Mike

 

[bimr]

Sometimes when the ground is unstable, a casing can be pushed down the bore hole to prevent the collapse of the hole walls.

In a river crossing one can assume with reasonable confidence that the directionally drilled pipe is subjected to the earth
pressure from the sediments above it combined with the water pressure.

However, it is not common to use a casing pipe for HDPE.

 

[cvg]

it is not uncommon to require a casing for a river crossing, regardless of the type of carrier pipe. An alternative when installed by trenching might be concrete encased. If you can drill deep enough under the river, you might get them to relax their requirements for the casing.

 

[rconner]

Yes (in multiple different fashions, not to mention any risks of pipe wall failure, third-party or external force damage), witness the 30 Jan 03 16:04 post of stanier at

http://www.eng-tips.com/viewthread.cfm?qid=42680,

now also the multiple specific revelations I’ve noticed at

http://hdpefailures.com/pipe failures.html,

and also applications of hdpe pipe and conduits that have been known involved in some cases of “necking down”, substantially in actual flow area/size (when the applied pulling and/or bending etc. stresses exceeded the local yield strength of the piping pulled). In the latter case (I guess with or assuming intermediate very good quality fusions allow this to occur) e.g. a 4” sized pipeline has been known to pass a hydro-test when its size is markedly reduced by over-pulling to say approximately 2” (though with great deficiency in intended carrying capacity etc., and clearly not “right”!)

Visual inspection or “Visual testing” (VT) was incidentally long promoted by the hdpe industry and its installation experts as the adequate quality control of fused joints. Revelations from the field as well as quite detailed research have indicated this is not reliable. Note the field findings of the reports including

http://www.oildom.com/PGJ/pgjarchive/March03/buttwelds3-03.pdf

, and the more recent reports and research

http://www.google.com/url?q=http://...QQFjAB&usg=AFQjCNE0DpNXiHZz5oGsP_iwOWacJ9MmOA

that conclude e.g.,

“Visual testing does not seem to reveal a subtle condition like lack of fusion in a butt fusion joint. It is a surface evaluation technique and does not represent the volumetric condition of a fusion joint. Research conducted to date shows that VT only detects certain unacceptable butt fusion joint conditions. The research indicates that VT will not detect many of the conditions that should not go into service.”

As cvg sort of intimates, and also e.g. ASCE MOP #89, “Pipeline Crossings”(1989) details, there are actually several intents/functions of casings that are sometimes preferred/specified by whatever authorities for different sorts of crossings, and at times for all piping materials.

All that being said, “over-kill” is relative and in the eyes of the beholder (I guess interpretation dependent on critical nature and risks of specific crossings), as I agree with bimr there are probably a whole lot of crossings out there that have not (at least yet) exhibited problems.