Movement of Buried Water Piping

[knight185]

At my plant we have underground raw water piping. The original piping is ductile iron. At one area the 24” header connects to a valve and then a reducer connects the other side of the valve to a 12” header. Several months ago we replaced the original 12” header with HDPE pipe. The HDPE pipe is connected to the valve via the 24x12 reducer and mechanical joints. Earlier this week we a had leak that pushed up through 10 feet of ground. Excavation revealed a leak at the mechanical joint where 24” side of the reducer connects to the valve. There are rods the travel the length of the original 24” ductile iron pipe and are welded to the valve body and then to the 12” flange. The rods broke away from the 12” flange. Some folks think the HDPE pipe moved a few inches away from the valve, causing the mechanical joint to unseat itself. I am wondering if the mechanical joint simply had a leak that, over the course of a few months, washed the soil away from it and cause the pipe to sag. Not sure.

Has anyone had any experience with underground piping moving? The water system didn’t experience any sudden pressure increases. Pressure was normal and then dropped quickly.

We are in the process of excavating the entire HDPE line to check for any movement of the pipe, thrust block, etc.

 

[BigInch]

In general pipe buried 10 feet below ground is unlikely to move, whatever type of pipe it is, unless it is running at very high temperatures. I am inclined to believe that the washout was responsbile for excessive spanning, collapse and movement, in that order. What kind of joints does the iron pipe have? It would help if you could attach a sketch of the configuration, or even better with some pictures too.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[knight185]

Don't have any sketches or photos handy at the moment but you have 12" HDPE Pipe---12x24---Reducer---24" Valve---24" Ductile Iron Pipe. Mechanical joints are used to connect everything together. The joint between the valve and the iron pipe appeared to be okay. The joint between the valve and the reducer was leaking.

I still think this joint was leaking and eventually let loose. The pipe is about 10 foot underground and the water shot several feet up above the ground level.

 

[rconner]

If the 24" reducer end is connected to the valve with a simple UNRESTRAINED mechanical joint, and with no external blocking of the reducer and/or closed valve thrust, I would think this arrangement could indeed be problematic. (Whereas the former 12" DIP could have acted in effect as a quite stiff/strong column, I'm not sure the same would be true of very low long-term strength/modulus plastic.)

 

[knight185]

The only restraints on the outside of the joint were rods that were welded together, but no kind of device that could really keep a closing force on the mechanical joint.

The other thing I wonder about is the inside diameter of the HDPE piping. My facility replaces a lot of old steel piping with HDPE of the same OD, we don't always account for the smaller ID. I wonder if the smaller ID of the 12" HDPE line could be causing a higher pressure at the valve. I wouldn't think the higher pressure would be enough to cause joint failure unless the joint was not properly assembled.

 

[rconner]

What has been posted is a little confusing. If rods are "welded across" joint involved, how did it "let loose"? What pressure, how many of what size/strength rods, and was valve ever closed with differential pressure e.g. From reducer side of gate or disk etc? Or, are you talking about a joint that is still fully inserted, but just leaking as if not firmly bolted up?

 

[rconner]

In re-reading your post, while I am not familiar with the practices you describe of “welding” rods to “valve”(s) and “flanges” as you wrote (and it appears some of same may not have been effective), it appears there may have at least been an effort to provide some sort of restraint across the reducer and valve to maybe a follower gland of the mechanical joint or something similar on the hdpe side. While there are obvious sources of thrust without same that could be applied to the 24" joint where a leak problem has also reportedly been observed (i.e. unbalanced pressure thrust on the annular/donut area of the sloped etc. reducer walls, and/or potential, up to dead end or bulkhead level thrust on the adjacent valve disk/gate if/when closed against same), whoever offered the thought that the hdpe might also pull some on the joint involved is also correct. The two mechanisms of the latter are as follows:

1. Pronounced “Poisson” effect of plastics, and particular in long-term (a straight run of pipe not subjected to other tensile forces shortens, or tries to shorten, significantly when pressure is applied)
2. If the hdpe pipeline was warmed by sun or installed in warm weather, and then at some point water is eventually introduced cooler than that installation level, there is a pronounced thermal shortening, or attempted shortening of plastics.

[As I believe Mr. BigInch indicated, the forces that are applied, however, would be obviated at least some by the soil pressures and friction at 10 feet of cover.]

 

[cvg]

tie rods generally don't break loose without extreme pressure spike or other large, longitudinal force. At some point in recent past, there must have been a pressure surge. It sounds like the 24" line, the valve and reducer were all restrained with tie rods. There should have been no movement unless the rods were broken. DIP can span great distances, so it is unlikely that a small void under a leak would have caused enough sagging to break the rods. A void under the pipe might have allowed the HDPE to sag which could have then pulled away from the reducer.