Pipe Failure

[Rob100]

We have a developer that built an embankment that has failed. Near this embankment, we have a buried 48" pipeline. The line is welded steel carrying municipal water. The question I have been asked is when will the pipe fail due to erosion. I have performed a simple beam analysis using the weight of the pipe and water. The pipeline is in a hillside that is quickly eroding away. What is the best approach to estimating how much soil can erode away? I'm not sure what impact deflection of the pipe will have on failure. This is obviously an emergency so there is no time to do ASME analysis of the piping. Does anyone have a quick a dirty hand calc to approx this?

 

[civilperson]

Your beam approach to span an eroded gully is probably the best estimate of when the lack of support causes distress.

 

[CheckMate]

The question that follows the when will it fail, but "what happens next?" With a 48" dia pipe there is a lot of water and it sounds like you already have an erosion situation. What is on the lower end of this failure?
Can the municipal supply be shut off ahead of time to reduce the weight? If the answer is no, then what are they going to do after the failure?
What is the damage that will occur after the failure?
What support can be temporarily supplied as the line is exposed for support until permanent bridging can be re-established?
Can the source of erosion be diverted to alleviate the problem?
You have many more questions that need to be addressed now, not just how long can we stand and watch it until we are into the secondary trouble.

 

[cvg]

I'm not sure I understand the problem fully. You say the contractor built an embankment and your pipe is near that embankment. Is your pipe in his embankment? Or was it constructed in a trench in natural ground at the base of his fill? Did the contractor place embankment fill, or is this a cut slope and your pipe is near the top of his cut slope / fill slope?

 

[GeoPaveTraffic]

cvg asks some good questions, answers to which will help us to provide better information.

Depending on those answers, you may be able to drive/drill some h-piles in place and weld some pipe supports to give the pipe a chance.

 

[Scott367]

I would be asking how to stop the erosion or protect the pipe against the unstopable erosion. Sounds like the client wants to know how long they can ignore the problem.

 

[LHA]

Run your beam analysis agian with h-piles (as GeoPaveTraffic suggests) or some other cradles in place. Do a couple of iterations and come up with the fewest supports to support the pipe. Instead of just supports, overexcavate and make them solid concrete, so they will act doubly as anti-ssep collars, or trench plugs. This will reduce piping along the outside of the pipe, which should also reduce subsurface erosion. Then, your just faced with matting the surface to prevent surface erosion. With this solution worked out, the question of "How much dirt can we risk lossing?" becomes irrelevant. You also have solid data to start cost estimating, and your client can either - or simultaneously - start budgetting for the fix, and/or initiate a lawsuit for the cost of the fix, and your time.

In the US, it is illegal to cause unmitigated accelerated erosion. So whoever did that has to mitigate the damage.

Engineering is the practice of the art of science - Steve