Wrapping PVC (glued) fittings prior to thrust block ??

[latchkey]

The contractor wants to wrap the PVC (4" sch 40 pipe w/sch 40 fittings) at the point of concrete thrust block contact. His reasoning is to prevent abraision at the point of contact. As an inspector, I see that since the wrap that is composed of 10 mil visqueen, with several wraps it becomes crinkled and wrinkled, provides room for movement, thus becomming a 'cushion' by not providing 100% contact between the pipe/fittings and the thrust block. This is the main pressure line for the site irrigation.
Come Monday I have to come up with an adaquate response for the management team. BTW, our civil engineer is away on vacation and not available for comment.

Please comment, your expert advice is sought....sl

 

[BRIS]

Our standard specification calls for wrapping with two layers - I am not convinced that here is any benefit.

The wet concrete will compress any wrinkles the amount of movement due to compression of the visqueen will be negligible and an order less than the movement in the thrust block to mobilise resistance. Both totally negligible compared to temperature movement.

 

[GregLamberson]

latchkey

I don't like thrust blocks at all and think they should be avoided when possible, which is most of the time. That being said, I agree 100% with BRIS, the issue you described is neglible.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BRIS]

I intended to ask why you have thrust blocks with solvent welded fittings?

 

[bimr]

I agree with the previous posters, you do not need thrust blocks at all.

 

[atook]

I agree on the thrustblocks probably unnecessary. I only spec thrustblocks for gasketed pipe, and I spec gasketed pipe for everything larger than 3-inch, since larger pipe is harder to glue properly.

 

[latchkey]

Thank you to all who responded. As I am not the engineer on this, I couldn't answer as to why have thrust blocks at all. Howevr, it's spec'd out. Interestingly, the 4" SDR 135 pipe sat in the open trench over the weekend. Temps in the low 100's. Today the 45's and 90's had pulled away from the concrete kickers AND the kickers (2) were split! The run is about 80'. I can see that it expanded and later contracted. So, I can see why you all disagreed with the kickers. Thank you very much! steve

 

[rconner]

Interesting questions and observations. If no thrust blocks or external anchorages are installed it would appear there will be some movements and perhaps even some (attempted) rebating movements of the fittings and piping relative to the soil mass, particularly in the thrust foci areas in response to thermal variations, Poisson/Bourdon effects of changing pressure conditions etc. If on the other hand effective external anchorages are installed one would think subsequent movements at least in the thrust foci areas would be more controlled (the thinking of this “designer”?); however, it would appear there are then some concerns and/or experience of abrasion and/or other stress concentrations in the block area.

I am not sure that with plastic materials (with very high thermal expansion coefficients, relatively high long-term Poisson’s ratio, relatively low long-term moduli, hardness, scratch and gouge resistance etc.) that such effects in either case would necessarily be inconsequential. I do know that after a discussion of many subjects, including the weakening effects of surface damages (or other notches etc.) in a July 1972 AWWA Journal article, “Designing PVC Pipe for Water-Distribution Systems”, the author Mr. R. T. Hucks JR then stated,

“Until such time that a wider knowledge of actual pipe operating conditions has been accumulated and the effects of surface damage is better understood, the design of PVC water distribution pipe must be conservative, and based upon the controlling stress causing failure, the cyclical (pressure)operating conditions.
Any method of design upon which an engineer can make his PVC pipe material selection must include (1) the effect of static conditions; (2) the effect of field damage (scratches); and (3) the effect of dynamic properties.”

While Mr. Hucks didn’t cover all aspects of piping in his article, he was perhaps trying to cover a lot of ground with these particular statements.

 

[BRIS]

If you use push fit spigot and socket joints on PVC pipe then you need a thrust block at bends. Expansion and contraction is taken by joint movement and you may need intermediate anchors to prevent the pipe shuffling itself down the trench.

If you use welded joints thermal movement results in the development of compression and tension forces in the pipe. Some are relieved by movement into and out of the earth at bends. Snaking the pipe in the trench also provides relief.

The concern is if you provide rigid thrust blocks on a welded pipe there is a stress concentration at the point of contact. This stress is due to longitude forces in the pipe and can result in buckling of the pipe/fitting wall at the point of contact.

In a normal thrust block situation with push in joints the thrust block resist internal hydraulic pressure which acts uniformly over the inside of the pipe wall and is resisted by the thrust block which acts uniformly over the outside of the pipe wall. The wall is in compression with no significant shear, buckling or compressive forces.

 

[latchkey]

Thank you all again, and for the detailed information. I love details....sl